Assessing the impact of temperature and humidity exposures during early infection stages on case-fatality of COVID-19: a modelling study in Europe

Background Although associations between key weather indicators (i.e. temperature and humidity) and COVID-19 mortality has been reported, the relationship between these exposures among different timing in early infection stages (from virus exposure up to a few days after symptom onset) and the probability of death after infection (also called case fatality rate, CFR) has yet to be determined. Methods We estimated the instantaneous CFR of eight European countries using Bayesian inference in conjunction with stochastic transmission models, taking account of delays in reporting the number of newly confirmed cases and deaths. The exposure lag response associations between fatality rate and weather conditions to which patients were exposed at different timing were obtained using distributed lag nonlinear models coupled with mixed effect models. Results Our results showed that the Odds Ratio (OR) of death is negatively associated with the temperature, with two maxima (OR=1.29 (95% CI, 1.23, 1.35) at -0.1 degrees Centigrade, OR=1.12 (95% CI, 1.08, 1.16) at 0.1 degrees Centigrade) occurred at the time of virus exposure and after symptom onset. Two minima (OR=0.81 (95% CI: 0.71, 0.92) at 23.2 degrees Centigrade, OR=0.71 (95% CI, 0.63, 0.80) at 21.7 degrees Centigrade) also occurred at these two distinct periods correspondingly. Low humidity (below 50%) during the early stages and high humidity (around 89%) after symptom onset were related to the lower fatality. Conclusion Environmental conditions may affect not only the initial viral load when exposure to viruses but also individuals immunity response around symptom onset. Warmer temperatures and higher humidity after symptom onset were related to the lower fatality.