Risk effects of low temperature and high humidity on the spread of COVID-19 in California: Time series study

Abstract Background Previous researches inferred that ‘summer’ might weaken COVID-19 transmission. However, with the warming weather coming, the COVID-19 pandemic was still intensifying. Methods This study aimed to investigate the associations between temperature, relative humidity, and COVID-19 cases using the Distributed Lag Non-linear Model (DLNM) from Jan 27th to July 15th, 2020, in California, US. Results It showed that as of July 15, 2020, there were 355 285 reported cases in California, where the temperature was between 6.33 °C and 30.72 °C and the relative humidity was between 23% and 100%. Temperature from 6.33 °C to 9 °C, relative humidity from 80–98% were the risky factors of COVID-19 transmission. It increased the risk of 95.4% at 6.33 °C (RR:1.954; CI: 1.032–3.701). It increased the risk of 70.3% when the humidity was 98% (RR: 1.703, CI: 1.049–2.765). When the temperature > 9 °C and the relative humidity < 80%, there was no statistical association. Conclusions This suggested that in winter with low temperature and high humidity, the spread of the COVID-19 would be severe due to weather factors. However, temperature and humidity were not related to the COVID-19 pandemic in summer. It did not mean that ‘summer’ would weaken the spread of COVID-19 in California. Therefore, special attention should be paid to the prevention and treatment of COVID-19 in the winter. And it cannot be ignored in summer, otherwise, it will also cause a counterattack against the epidemic.