Key Message
In the present research, both single- and multi-pollutant models indicated that the level of PM2.5 was significantly related to the daily number of influenza cases in Khon Kaen, Thailand, prior to the COVID-19 outbreak. Additionally, a significant risk difference was observed between the pre-outbreak and the pandemic periods due to the reduction in air pollutant concentrations because of lockdown measures to control the spread of COVID-19. These findings could be useful for developing environmental policies and strategies accordingly to mitigate respiratory health issues.