Meteorological Conditions And Influenza-Related Emergency Department Visits: A Retrospective Analysis Of Syndromic Surveillance Data

Abstract

Introduction: Lower temperatures and drier air are associated with seasonal influenza epidemics and influenza-related mortality and hospitalizations in temperate climates. However, for support of effective public health response, emergency department (ED) visits are a timelier indicator of outcomes than mortality or hospitalizations because ED data are often collected for syndromic surveillance. We evaluated the relationship between weather conditions and influenza-related ED visits in a large northeast US city. Methods: Using syndromic surveillance data, we created a scaled metric, known as ILI+, by multiplying the daily number of influenza-like illness (ILI) ED visits by US Centers for Disease Control and Prevention (CDC) regional influenza positive proportions. This scaled metric can provide a more specific indicator of influenza activity than ILI alone. We linked daily ILI+ to weather data collected at airports located in and near the study city. Negative binomial generalized additive models with autoregressive terms were used to examine lags up to 28 days prior, were adjusted for season, time trends, and day of week, and were offset by the daily total number of ED visits. Results: During CDC influenza seasons (week 40 to week 20 of the next year) between Jan 2006 – Sept 2014, there were 496,499 ILI ED visits (3.2% of all ED visits). Colder, drier outdoor conditions up to 17 days prior were associated with increased ED visits. Rate ratios for temperature and absolute humidity were similar and strongest for lags 8-14 days, so we focused on the moving average of lags 8-14 days. Each 1°C decrease in temperature and each 0.5 g/m^3 decrease in absolute humidity increased daily ILI+ ED visits by 1.2% (95% confidence interval (CI): 1.0%, 1.4%) and 1.6% (95% CI: 1.1%, 2.0%), respectively. Discussion: Syndromic surveillance can provide early detection of weather-related health effects. Lower temperatures and drier air in the prior 3 weeks increased ED visits for influenza.