A social ecological approach to identify the barriers and facilitators to COVID-19 vaccination acceptance: A scoping review

Abstract

Background

COVID-19 is an infectious disease caused by the SARS-CoV-2 virus that has caused substantial impact on population health, healthcare, and social and economic systems around the world. Several vaccines have been developed to control the pandemic with varying effectiveness and safety profiles. One of the biggest obstacles to implementing successful vaccination programmes is vaccine hesitancy stemming from concerns about effectiveness and safety. This review aims to identify the factors influencing COVID-19 vaccine hesitancy and acceptance and to organize the factors using the social ecological framework.

Methods

We adopted the five-stage methodological framework developed by Arksey and O’Malley to guide this scoping review. Selection criteria was based on the PICo (Population, Phenomenon of interest and Context) framework. Factors associated with acceptance and hesitancy were grouped into the following: intrapersonal, interpersonal, institutional, community, and public policy factors using the social ecological framework.

Results

Fifty-one studies fulfilled this review’s inclusion criteria. Most studies were conducted in Europe and North America, followed by Asia and the Middle East. COVID-19 vaccine acceptance and hesitancy rates varied across countries. Some common demographic factors associated with hesitancy were younger age, being female, having lower than college education, and having a lower income level. Most of the barriers and facilitators to acceptance of the COVID-19 vaccines were intrapersonal factors, such as personal characteristics and preferences, concerns with COVID-19 vaccines, history/perception of general vaccination, and knowledge of COVID-19 and health. The remaining interpersonal, institution, community, and public policy factors were grouped into factors identified as barriers and facilitators.

Conclusion

Our review identified barriers and facilitators of vaccine acceptance and hesitancy and organised them using the social ecological framework. While some barriers and facilitators such as vaccine safety are universal, differentiated barriers might exist for different target groups, which need to be understood if they are to be addressed to maximize vaccine acceptance.

Introduction

COVID-19 is an infectious disease caused by the SARS-CoV-2 virus that has resulted in significant impact on the health of the world’s population, with over 500 million confirmed cases and over 6.2 million deaths worldwide as of 3^rd June 2022 [1]. Other consequences include causing high levels of psychological distress as well as financial challenges for countries and their healthcare systems [2, 3]. Hence, measures such as lockdowns, social distancing, and travel restrictions were imposed by many countries in an attempt to reduce the spread of infections. While such measures can help [4], they are not long-term solutions.

Vaccinations can have an integral role in restoring normalcy [5]. While several viable candidates have been developed, approved, and distributed, uptake of these vaccines can be hindered by vaccine hesitancy. A systematic review on COVID-19 vaccine acceptance rate found varying rates among the public and health care professionals in different countries [6]. The updated review also found variation of acceptance rates in different regions, with the Middle East/North Africa, Europe and central Asia, and Western/Central Africa having more COVID-19 vaccine hesitancy [7]. Common reasons cited for vaccine hesitancies are concerns with the safety or effectiveness of the vaccines [8–13].

On the other hand, a study using large-scale retrospective data around the world found that confidence in the importance of vaccines promoted vaccine uptake acceptance, whereas those in minority religious groups were more likely to be hesitant [14]. An individual’s political affiliation is also among the contributing factors to COVID-19 vaccine hesitancy [15, 16]. Clearly, there is a need to consider the human factor to ensure widespread acceptance of COVID-19 vaccines [5]. Recommendations from the World Health Organization (WHO) centered around the need to improve knowledge regarding vaccine safety and long-term effectiveness, ensure accessibility, and employing targeted interventions to address the concerns of specific populations [17].

To better understand the factors that posed as barriers or facilitated acceptance of COVID-19 vaccines in the general public adult population, we conducted a scoping review using the social ecological framework to segment the level of influences: Intrapersonal factors, interpersonal processes, institutional factors, community factors, and public policy [18, 19]. As vaccine hesitancy is a complex issue that entails much more than one’s attitude or behaviour [20, 21], using the social ecological model could enhance understanding of external factors such as access to the vaccines that involve wider system-related factors. The identified factors would also be organized into barriers and facilitators to clarify if their influences on the acceptance or hesitancy of COVID-19 vaccination are mutually exclusive or are more complexly intertwined.

While there have been other scoping reviews conducted on the determinants to vaccine acceptance or hesitancy [11–13, 22], our review focused on studies conducted on the general adult population that took place after the release of major COVID-19 vaccine clinical trial results to account for possible shifting reasons for vaccine acceptance and hesitancy. This review contributes by taking time sensitivity of the evolving barriers and facilitators into consideration [23].

Methods

We adopted the five-stage methodological framework developed by Arksey and O’Malley [24] with advancements to the framework proposed by Levac, Colquhoun and O’Brien [25] to guide this scoping review. In order to capture a variety of study types, the scoping review methodology was used. Quality assessments on the studies were not performed to allow for inclusion of more studies, as our purpose was to scope factors that could pose as barriers or facilitators to COVID-19 vaccine acceptance [24].

The research question

The research question was refined through reviewing the literature on acceptance of the COVID-19 vaccines: What are the barriers and facilitators that affect the acceptance of COVID-19 vaccines among adults in the general public?

Identifying relevant studies

The authors developed a search strategy with a medical librarian guided by the following key terms: COVID-19 or nCoV* or 2019nCoV or 19nCoV or COVID19* or COVID or SARS-COV-2 or SARSCOV-2 or SARSCOV2 or “Severe Acute Respiratory Syndrome Coronavirus 2” or “Severe Acute Respiratory Syndrome Corona Virus 2” [26] and Vaccination Refusal or Anti-Vaccination Movement or Mass Vaccination or Vaccination Coverage (hesitancy or acceptance or preference or rejection or anti-vaccination or attitude or barrier or facilitator or intention). The search strategy was adapted for PubMed, Medline, Embase, PsycInfo, and CINAHL and conducted on April 14, 2021. S1 Table shows the full search strategy in the respective databases. Reference lists of review papers found were also searched.

Study selection

Selection criteria was based on the PICo (Population, Phenomenon of interest and Context) framework [27]. Table 1 presents details of the criteria. Time frame on the data collection (study) period rather than publication date was imposed for two reasons. Firstly, a recent scoping review reported on attitudes towards COVID-19 vaccination using the social-ecological model had included studies conducted before September 2020 [22]. Secondly, as people might change their minds with new information, we wanted to capture factors and perspectives that were more aligned with the actual COVID-19 vaccine situation after publication of the Pfizer and Moderna phase I/II trials data in the summer of 2020.

[Figure omitted. See PDF.]

The title/abstract screening were conducted by three reviewers (BT, PL, JG) independently, with one main reviewer (BT) going through all titles and abstracts, while PL and JG shared the task as second reviewers. Any disagreements between two reviewers were discussed and resolved by the third reviewer (BT, PL, JG). During the full text review, each full text was again screened by two reviewers (combination of BT/PL, BT/JG, PL/JG) and disagreements were resolved by the third. As reported by Levac and colleagues [25], the screening process was iterative in order to refine the inclusion criteria. To calibrate our understanding during the screening process, disagreements were reviewed on a regular basis.

Charting the data

The data extraction form was developed and pilot-tested before the start of the extraction phase. The extraction fields included publication details, study design and population, rates of COVID-19 vaccine acceptance/hesitancy, and reported factors influencing COVID-19 vaccine acceptance and hesitancy. Similar to studies that examined such factors using barriers and facilitators [28, 29], we defined factors associated with acceptance as facilitators and factors that deterred acceptance of the vaccination as barriers. These factors were then categorized using the social ecological framework [19] into intrapersonal factors, interpersonal processes, institutional/organizational factors, community factors, and public policy.

Intrapersonal factors include one’s knowledge, attitudes, behavior, self-concepts, and skills, whereas interpersonal factors are the influences from formal and informal social networks [19]. Institutional factors describe “social institutions and organization characteristics, and formal (and informal) rules and regulations for operations” [19] that impact people’s acceptance of the COVID-19 vaccines and vaccination. Community factors refer to relationships among organizations and groups with boundaries defined by geographical and political terms, whereas public policy refers to policies or programs that promote or deter certain behaviors [19]. To better conceptualize each level in the social ecological model, sub-categories defined in other vaccine studies using the framework were adapted to guide allocation of the extracted factors [30, 31]. To ensure objectivity, data from each included article was extracted by one reviewer independently (BT, JG, or PL) and 20% of the extracted data were cross-checked by a second reviewer (BT, JG, or PL). The screening and data extraction were conducted in Covidence systematic review software [32]. S3 and S4 Tables show the data extracted and the excluded full text with reasons respectively.

Collation, summarizing, and reporting the results

The results were collated and summarized descriptively by the three reviewers (BT, JG, PL). The factors and themes (when necessary) identified were classified under barriers or facilitators in the social ecological framework. These were shared and discussed with the remaining research team before finalization. Preferred Reporting Items for Systematic Reviews and Meta-Analyses Extension for Scoping Review (PRISMAScR) was used to report this scoping review (see S5 Table) [33].

Results

A total of 1066 unique citations were found using our search strategy. After the title/abstract screening, 233 citations were included for full text screening, of which 51 articles fulfilled the inclusion criteria for synthesis. Most of these articles were published in 2021(n = 46, 90%), whereas the rest were published in 2020. Fig 1 shows the PRISMA flow diagram of the screening process for the review [34].

[Figure omitted. See PDF.]

Study characteristics

A Majority of the studies were cross-sectional studies (n = 32, 63%), with data collected via surveys (n = 45, 88%). Most of the studies had sample sizes over 1000 (n = 32, 63%) and included participants from Europe (n = 15, 29%), North America (n = 15, 29%), Asia (n = 9, 18%), and the Middle East (n = 5, 10%). In addition, a majority of the studies included adults from 18 years and above, while 6 studies included participants from 15–16 years [35–40]. One study was conducted solely on older adults [41]. Nineteen studies reported the mean age of their overall study participants, which were mostly in the 40s. Table 2 shows study characteristics (S2 Table provides study characteristics details).

[Figure omitted. See PDF.]

Acceptance/Hesitancy rate

Thirty-seven of the 51 studies reported COVID-19 vaccine acceptance and/or hesitancy rates. Most of these studies assessed one’s willingness or intentions to take the COVID-19 vaccines when available, using Likert-type scales, instead of validated scales such as the Vaccination Attitudes Examination (VAX) Scale [42, 43]. One study measured trust in the vaccines [44]. For the purpose of our study, the hesitancy rates included those who: (a) reported unwillingness to accept the vaccination, (b) were strongly hesitant, or (c) reported being very unlikely to accept COVID-19 vaccination.

Global acceptance rates ranged from 13.1–91%, and hesitancy rates ranged from 7–86.6%. Studies from Spain (77.6%), Germany (78.2%), China (81.3–88.6%), and India (83.6%) reported acceptance rates over 70%. Some studies in Italy [45], UK [35, 40, 46], and USA [47] also reported acceptance rates over 70%. On the other hand, studies from Hong Kong [48] and several middle Eastern countries [36] reported acceptance rates of below 30%. Acceptance and/or hesitancy rates could not be determined in 13 studies, as they were experimental studies or social media studies that measured positive and negative sentiments towards the vaccines, or were studies that presented only stratified results (e.g., gender, ethnic). Table 3 summarizes the acceptance and hesitancy rates by country.

[Figure omitted. See PDF.]

Barriers and facilitators using social ecological framework

Socio-demographic determinants.

In general, most studies found that lower age, being female, having lower than college education, having a lower income level, and having or living with school age children were commonly cited determinants of vaccine hesitancy and rejection. A minority of the studies found opposite trends. For example, two studies found that males were more hesitant than females [49, 50], whereas one study found that higher education was a barrier [48] and another where having the least education was a facilitator [51]. With regards to age, a positive trend of COVID-19 vaccine acceptance among older adults above 60 to 70 were reported in several studies [9, 10, 38, 48, 52–54]. However, one study conducted in Qatar found that being older than 65 was a significant factor in vaccine hesitancy, compared to the younger participants surveyed [43]. Interestingly, a discrete choice experiment study found that those aged 55 and above were significantly more likely to reject a vaccine with 50% protection, while being significantly more likely to accept a vaccine with 90% protection, when compared to the younger participants [40].

In addition, a few studies that identified ethnic minority as a determinant of vaccine hesitancy were conducted in the UK and USA [10, 35, 46, 47, 52, 55–57]. Although few studies measured political leanings, their results were consistent, where those identifying with conservative ideology tended to be more hesitant [44, 58], compared to those identifying with liberal/democratic political ideology [44, 52, 58, 59]. Having comorbidities could either be a barrier, when one has compromised immunity or conditions in which the vaccine was not recommended [44, 60], or a facilitator [10, 36, 41, 48–50, 53, 54, 61, 62] where one identifies as being vulnerable to the severe effects of COVID-19. Likewise, perceived fair or good health could be a barrier [9, 52] or perceived reasonable health as a facilitator [61] to one’s willingness to be vaccinated. Current smokers [65, 72], and those who have psychological distress, were less likely to accept the vaccine [56]. Table 4 shows a full list of the social demographic factors identified.

[Figure omitted. See PDF.]

Intrapersonal factors.

As there were many identified factors in this category, they were further categorized into broader themes, namely: Individual characteristics and preferences, concerns with COVID-19 vaccines, history/experience of vaccination, and knowledge/perception of COVID-19 and health-related information. Fig 2 summarizes factors/themes identified in the social ecological framework.

[Figure omitted. See PDF.]

Individual characteristics and preferences. Those who have had COVID-19 infections and believed that they have acquired immunity [55, 60], and those who preferred to acquire immunity naturally, were less likely to accept the vaccination [37, 38, 42–44, 50, 65, 72]. Those who experienced negative emotions (felt agitated, sad, or anxious) or adhered less with health measures and guidelines, such as wearing a mask and safe distancing, or not having been quarantined, were also less likely to accept the vaccine [42, 46, 49, 61]. Having a strong perception of one’s autonomy [71] and not believing that being vaccinated was one’s civic duty as a citizen [47] were barriers to accepting the vaccines as well. On the other hand, the belief that getting vaccinated protected others or was one’s social responsibility was associated with acceptance towards COVID-19 vaccination [29, 35, 47, 50–53, 70, 73, 74]. The desire to return to normalcy also facilitated acceptance of the vaccines [35, 54, 74, 75]. Lastly, one study conducted in Saudi Arabia found that those believing in mandatory vaccinations were more likely to accept the COVID-19 vaccination [69].

Concerns with COVID-19 vaccines. Our findings identified that major barriers to acceptance of COVID-19 vaccines were doubts or mistrusts on their effectiveness and benefits [9, 29, 38, 42–44, 50, 51, 54, 55, 60, 62, 65, 66, 72, 74, 76, 77], including not having enough information or evidence [38, 65, 66, 74]. Perceived risk of the vaccines from their potential side effects or unforeseen longer-term effects were also identified as barriers in many studies [9, 10, 29, 35, 38, 42, 48–50, 54, 55, 58, 60, 62, 65, 72, 73, 76–78]. Other perceived barriers included concerns over the following: Requiring more than one dose or booster doses, convenience (e.g., timing of vaccination appointment), not being able to receive proof of vaccination, or the need to submit personal information etc. [29, 40, 49, 52, 78]. Conversely, perceiving higher benefits than risks, such as high vaccine efficacy, facilitated acceptance of the vaccines [29, 35, 38, 41, 48, 50, 52–54, 65, 67, 68, 71, 73, 78]. Accessibility influences acceptance/hesitancy as well, with studies suggesting that ease of access relates to higher acceptance [51, 53, 59, 68].

History/experience of vaccination. Those who had a history of rejecting vaccines, including flu vaccines and those who mistrusted vaccines in general, were less likely to accept the COVID-19 vaccines [9, 35, 38, 42, 44, 50, 54, 55, 60, 61, 66, 69, 72, 76, 78], whilst those who had a history of being vaccinated were more likely to accept them [9, 37, 38, 43, 48, 52, 59, 61, 65, 69, 73, 74]. Factors such as having had adverse reactions or bad experiences with previous vaccinations [9, 37, 38, 74], and being fearful of needles or injection [38, 50, 54, 55], contributed to COVID-19 vaccine hesitancy.

Knowledge/perception on COVID-19 and health-related information. Those who perceived a high risk of contracting COVID-19, and/or possible severe consequences from COVID-19, were more likely to accept the vaccination [41, 44, 46, 47, 51, 52, 54, 59, 61, 65, 66, 68, 69, 73, 78], whereas those who perceived a low or non-existing risk of infection, or believed that they were less likely to develop complications, were less likely to accept the COVID-19 vaccination [52, 54, 55, 57, 61, 65, 77]. The latter group included those who believed that the COVID-19 symptoms were mild or had been exaggerated. In addition, belief in conspiracy theories (e.g., COVID-19 is hoax, vaccines are used to control or kill people) was also found to be a barrier to COVID-19 vaccination acceptance [36, 37, 46, 57, 60, 62, 66]. Table 5 shows full list of the intrapersonal factors.

[Figure omitted. See PDF.]

Interpersonal factors.

Influence from those who were vaccinated, influence from those infected with COVID-19, and normative influences from family and friends were three factors found that impact one’s perception and attitude toward COVID-19 vaccination acceptance. Knowing someone with a serious vaccine reaction [52] or not knowing anyone close who was affected by COVID-19 [64] were barriers to one’s acceptance, whilst knowing someone who was infected [66], or knowing people who had been or intended to get vaccinated [29, 48, 54], facilitated acceptance. In addition, trusting information or valuing opinions of family, friends, and acquaintances could either be a barrier [37] or a facilitator [38, 71], depending on their own views on hesitancy or acceptance. For example, one study found that the opinion of family and friends was significantly correlated with participants’ willingness to take a COVID-19 vaccine [38].

Institutional/Organizational factors.

Four institutional/organizational factors were found to influence one’s acceptance of the vaccine: (a) Trust/confidence in governments/authorities, (b) trust/confidence in science, and/or healthcare/healthcare personnel, (c) development of COVID-19 vaccines, and (d) impact of social media vs. legacy media (e.g., newspapers, radio, television). Studies that measured social media responses were included in this category.

Low trust or mistrust toward the government and health authorities, such as WHO and the Centres for Disease Control and Prevention (CDC), could pose as a barrier to one’s acceptance of the COVID-19 vaccination [37, 40, 43, 44, 47, 52, 55, 61, 63, 66, 74], whereas high trust and confidence in the government and health authorities promoted acceptance of the vaccines [10, 37, 44, 48, 52]. Similarly, low trust or mistrust of the pharmaceutical industry and healthcare providers would make one less likely to accept the COVID-19 vaccines [44, 46, 47, 52, 61, 66, 76, 77], including those who were concerned with profiteering by the pharmaceutical companies [29, 42, 72]. A lack of recommendation by a doctor has been cited as a reason for not intending to take the COVID-19 vaccine [55], whereas having a recommendation from a doctor influenced vaccine acceptance [67, 78].

In addition, the speedy development of the vaccines [29, 55, 60, 76, 77], the manufacturing country [48, 77, 81], and the perceived lack of information or clear data (e.g., inconsistent/ contradictory/delays and trial pauses) contributed to people’s hesitancy [9, 60, 61, 79]. While trusting alternative sources of information, and exposure to coverage on WhatsApp, blogs, and social media were more likely to increase one’s hesitancy toward the vaccines [37, 40, 57], one study found that frequent exposure to positive social media messages facilitated one’s acceptance [48]. As expected, those who trusted official sources of information and legacy media were less likely to be hesitant toward the vaccines [37, 44, 57, 75, 79]. A point to note is that those who thought that the media over-reported on the side effects of the vaccines facilitated their acceptance of the vaccines [41].

Community factor and public policy.

Low magnitude of community spread (e.g., number of confirmed or suspected cases in the county) could pose as a barrier to vaccine acceptance [78]. In addition, rate of community vaccination could either result in low perceived risk, which would be a barrier [50] or a facilitator, if that nudged one into getting vaccinated [67]. Meanwhile, the choice of the vaccination venue could either be a barrier [40, 49] or a facilitator [40] to one’s willingness to be vaccinated, depending on if their preferences were met. Cost of the vaccine [39, 78] or associated cost with the vaccines [55] could also be a barrier to some, especially for those without health insurance [55].

The effects of politics have spilled over to influence people on their hesitancy or acceptance of the vaccines. Those who believed that political pressure had influenced the development speed of the vaccine were more likely to be hesitant toward accepting the vaccines, and depending on endorsement of one’s political/public figure of choice, this could either be a barrier [58, 66, 76, 79, 81, 82] or facilitator [58, 82] to accepting COVID-19 vaccination. Table 6 presents a full list of other factors (interpersonal, institutional, community, public policy).

[Figure omitted. See PDF.]

Discussion

COVID-19 vaccine acceptance and uptake are important factors that could help curb the spread and severity of the disease. Our review distils the factors reported around the world into various levels in the social ecological model revealing the environmental influencing factors at play [19, 83], upon which targeted interventions or policies could be considered. By organizing the factors into barriers and facilitators, the comparison highlighted that the same factor could either promote or deter vaccine acceptance.

In consensus with other reviews, demographic factors such as being female, being younger, having lower education, or having a low income contributed to COVID 19 vaccine hesitancy [11–13, 22, 84]. Similar to the findings by Al-Jayyousi et al [22], our results show that most factors identified are related to intrapersonal level on one’s knowledge, attitudes, behavior, self-concepts, and skills. These factors are a culmination of one’s experiences and interactions with the environment, which would be difficult to influence quickly. For example, pre-existing factors that impact vaccine acceptance or hesitancy include history and perception of general vaccinations, knowledge of COVID-19 and health-related information, belief in conspiracy theories, as well as personal characteristics and preferences [22].

A major intrapersonal factor that is unique to COVID-19 vaccines is the concern regarding their effectiveness and potential side effects, including their long-term safety, which were also identified in other reviews [11, 12, 22, 76]. These are valid concerns given the accelerated rate of the COVID-19 vaccine development and the lack of long-term safety data [85, 86]. Although many benefitted, and are still benefitting from this unprecedented speed in the history of vaccine development, others cited it as a concern that led to their hesitancy [29, 55, 60, 76, 77]. A recent multi-country survey that tested acceptance of four hypothetical COVID-19 vaccines with varying efficacy and safety profiles found that higher efficacy and lower risks increased the acceptance level among study participants [87]. An interesting point to note from this study was that those believing that new vaccines are riskier than older vaccines were less likely to accept any of the new hypothetical vaccines [87].

Besides intrapersonal factors, common factors found in COVID-19 vaccine acceptance or hesitancy can be broadly summarized into trust in authorities (government/health care including pharmaceutical) and trust in legacy media versus social media [11, 12, 22, 76]. In our review, such factors were categorised under institutional factors. Citing concerns on effectiveness and safety of a vaccine imply some level of doubt in the authorities [88]. Trust and confidence in any authorities stem from historic and existing systems that could not be addressed instantly but would affect people’s attitude towards the current recommended vaccines [21, 88, 89]. Understanding weaknesses in the system and investing in better healthcare structure would be longer-range goals that could ultimately address people’s trust issues [21]. For more immediate results, communicating consistent information on efficacy and safety of the COVID-19 vaccines could impact people’s acceptance of the vaccines [87], which might help combat misinformation in the media and social media.

While influence from family and friends was briefly mentioned in the other reviews, our findings suggested that interpersonal level of influence could play a substantial role in swaying one toward or away from acceptance of the COVID-19 vaccines. One would be more hesitant if they knew someone who had a serious adverse reaction from the vaccine [52] or did not know someone close that was affected by the disease [64], whereas knowing people who were vaccinated, especially one’s role model [29, 48, 54], or knowing someone infected with COVID-19 [66], would facilitate vaccine acceptance. However, it could also be the case that individuals tend to socialise with others like them (i.e., someone pro-vaccination would likely socialise with other pro-vaccination individuals). Two studies found that valuing opinions of family or friends who share the same views/beliefs facilitated COVID-19 vaccine acceptance [38, 71]. Hence, the exact nature of the role played by family and friends in vaccination decisions should be explored further in future research so that policies or programs that target those who are hesitant could consider extending beyond that individual.

As demonstrated in the above example, our findings highlighted that even though some factors seemed to present a clearer direction for intervention or policy, others were more context-dependent and not clearly a barrier or facilitator. For instance, having comorbidities or belonging to a high-risk group could be a positive factor that motivates people to be vaccinated [10, 36, 41, 48–50, 53, 54, 61, 62]. Conversely, the same factor presented as a barrier for others who were likely concerned with how the vaccine would impact their medical conditions or health [44, 60]. However, this could be due to an earlier advisory which cautioned against vaccination for those with certain medical conditions, which has since changed [90, 91]. Perceived good health could also pose either as a barrier [9, 52] or a facilitator [61] to one’s willingness to be vaccinated. In short, having contextual information on the target population will be crucial to understand factors that pose as barriers and facilitators.

Implications

Although organizing the factors by respective levels of influence in the social ecological framework provided some distinctions that could inform areas for potential interventions or policies, they are still very much intertwined and pose more questions, such as the exact nature of the role played by family and friends in vaccination decisions. Further research on this relationship might make identifying effective strategies to overcome barriers easier.

Consequently, the need to understand the context, especially of the barriers, should be emphasized, since the same factor could be a facilitator to some while a barrier to others. Structural barriers [92] such as the community and public policy factors affect access and could impact people’s acceptance to be vaccinated. Attitudinal barriers [92] such as the factors identified in the intrapersonal, interpersonal, and institutional/organizational levels played a major role in influencing one’s acceptance or hesitancy but are more complex to address. Some of these attitudinal barriers could stem from institutional or policy level gaps that would only be known if time and effort were taken to understand them [21]. This could be achieved through engaging targeted sub-groups, groups, or communities through partnership [92], such as through dialogues.

In addition, COVID-19 is the first pandemic in the 21^st century with unprecedented world-wide aftermath, as well as having on-going impacts on population health and economics [93]. As parts of the world are beginning to recover from the aftermath of the initial COVID-19 infection, COVID-19 has been shown to be a moving target that would continue to influence people’s views and acceptance of the vaccines. With scientists warning of more of such infectious diseases in the future [94, 95], it might be important to study the current dynamic response of people’s acceptance and hesitancy toward COVID-19 vaccines during different periods of the pandemic. For example, the findings from this review provide some insights into the general population’s views on accepting a newly developed vaccine after results of the trials confirmed they were effective and safe. Insights gathered could serve as a guide to future response plans for new infectious disease outbreaks (e.g., by pre-emptively addressing concerns before commencing a nationwide vaccination effort).

Lastly, COVID-19 vaccine rollout prioritized those most at risk, such as healthcare workers and older adults; the latter are the most vulnerable to severe infection and death. As people’s needs and situations are heterogeneous, a customized approach to different segments of the population has proven to be both pragmatic and essential. Seeking to understand which factors pose as barriers or facilitators, and for which populations, could help inform context-relevant policies or programs.

Limitations

Our findings have several limitations. It is possible that some studies have been missed by our search strategy, due to adopting a more general search strategy. Summarizing the heterogeneous studies was challenging, especially on what vaccine hesitancy entails, which was not explored in depth due to the complexity of the ongoing discussion surrounding the term vaccine hesitancy; thus, we determined it to be beyond the scope of our study. For instance, most studies developed their own surveys to measure vaccine acceptance or hesitancy and the lack of methodological equivalence could cause differences in findings. In addition, we have several inherent limitations when using the social-ecological framework, as classifying factors into the five categories were subjective. Although we minimised the subjectivity through defining sub-categories of each factor in the framework a priori, and performed cross checks to calibrate our understanding and agreement, we acknowledge that subjectivity may not have been completely eliminated.

While there are limitations in using barriers and facilitators, due to the overlapping of factors (as well as not clearly addressing the interrelatedness of those factors) [23], the social- ecological framework has helped to frame those realms, which provided some clarity and insights. The above limitations notwithstanding, our review has identified important barriers and facilitators of vaccine acceptance and hesitancy.

Conclusion

Our review has identified barriers and facilitators of vaccine acceptance and hesitancy and organised them using the social ecological framework. These factors are context-, population-, and even sub-population-dependent, which could present either as barriers or facilitators. It also shows that factors associated with COVID-19 vaccine acceptance and hesitancy could stem from different levels of influence that are intertwined. Our findings present a general scope of barriers or facilitators that should be considered when developing programs or policies to promote acceptance and uptake of the COVID-19 vaccines, while highlighting the need to also consider the varying contexts experienced by different population.

Supporting information

S1 Table. Databases search strategy.

https://doi.org/10.1371/journal.pone.0272642.s001

(DOCX)

S2 Table. Study characteristics details.

https://doi.org/10.1371/journal.pone.0272642.s002

(DOCX)

S3 Table. Data extraction sheet.

https://doi.org/10.1371/journal.pone.0272642.s003

(XLSX)

S4 Table. Excluded studies from full text screening.

https://doi.org/10.1371/journal.pone.0272642.s004

(DOCX)

S5 Table. Preferred reporting items for systematic reviews and meta-analyses extension for scoping reviews (PRISMA-ScR) checklist.

https://doi.org/10.1371/journal.pone.0272642.s005

Acknowledgments

We would like to express our sincere thanks to Ms Yasmin Lynda Munro at the Nanyang Technological University, Lee Kong Chian School of Medicine (LKCMedicine) Medical Library, for her advice and assistance on the search strategy and the database searches.

Citation: Lun P, Gao J, Tang B, Yu CC, Jabbar KA, Low JA, et al. (2022) A social ecological approach to identify the barriers and facilitators to COVID-19 vaccination acceptance: A scoping review. PLoS ONE 17(10): e0272642. https://doi.org/10.1371/journal.pone.0272642

About the Authors:

Penny Lun

Roles: Data curation, Formal analysis, Investigation, Project administration, Visualization, Writing – original draft, Writing – review & editing

E-mail: [email protected]

Affiliation: Geriatric Education and Research Institute Limited, Singapore, Singapore

https://orcid.org/0000-0001-8125-7411

Jonathan Gao

Roles: Data curation, Formal analysis, Investigation, Visualization, Writing – original draft, Writing – review & editing

Affiliation: Geriatric Education and Research Institute Limited, Singapore, Singapore

Bernard Tang

Roles: Data curation, Formal analysis, Investigation, Validation, Writing – review & editing

Affiliation: Geriatric Education and Research Institute Limited, Singapore, Singapore

Chou Chuen Yu

Roles: Conceptualization, Supervision, Writing – review & editing

Affiliation: Geriatric Education and Research Institute Limited, Singapore, Singapore

Khalid Abdul Jabbar

Roles: Writing – review & editing

Affiliation: Geriatric Education and Research Institute Limited, Singapore, Singapore

James Alvin Low

Roles: Conceptualization, Supervision, Writing – review & editing

Affiliations Geriatric Education and Research Institute Limited, Singapore, Singapore, Geriatric Medicine, Khoo Teck Puat Hospital, Singapore, Singapore, Lee Kong Chian School of Medicine, Nanyang Technological University, Singapore, Singapore

Pradeep Paul George

Roles: Conceptualization, Supervision, Writing – review & editing

Affiliations Geriatric Education and Research Institute Limited, Singapore, Singapore, Lee Kong Chian School of Medicine, Nanyang Technological University, Singapore, Singapore, Health Services and Outcomes Research, National Healthcare Group, Singapore, Singapore, Faculty of Health Sciences, University of Adelaide, Adelaide, Australia

https://orcid.org/0000-0003-4743-1425

References

1. WHO Coronavirus (COVID-19) Dashboard [Internet]. [cited 2022 Jan 27]. Available from: https://covid19.who.int

2. Xiong J, Lipsitz O, Nasri F, Lui LMW, Gill H, Phan L, et al. Impact of COVID-19 pandemic on mental health in the general population: A systematic review. Journal of Affective Disorders. 2020 Dec;277:55–64. pmid:32799105

3. Kaye AD, Okeagu CN, Pham AD, Silva RA, Hurley JJ, Arron BL, et al. Economic impact of COVID-19 pandemic on healthcare facilities and systems: International perspectives. Best Practice & Research Clinical Anaesthesiology. 2021 Oct 1;35(3):293–306. pmid:34511220

4. Rawson T, Huntingford C, Bonsall MB. Temporary “Circuit Breaker” Lockdowns Could Effectively Delay a COVID-19 Second Wave Infection Peak to Early Spring. Front Public Health. 2020 Dec 7;8:614945. pmid:33365299

5. Schoch-Spana M, Brunson EK, Long R, Ruth A, Ravi SJ, Trotochaud M, et al. The public’s role in COVID-19 vaccination: Human-centered recommendations to enhance pandemic vaccine awareness, access, and acceptance in the United States. Vaccine. 2021 Sep;39(40):6004–12. pmid:33160755

6. Sallam M. COVID-19 Vaccine Hesitancy Worldwide: A Concise Systematic Review of Vaccine Acceptance Rates. Vaccines (Basel). 2021 Feb 16;9(2):160. pmid:33669441

7. Sallam M, Al-Sanafi M, Sallam M. A Global Map of COVID-19 Vaccine Acceptance Rates per Country: An Updated Concise Narrative Review. J Multidiscip Healthc. 2022 Jan 11;15:21–45. pmid:35046661

8. Tan M, Straughan PT, Lim W, Cheong G. Special report on COVID-19 vaccination trends among older adults in Singapore. ROSA Research Briefs. 2021 Jul 1;1–15.

9. Gerussi V, Peghin M, Palese A, Bressan V, Visintini E, Bontempo G, et al. Vaccine Hesitancy among Italian Patients Recovered from COVID-19 Infection towards Influenza and Sars-Cov-2 Vaccination. Vaccines. 2021 Feb 18;9(2):172. pmid:33670661

10. Daly M, Robinson E. Willingness to Vaccinate Against COVID-19 in the U.S.: Representative Longitudinal Evidence From April to October 2020. American Journal of Preventive Medicine. 2021 Jun;60(6):766–73. pmid:33773862

11. Biswas MR, Alzubaidi MS, Shah U, Abd-Alrazaq AA, Shah Z. A Scoping Review to Find Out Worldwide COVID-19 Vaccine Hesitancy and Its Underlying Determinants. Vaccines (Basel). 2021 Oct 25;9(11):1243. pmid:34835174

12. Joshi A, Kaur M, Kaur R, Grover A, Nash D, El-Mohandes A. Predictors of COVID-19 Vaccine Acceptance, Intention, and Hesitancy: A Scoping Review. Front Public Health. 2021 Aug 13;9:698111. pmid:34485229

13. Aw J, Seng JJB, Seah SSY, Low LL. COVID-19 Vaccine Hesitancy—A Scoping Review of Literature in High-Income Countries. Vaccines. 2021 Aug 13;9(8):900. pmid:34452026

14. de Figueiredo A, Simas C, Karafillakis E, Paterson P, Larson HJ. Mapping global trends in vaccine confidence and investigating barriers to vaccine uptake: a large-scale retrospective temporal modelling study. The Lancet. 2020 Sep;396(10255):898–908. pmid:32919524

15. Kaufman J, Bagot KL, Tuckerman J, Biezen R, Oliver J, Jos C, et al. Qualitative exploration of intentions, concerns and information needs of vaccine‐hesitant adults initially prioritised to receive COVID‐19 vaccines in Australia. Australian and New Zealand Journal of Public Health. 2021 Dec 13;1753–6405.13184. pmid:34897909

16. El-Mohandes A, White TM, Wyka K, Rauh L, Rabin K, Kimball SH, et al. COVID-19 vaccine acceptance among adults in four major US metropolitan areas and nationwide. Sci Rep. 2021 Dec;11(1):21844. pmid:34737319

17. World Health Organization SAGE Working Group. Report of the SAGE Working Group on Vaccine Hesitancy [Internet]. WHO; 2014 [cited 2021 Dec 20]. Available from: https://www.who.int/immunization/sage/meetings/2014/october/1_Report_WORKING_GROUP_vaccine_hesitancy_final.pdf

18. Bronfenbrenner U. Toward an experimental ecology of human development. American Psychologist. 1977;32(7):513–31.

19. McLeroy KR, Bibeau D, Steckler A, Glanz K. An Ecological Perspective on Health Promotion Programs. Health Education Quarterly. 1988 Dec;15(4):351–77. pmid:3068205

20. Bedford H, Attwell K, Danchin M, Marshall H, Corben P, Leask J. Vaccine hesitancy, refusal and access barriers: The need for clarity in terminology. Vaccine. 2018 Oct 22;36(44):6556–8. pmid:28830694

21. Attwell K, Hannah A, Leask J. COVID-19: talk of ‘vaccine hesitancy’ lets governments off the hook. Nature. 2022 Feb;602(7898):574–7. pmid:35194212

22. Al-Jayyousi GF, Sherbash MAM, Ali LAM, El-Heneidy A, Alhussaini NWZ, Elhassan MEA, et al. Factors Influencing Public Attitudes towards COVID-19 Vaccination: A Scoping Review Informed by the Socio-Ecological Model. Vaccines. 2021 May 24;9(6):548. pmid:34073757

23. Bach‐Mortensen AM, Verboom B. Barriers and facilitators systematic reviews in health: A methodological review and recommendations for reviewers. Res Syn Meth. 2020 Nov;11(6):743–59. pmid:32845574

24. Arksey H , O’Malley L. Scoping studies: towards a methodological framework. International Journal of Social Research Methodology. 2005 Feb;8(1):19–32.

25. Levac D, Colquhoun H, O’Brien KK. Scoping studies: advancing the methodology. Implementation Sci. 2010 Dec;5(1):69. pmid:20854677

26. CADTH COVID-19 Search Strings [Internet]. CADTH Covid-19 Evidence Portal. [cited 2021 Sep 1]. Available from: https://covid.cadth.ca/literature-searching-tools/cadth-covid-19-search-strings/

27. Richardson WS, Wilson MC, Nishikawa J, Hayward RS. The well-built clinical question: a key to evidence-based decisions. ACP J Club. 1995 Dec;123(3):A12–13. pmid:7582737

28. Rehman K, Hakim M, Arif N, Islam SU, Saboor A, Asif M, et al. COVID-19 vaccine acceptance, barriers and facilitators among healthcare workers in Pakistan [Internet]. In Review; 2021 Apr [cited 2022 May 3]. Available from: https://www.researchsquare.com/article/rs-431543/v1

29. Kumari A, Ranjan P, Chopra S, Kaur D, Kaur T, Upadhyay AD, et al. Knowledge, barriers and facilitators regarding COVID-19 vaccine and vaccination programme among the general population: A cross-sectional survey from one thousand two hundred and forty-nine participants. Diabetes Metab Syndr. 2021;15(3):987–92. pmid:33984818

30. Latkin C, Dayton LA, Yi G, Konstantopoulos A, Park J, Maulsby C, et al. COVID-19 vaccine intentions in the United States, a social-ecological framework. Vaccine. 2021 Apr;39(16):2288–94. pmid:33771392

31. Kumar S, Quinn SC, Kim KH, Musa D, Hilyard KM, Freimuth VS. The Social Ecological Model as a Framework for Determinants of 2009 H1N1 Influenza Vaccine Uptake in the United States. Health Educ Behav. 2012 Apr;39(2):229–43. pmid:21984692

32. Covidence systematic review software, Veritas Health Innovation, Melbourne, Australia. Available at www.covidence.org

33. Tricco AC, Lillie E, Zarin W, O’Brien KK, Colquhoun H, Levac D, et al. PRISMA Extension for Scoping Reviews (PRISMA-ScR): Checklist and Explanation. Ann Intern Med. 2018 Oct 2;169(7):467–73. pmid:30178033

34. Moher D, Liberati A, Tetzlaff J, Altman DG, The PRISMA Group. Preferred Reporting Items for Systematic Reviews and Meta-Analyses: The PRISMA Statement. PLoS Med. 2009 Jul 21;6(7):e1000097. pmid:19621072

35. Robertson E, Reeve KS, Niedzwiedz CL, Moore J, Blake M, Green M, et al. Predictors of COVID-19 vaccine hesitancy in the UK household longitudinal study. Brain, Behavior, and Immunity. 2021 May 1;94:41–50. pmid:33713824

36. Sallam M, Dababseh D, Eid H, Al-Mahzoum K, Al-Haidar A, Taim D, et al. High Rates of COVID-19 Vaccine Hesitancy and Its Association with Conspiracy Beliefs: A Study in Jordan and Kuwait among Other Arab Countries. Vaccines (Basel). 2021 Jan 12;9(1):42. pmid:33445581

37. Petravić L, Arh R, Gabrovec T, Jazbec L, Rupčić N, Starešinič N, et al. Factors Affecting Attitudes towards COVID-19 Vaccination: An Online Survey in Slovenia. Vaccines. 2021 Mar;9(3):247. pmid:33808958

38. Cordina M, Lauri MA, Lauri J. Attitudes towards COVID-19 vaccination, vaccine hesitancy and intention to take the vaccine. Pharm Pract (Granada). 2021;19(1):2317.

39. La Vecchia C, Negri E, Alicandro G, Scarpino V. Attitudes towards influenza vaccine and a potential COVID-19 vaccine in Italy and differences across occupational groups, September 2020. Med Lav. 2020 Nov 17;111(6):445–8. pmid:33311419

40. McPhedran R, Toombs B. Efficacy or delivery? An online Discrete Choice Experiment to explore preferences for COVID-19 vaccines in the UK. Economics Letters. 2021 Mar 1;200:109747. pmid:33551522

41. Malesza M, Wittmann E. Acceptance and Intake of COVID-19 Vaccines among Older Germans. J Clin Med. 2021 Mar 30;10(7):1388. pmid:33808414

42. Paul E, Steptoe A, Fancourt D. Attitudes towards vaccines and intention to vaccinate against COVID-19: Implications for public health communications. Lancet Reg Health Eur. 2021 Feb;1:100012. pmid:33954296

43. Alabdulla M, Reagu SM, Al-Khal A, Elzain M, Jones RM. COVID-19 vaccine hesitancy and attitudes in Qatar: A national cross-sectional survey of a migrant-majority population. Influenza Other Respir Viruses. 2021 May;15(3):361–70 pmid:33605010

44. Latkin CA, Dayton L, Yi G, Konstantopoulos A, Boodram B. Trust in a COVID-19 vaccine in the U.S.: A social-ecological perspective. Soc Sci Med. 2021 Feb;270:113684. pmid:33485008

45. Biasio LR, Bonaccorsi G, Lorini C, Mazzini D, Pecorelli S. Italian Adults’ Likelihood of Getting COVID-19 Vaccine: A Second Online Survey. Vaccines. 2021 Mar 17;9(3):268. pmid:33802873

46. Freeman D, Loe BS, Chadwick A, Vaccari C, Waite F, Rosebrock L, et al. COVID-19 vaccine hesitancy in the UK: the Oxford coronavirus explanations, attitudes, and narratives survey (Oceans) II. Psychol Med. 2020 Dec 11;1–15.

47. Benis A, Seidmann A, Ashkenazi S. Reasons for Taking the COVID-19 Vaccine by US Social Media Users. Vaccines (Basel). 2021 Mar 29;9(4):315. pmid:33805283

48. Yu Y, Lau JTF, Lau MMC, Wong MCS, Chan PKS. Understanding the Prevalence and Associated Factors of Behavioral Intention of COVID-19 Vaccination Under Specific Scenarios Combining Effectiveness, Safety, and Cost in the Hong Kong Chinese General Population. International Journal of Health Policy and Management [Internet]. 2021 Jan 18 [cited 2022 Jan 6];0. Available from: https://www.ijhpm.com/article_4003.html

49. Craig BM. United States COVID-19 Vaccination Preferences (CVP): 2020 Hindsight. Patient. 2021 May;14(3):309–18. pmid:33783724

50. Liu D, Luo L, Xie F, Yu Z, Ma ZF, Wang Y, et al. Factors associated with the willingness and acceptance of SARS-CoV-2 vaccine from adult subjects in China. Human Vaccines Immunotherapeutics. 2021 Mar 24;1–10 pmid:33759691

51. Dorman C, Perera A, Condon C, Chau C, Qian J, Kalk K, et al. Factors Associated with Willingness to be Vaccinated Against COVID-19 in a Large Convenience Sample. J Community Health. 2021 Oct;46(5):1013–9. pmid:33835369

52. Salmon DA, Dudley MZ, Brewer J, Kan L, Gerber JE, Budigan H, et al. COVID-19 vaccination attitudes, values and intentions among United States adults prior to emergency use authorization. Vaccine. 2021 May 6;39(19):2698–711. pmid:33781601

53. Machida M, Nakamura I, Kojima T, Saito R, Nakaya T, Hanibuchi T, et al. Acceptance of a COVID-19 Vaccine in Japan during the COVID-19 Pandemic. Vaccines (Basel). 2021 Mar 3;9(3):210. pmid:33802285

54. Yoda T, Katsuyama H. Willingness to Receive COVID-19 Vaccination in Japan. Vaccines (Basel). 2021 Jan 14;9(1):48. pmid:33466675

55. Nguyen KH. COVID-19 Vaccination Intent, Perceptions, and Reasons for Not Vaccinating Among Groups Prioritized for Early Vaccination—United States, September and December 2020. MMWR Morb Mortal Wkly Rep [Internet]. 2021 [cited 2021 Sep 23];70. Available from: https://www.cdc.gov/mmwr/volumes/70/wr/mm7006e3.htm

56. Batty GD, Deary IJ, Fawns-Ritchie C, Gale CR, Altschul D. Pre-pandemic cognitive function and COVID-19 vaccine hesitancy: cohort study. Brain Behav Immun. 2021 Aug;96:100–5. pmid:34022372

57. Allington D, McAndrew S, Moxham-Hall V, Duffy B. Coronavirus conspiracy suspicions, general vaccine attitudes, trust and coronavirus information source as predictors of vaccine hesitancy among UK residents during the COVID-19 pandemic. Psychol Med. 2021 Apr 12;1–12.

58. Kaplan RM, Milstein A. Influence of a COVID-19 vaccine’s effectiveness and safety profile on vaccination acceptance. Proc Natl Acad Sci U S A. 2021 Mar 9;118(10):e2021726118. pmid:33619178

59. Meier BP, Dillard AJ, Lappas CM. Predictors of the intention to receive a SARS-CoV-2 vaccine. Journal of Public Health. 2021 Mar 3;fdab013.

60. Eguia H, Vinciarelli F, Bosque-Prous M, Kristensen T, Saigí-Rubió F. Spain’s Hesitation at the Gates of a COVID-19 Vaccine. Vaccines (Basel). 2021 Feb 18;9(2):170. pmid:33670621

61. Soares P, Rocha JV, Moniz M, Gama A, Laires PA, Pedro AR, et al. Factors Associated with COVID-19 Vaccine Hesitancy. Vaccines (Basel). 2021 Mar 22;9(3):300. pmid:33810131

62. Ditekemena JD, Nkamba DM, Mutwadi A, Mavoko HM, Siewe Fodjo JN, Luhata C, et al. COVID-19 Vaccine Acceptance in the Democratic Republic of Congo: A Cross-Sectional Survey. Vaccines (Basel). 2021 Feb 14;9(2):153. pmid:33672938

63. Green MS, Abdullah R, Vered S, Nitzan D. A study of ethnic, gender and educational differences in attitudes toward COVID-19 vaccines in Israel–implications for vaccination implementation policies. Israel Journal of Health Policy Research. 2021 Mar 19;10(1):26. pmid:33741063

64. Mercadante AR, Law AV. Will they, or Won’t they? Examining patients’ vaccine intention for flu and COVID-19 using the Health Belief Model. Res Social Adm Pharm. 2021 Sep;17(9):1596–605. pmid:33431259

65. Alqudeimat Y, Alenezi D, AlHajri B, Alfouzan H, Almokhaizeem Z, Altamimi S, et al. Acceptance of a COVID-19 Vaccine and Its Related Determinants among the General Adult Population in Kuwait. MPP. 2021;30(3):262–71. pmid:33486492

66. Al-Qerem WA, Jarab AS. COVID-19 Vaccination Acceptance and Its Associated Factors Among a Middle Eastern Population. Frontiers in Public Health. 2021;9:34. pmid:33643995

67. Tran VD, Pak TV, Gribkova EI, Galkina GA, Loskutova EE, Dorofeeva VV, et al. Determinants of COVID-19 vaccine acceptance in a high infection-rate country: a cross-sectional study in Russia. Pharm Pract (Granada). 2021 Mar;19(1):2276.

68. Zampetakis LA, Melas C. The health belief model predicts vaccination intentions against COVID-19: A survey experiment approach. Appl Psychol Health Well Being. 2021 May;13(2):469–84. pmid:33634930

69. Alfageeh EI, Alshareef N, Angawi K, Alhazmi F, Chirwa GC. Acceptability of a COVID-19 Vaccine among the Saudi Population. Vaccines (Basel). 2021 Mar 5;9(3):226. pmid:33807732

70. Loomba S, de Figueiredo A, Piatek SJ, de Graaf K, Larson HJ. Measuring the impact of COVID-19 vaccine misinformation on vaccination intent in the UK and USA. Nat Hum Behav. 2021 Mar;5(3):337–48. pmid:33547453

71. Lueck JA, Spiers A. Which Beliefs Predict Intention to Get Vaccinated against COVID-19? A Mixed-Methods Reasoned Action Approach Applied to Health Communication. J Health Commun. 2020 Oct 2;25(10):790–8. pmid:33719876

72. Jackson SE, Paul E, Brown J, Steptoe A, Fancourt D. Negative Vaccine Attitudes and Intentions to Vaccinate Against Covid-19 in Relation to Smoking Status: A Population Survey of UK Adults. Nicotine Tob Res. 2021 Aug 18;23(9):1623–8. pmid:33751125

73. Chu H, Liu S. Integrating health behavior theories to predict American’s intention to receive a COVID-19 vaccine. Patient Educ Couns. 2021 Aug;104(8):1878–86. pmid:33632632

74. Benham JL, Lang R, Kovacs Burns K, MacKean G, Léveillé T, McCormack B, et al. Attitudes, current behaviours and barriers to public health measures that reduce COVID-19 transmission: A qualitative study to inform public health messaging. PLoS One. 2021;16(2):e0246941. pmid:33606782

75. Radic A, Koo B, Gil-Cordero E, Cabrera-Sánchez JP, Han H. Intention to Take COVID-19 Vaccine as a Precondition for International Travel: Application of Extended Norm-Activation Model. Int J Environ Res Public Health. 2021 Mar 17;18(6):3104. pmid:33803023

76. Griffith J, Marani H, Monkman H. COVID-19 Vaccine Hesitancy in Canada: Content Analysis of Tweets Using the Theoretical Domains Framework. Journal of Medical Internet Research. 2021 Apr 13;23(4):e26874. pmid:33769946

77. Praveen SV, Ittamalla R, Deepak G. Analyzing the attitude of Indian citizens towards COVID-19 vaccine—A text analytics study. Diabetes Metab Syndr. 2021 Apr;15(2):595–9. pmid:33714134

78. Wang J, Lu X, Lai X, Lyu Y, Zhang H, Fenghuang Y, et al. The Changing Acceptance of COVID-19 Vaccination in Different Epidemic Phases in China: A Longitudinal Study. Vaccines (Basel). 2021 Feb 25;9(3):191. pmid:33668923

79. Hussain A, Tahir A, Hussain Z, Sheikh Z, Gogate M, Dashtipour K, et al. Artificial Intelligence-Enabled Analysis of Public Attitudes on Facebook and Twitter Toward COVID-19 Vaccines in the United Kingdom and the United States: Observational Study. J Med Internet Res. 2021 Apr 5;23(4):e26627. pmid:33724919

80. Yin F, Wu Z, Xia X, Ji M, Wang Y, Hu Z. Unfolding the Determinants of COVID-19 Vaccine Acceptance in China. J Med Internet Res. 2021 Jan 15;23(1):e26089. pmid:33400682

81. Gramacho WG, Turgeon M. When politics collides with public health: COVID-19 vaccine country of origin and vaccination acceptance in Brazil. Vaccine. 2021 May 6;39(19):2608–12. pmid:33846045

82. Bokemper SE, Huber GA, Gerber AS, James EK, Omer SB. Timing of COVID-19 vaccine approval and endorsement by public figures. Vaccine. 2021 Jan 29;39(5):825–9. pmid:33390295

83. Richard L, Gauvin L, Raine K. Ecological Models Revisited: Their Uses and Evolution in Health Promotion Over Two Decades. Annu Rev Public Health. 2011 Apr 21;32(1):307–26. pmid:21219155

84. Hassan W, Kazmi SK, Tahir MJ, Ullah I, Royan HA, Fahriani M, et al. Global acceptance and hesitancy of COVID-19 vaccination: A narrative review. Narra J [Internet]. 2021 Dec 1 [cited 2022 May 4];1(3). Available from: https://narraj.org/main/article/view/57

85. Kaur RJ, Dutta S, Bhardwaj P, Charan J, Dhingra S, Mitra P, et al. Adverse Events Reported From COVID-19 Vaccine Trials: A Systematic Review. Ind J Clin Biochem. 2021 Oct;36(4):427–39. pmid:33814753

86. WHO. Coronavirus disease (COVID-19): Vaccines safety [Internet]. [cited 2022 Jan 6]. Available from: https://www.who.int/news-room/questions-and-answers/item/coronavirus-disease-(covid-19)-vaccines-safety

87. Rosiello DF, Anwar S, Yufika A, Adam RY, Ismaeil MIH, Ismail AY, et al. Acceptance of COVID-19 vaccination at different hypothetical efficacy and safety levels in ten countries in Asia, Africa, and South America. Narra J [Internet]. 2021 Dec 1 [cited 2022 May 4];1(3). Available from: https://narraj.org/main/article/view/55

88. Dubé È, Ward JK, Verger P, MacDonald NE. Vaccine Hesitancy, Acceptance, and Anti-Vaccination: Trends and Future Prospects for Public Health. Annu Rev Public Health. 2021 Apr 1;42(1):175–91.

89. Lazarus JV, Ratzan SC, Palayew A, Gostin LO, Larson HJ, Rabin K, et al. A global survey of potential acceptance of a COVID-19 vaccine. Nat Med. 2021 Feb;27(2):225–8. pmid:33082575

90. COVID-19 Vaccines for Moderately or Severely Immunocompromised People | CDC [Internet]. [cited 2022 Jan 5]. Available from: https://www.cdc.gov/coronavirus/2019-ncov/vaccines/recommendations/immuno.html

91. WHO. Interim recommendations for an extended primary series with an additional vaccine dose for COVID-19 vaccination in immunocompromised persons [Internet]. [cited 2022 Jan 7]. Available from: https://www.who.int/publications-detail-redirect/WHO-2019-nCoV-vaccines-SAGE_recommendation-immunocompromised-persons

92. Fisk RJ. Barriers to vaccination for coronavirus disease 2019 (COVID-19) control: experience from the United States. Global Health Journal. 2021 Mar;5(1):51–5. pmid:33585053

93. Tong S, Ebi K, Olsen J. Infectious disease, the climate, and the future. Environmental Epidemiology. 2021 Apr;5(2):e133. pmid:33870009

94. Morens DM, Fauci AS. Emerging Pandemic Diseases: How We Got to COVID-19. Cell. 2020 Sep;182(5):1077–92. pmid:32846157

95. Baker RE, Mahmud AS, Miller IF, Rajeev M, Rasambainarivo F, Rice BL, et al. Infectious disease in an era of global change. Nat Rev Microbiol. 2022 Apr;20(4):193–205. pmid:34646006