Short-term wildfire-related PM_2.5 exposure and risk of influenza in Brazil: A nationwide study

Wildfires are a significant source of fine particulate matter (PM_2.5) and while emerging evidence has linked wildfire smoke to various respiratory and cardiovascular diseases, its impact on influenza remains largely unexplored, particularly in Brazil, where wildfires are frequent and severe. This study quantifies the association between wildfire-related PM_2.5 and influenza cases in Brazil and estimates the fraction of influenza cases attributable to wildfire-related PM_2.5. Using daily surveillance data from 510 immediate-regions (2009-2019), a space-time-stratified case-crossover design with a distributed lag model was applied. Of 374,110 reported influenza cases (median age: 21 years), 51.4 % were females and the median daily wildfire-related PM_2.5 concentration was 2.2 µg/m³. A 3.4 µg/m³ increase in wildfire-related PM_2.5 was associated with a 13.2 % rise in influenza cases over a 7-day lag period (OR [odds ratio]: 1.132, 95 % CI [confidence interval]: 1.090-1.176). This effect was stronger among females, teenagers, white individuals, residents of the Southeast region and high population density areas. The association was more pronounced on weekdays, among non-hospitalized individuals with influenza, and among individuals unvaccinated for influenza. Wildfire-related PM_2.5 could be attributable to 11.8 % of influenza cases, equating to 43,883 additional cases (95 % CI: 23,265-62,970). These findings highlight the influenza risk of wildfire emissions in vulnerable populations and underscore the need for targeted prevention strategies, particularly vaccination and timely healthcare access.