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1. Introduction
Annual influenza vaccination has been recommended since 2010 for all U.S. adults by the country’s Advisory Committee on Immunization Practices (ACIP) [1], but little significant progress in increasing coverage has been made in recent years. The percentage of adults in the U.S. getting seasonal influenza vaccination has not significantly changed since 2013 and remains far below the 70% target established by the nation’s Healthy People 2020 program. Healthy People provides science-based, 10-year national objectives for improving the health of all Americans [2]. According to U.S. Centers for Disease Control and Prevention estimates (CDC), 42.2% of adults received an influenza vaccination in 2013-2014, 43.6% in 2014-2015, and 41.7% in 2015-2016 [3,4,5,6]. Early season coverage data suggested only about four of 10 U.S. adults would receive an influenza vaccination in 2016-2017 [3]. Even among U.S. adults with medical conditions (e.g., pulmonary disease, diabetes, asthma, or heart disease) influenza vaccination coverage is quite low, placing them at risk of influenza complications. While the recommendation has existed since the early 1960s, only about half of U.S. adults with high-risk conditions report getting a seasonal influenza vaccination [7,8].
Recent reviews provide insights into potential facilitators and barriers to influenza vaccination. Schmid et al. systematically reviewed 470 articles published between 2005 and 2016, involving a wide range of countries, and found sociodemographic variables, particularly gender and age, were the most reported yet the most inconsistent predictors of vaccination [9]. Stronger, more consistent relationships were found with level of vaccine confidence, influenza complacency, physician recommendation, past influenza illness, and vaccination experience. This is consistent with recent reviews involving both U.S. and European adults, which have generally found seasonal influenza vaccination associated with healthcare provider recommendations, perceptions of susceptibility to influenza and its severity, believing in influenza vaccine effectiveness, negative influenza experiences, and no negative seasonal influenza vaccination experiences [10,11,12]. Non-vaccination has generally been associated with the opposite [10,11,12]. Overall, surveys of U.S. adults, while primarily focused on 2009 H1N1 vaccination, have also found healthcare provider recommendations, susceptibility and severity perceptions, and past experience with influenza illness and vaccination correlated with seasonal influenza vaccination [13,14,15,16,17]. Racial and ethnic differences also have been frequently documented [18,19,20], with the 2015-2016 seasonal vaccination rates ranging from 44.5% for non-Hispanic Whites to 36.6% for non-Hispanic...