Our study comprehensively explored the role of multiple meteorological factors on influenza activity in the largest municipality in China. The correlations between mean temperature, wind speed, sunshine and influenza activity were illustrated with inverted J-shaped curve. The relation between relative humidity and influenza activity was described as J-shaped curve. The relationship between aggregate rainfall and influenza activity was illustrated with U-shaped curve. No significant effect was observed for atmosphere pressure.
Consistent with previous studies 10–12, 15, 16, we found that the mean temperature was inversely associated with influenza activity. The influenza activity increased significantly with a lower temperature below 18℃. Laboratory studies showed that low temperature may promote the spread of influenza by lengthening the survival of influenza virus, enhancing the transmissibility of influenza virus and increasing the host susceptibility 17, 18. Moreover, people are likely to spend more time indoor under cold condition, so the indoor environment-virus-host interactions substantially increase the opportunity of influenza transmission 7, 19.
A significant wet effect on influenza activity was observed in Chongqing, which is consistent with previous studies 12, 20–23. Experimental study has shown stronger infectivity of influenza virus in a high relative humidity 24. High humidity may bring forth droplets that bind to influenza virus, increasing the concentration of virus in the air around the infection source 25. However, many studies have demonstrated that “dry” condition played a critical role in influenza transmission 11, 16, 21, 26. Laboratory study has shown that the influenza virus is active when relative humidity is below 50%, especially between 20% and 35% 27. The lack of association between the “dry” condition with influenza activity in our study may partly be explained by the fact that no exposure to extremely dry condition of relative humidity below 50% was observed during the whole study period. The relative humidity was high all year around in Chongqing, with minimum weekly average relative humidity of 57.07% during 2012–2019. Our finding indicates that dehumidifying the indoor air especially on extreme moist days may be useful to reduce the spread of influenza in Chongqing.
Previous studies have inconsistent findings on the association between rainfall and influenza. Many studies reported increased influenza circulation during the rainy seasons 28, 29, while others reported no or contradicting effects of rainfall 11, 30. In general, our study agrees with the former. We found that extreme rainfall increased the risk of influenza activity with high relative risk of 40.37, which was much higher than rainless effect (relative risk was 2.61). Rainfall may lead to indoor crowding and consequently increase the probability for close contact which could speed the transmission of influenza virus 31. Previous study also indicated that low level precipitation could increase the amount of virus particulate in the air, then increased the risk of virus infection 32. In the future, more studies are needed to fully solve the inconsistence in the association between rainfall and influenza.
The understanding of effects of sunshine, wind speed and atmosphere pressure on influenza is still limited. We found that long sunshine decrease the risk of influenza activity. It has been proposed that sunshine could affect the influenza activity through the mediation effect of Vitamin D synthesis on individuals’ immune response to infection 33, 34, but is remains unverified. In addition, our study showed that low wind speed increases the risk of influenza activity in Chongqing, which was consistent with previous study 35. Regarding atmosphere pressure, we did not find significant effect on influenza activity in Chongqing. Future studies are needed to fully understand the roles of these meteorological factors on influenza activity.
Several limitations exist in this study. First, the meteorological data were taken from fixed monitoring sites rather than individual exposure measures, which may create measurement errors in the exposure. However, these errors are likely to be random. Second, we did not account for the effect of other factors such as socioeconomic condition, host susceptibility and vaccination status on the association between meteorological factors and influenza activity because these data were not obtained. Third, we didn’t explore the effects of age and sex on the associations between meteorological factors and influenza activity. We plan to address this limitation in future work.