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Abstract
Data were acquired from the State of Massachusetts, Division of Health Care Finance and Policy, Executive Office of Health and Human Services and included patient town of residence, date of visit, age, sex, and the primary and five associated diagnostic codes (International Classification of Disease, Version 9 Clinical Modification (ICD-9-CM)) associated with each visit. No information regarding the length of time between onset of illness and the ER visit was available for individuals included in the study. Since a mean incubation period of 2 days (range 1-4) has been reported for influenza infection [27], we chose to evaluate point estimates for the association between ER influenza visit and an extreme precipitation event within a total lag period ranging from 0 to 6 days, as well as at each individual day lag 0-6. [ Table Omitted - see PDF ] Table 2 Adjusted odds ratios of influenza emergency room visits associated with extreme precipitation (?99th percentile) within 0-6 days, stratified by individual level characteristics OR (95% CI) p Age group 0-4 1.12 (0.93, 1.35) 5-18 1.32 (1.14, 1.52) 19-64 1.23 (1.15, 1.32) 65+ 1.11 (0.85, 1.45) 0.398 Gender Male 1.23 (1.13, 1.34) Female 1.23 (1.13, 1.33) 0.987 Race White 1.24 (1.04, 1.48) Black 1.48 (1.30, 1.68) Other 1.18 (1.10, 1.27) 0.035 Temperature < 40 °F 1.20 (1.12, 1.28) ? 40 °F 1.38 (1.21, 1.59) 0.156 Season Winter (Dec-Feb) 1.22 (1.13, 1.32) Spring (Mar-May) 1.23 (1.11, 1.36) Summer (Jun-Aug) 1.18 (0.69, 1.99) Fall (Sept-Nov) 1.31 (1.00, 1.72) 0.466 Discussion To our knowledge this case-crossover study was the first to investigate the association between the occurrence of extreme precipitation events and ER visits for influenza in a northern temperate climate. Other studies have also suggested the presence of differential host susceptibility such as socioeconomic vulnerabilities and potentially lower serum vitamin D concentrations among African Americans [37, 38], factors that could lead to more ER visits for influenza among this population, but were unavailable for assessment within this study.