Impact of the COVID-19 epidemic on the association between meteorological factors and influenza in Xiamen, China

Objective It is still unclear whether the coronavirus disease 2019(COVID-19) epidemic affects the association between meteorological factors and influenza, and this study aims to investigate this issue and its impact characteristics, so as to provide a basis for accurate assessment of the risk effect of meteorological factors on influenza in the future.

Methods The daily data of meteorological factors and influenza in Xiamen were analyzed in three datasets: the whole year period (from January 1, 2010 to December 31, 2021), the non-COVID-19 epidemic period (from January 1, 2010 to January 21, 2020), and the COVID-19 epidemic period (from January 22, 2020 to December 31, 2021).Association between meteorological factors and influenza were analyzed through the distributed lag non-linear model (DLNM) of R 4.2.1 software.

Results Low (< 42%) and high (>85%) relative humidity (RH) were risk factors for influenza during the non-COVID-19 epidemic period.The cumulative risk effect of high RH increased with the lag time, first increasing and then decreasing with the increase of RH.with the most significant effect when RH reached 93% and lagged for 14 days (relative riskRR=1.41).During the COVID-19 epidemic period, the cumulative risk effect of RH < 75% on influenza increased with the reduction of RH and the increase of lag time, with the most significant effect when RH decreased to 35% and lagged for 14 days (RR=1.86×10⁶).During the non-COVID-19 epidemic period, the cumulative risk effect of precipitation (PRE) at 25-75 mm on influenza increased first and then decreased and increased with the increase of lag time, with the most significant effect when PRE reached 40 mm and lagged for 14 days (RR=1.49).The cumulative risk effect of PRE on influenza increased with the increase of PRE during the COVID-19 epidemic period.The sharp reduction in the number of influenza cases during the COVID-19 epidemic period seriously affected the association between meteorological factors and influenza, leading to the significant difference in analysis result between this period and the other two periods, while it had little impact on the research result of the whole year period.

Conclusion The COVID-19 lockdown policy leads to a sharp reduction in the number of influenza cases during the COVID-19 epidemic period in Xiamen, which significantly affects the objectivity and scientificity of the association analysis between meteorological factors and influenza, but with limited impact on long-term and multi-year analysis result.In addition, extreme weather such as RH and PRE also affect the analysis of the association between meteorological factors and influenza.