  Objectives: We aim to assess the impact of temperature and relative humidity
on the transmission of COVID-19 across communities after accounting for
community-level factors such as demographics, socioeconomic status, and human
mobility status. Design: A retrospective cross-sectional regression analysis
via the Fama-MacBeth procedure is adopted. Setting: We use the data for
COVID-19 daily symptom-onset cases for 100 Chinese cities and COVID-19 daily
confirmed cases for 1,005 U.S. counties. Participants: A total of 69,498 cases
in China and 740,843 cases in the U.S. are used for calculating the effective
reproductive numbers. Primary outcome measures: Regression analysis of the
impact of temperature and relative humidity on the effective reproductive
number (R value). Results: Statistically significant negative correlations are
found between temperature/relative humidity and the effective reproductive
number (R value) in both China and the U.S. Conclusions: Higher temperature and
higher relative humidity potentially suppress the transmission of COVID-19.
Specifically, an increase in temperature by 1 degree Celsius is associated with
a reduction in the R value of COVID-19 by 0.026 (95% CI [-0.0395,-0.0125]) in
China and by 0.020 (95% CI [-0.0311, -0.0096]) in the U.S.; an increase in
relative humidity by 1% is associated with a reduction in the R value by 0.0076
(95% CI [-0.0108,-0.0045]) in China and by 0.0080 (95% CI [-0.0150,-0.0010]) in
the U.S. Therefore, the potential impact of temperature/relative humidity on
the effective reproductive number alone is not strong enough to stop the
pandemic.
