Impact of mitigating interventions and temperature on the instantaneous reproduction number in the COVID-19 pandemic among 30 US metropolitan areas

BACKGROUND: After more than six months into the coronavirus disease (COVID-19) pandemic, as of August 10, 2020, over 734,664 people had died worldwide. The current study aims to evaluate how mitigating interventions affected the epidemic process in the 30 largest metropolitan areas in the US and whether temperature played a role in the epidemic process. METHODS: Publicly available data for the time series of COVID-19 cases and deaths and weather were analyzed at the metropolitan level. The time-varying reproductive numbers (R(t)) based on retrospective moving average were used to explore the trends. Student t-tests were used to compare temperature and peak R(t) cross-sectionally. RESULTS: We found that virus transmissibility, measured by instantaneous reproduction number (R(t)), had declined since the end of March for all areas and almost all of them reached a R(t) of 1 or below after April 15, 2020. The timing of the main decline was concurrent with the implementation of mitigating interventions. However, the R(t)s remained around 1 for most areas since then and there were some small and short rebounds in some areas, suggesting a persistent epidemic in those areas when interventions were relaxed. Cities with warm temperature also tended to have a lower peak R(t) than that of cities with cold temperature. However, they were not statistically significant and large geographic variations existed. CONCLUSIONS: Aggressive interventions might have mitigated the current pandemic of COVID-19, while temperature might have weak effects on the virus transmission. We may need to prepare for a possible return of the coronavirus outbreak.