Background: Previous researches inferred that high temperatures and high humidity might weaken COVID-19 transmission. However, with the warming weather coming, the COVID-19 pandemic is still intensifying.
Methods: This study aims to evaluate the associations between daily 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: There was a statistically significant difference between COVID-19 and temperature from 6 °C to 9 °C, relative humidity from 80% to 98%. It increased the risk of 95.4% at 6 °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). At low temperature group, it increased the risk of 46.3% (RR = 1.463, 95%CI: 1.054-2.030) on lag 0-4 days. At high humidity group, it increased the risk of 42.3% (RR = 1.423, 95%CI: 1.070-1.892) on lag 0-6 days.
Conclusions: We found that low temperature and high humidity were the risky factors of COVID-19 transmission, and higher temperature and lower humidity had no effect on the transmission of COVID-19, which indicated that it might not slow down due to weather factors in summer in the Mediterranean climate.