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Social Distancing, Population Density, Temperature and SARS-CoV-2 Reproduction
Quick Takes
- At the height of the pandemic, a 50% decrease in visits to nonessential businesses was associated with a 46% decrease in SARS-CoV-2 transmission.
- Wet-bulb temperature, a metric incorporating temperature and humidity, exhibited a nonlinear relationship with transmission. Transmission was greatest at freezing temperatures.
Study Questions:
How are social distancing and weather associated with changing values of the instantaneous reproduction number (Rt) for severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) across counties in the United States?
Methods:
This cohort study examined counties representing state capitals and cities with ≥100,000 residents, in which ≥1 case of COVID-19 had been identified as of February 25, 2020. The outcome was the estimated Rt of SARS-CoV-2 in each county. Daily incident case counts of coronavirus disease 2019 (COVID-19) between February 25, 2020 and April 23, 2020 were obtained from the New York Times. Exposure variables were social distancing practice, based on daily cellular telephone movement and percentage change in visits to nonessential businesses; population density; and daily mean wet-bulb temperature, a metric that incorporates temperature and humidity. Covariates considered as potential confounders included proportion of residents >65 years, with incomes <200% of the poverty level, and with diabetes.
Results:
A total of 211 counties, representing 46 states and the District of Columbia and comprising 54.8% of the US population, met inclusion criteria. The mean reduction in visits to nonessential business by mid-April was 68.7% ± 7.9%. Median case incidence and fatality rates per 100,000 people were approximately 10 times higher for the top decile of densely populated counties (1,185.2 cases; 43.7 deaths) than for the lowest density quartile (121.4 cases; 4.2 deaths). Mean Rt in the first 2 weeks was 5.7 ± 2.5 in the top decile compared with 3.1 ± 1.2 in the lowest quartile. In multivariable analysis, a 50% decrease in visits to nonessential businesses was associated with a 46% decrease in Rt (95% confidence interval, 43%-49%). With a 70% reduction in visits to nonessential business, 202 counties (95.7%) were estimated to fall below a threshold Rt of 1.0 (i.e., each patient transmitted to <1 susceptible person). Wet-bulb temperature exhibited a nonlinear relationship with Rt. Rt was highest at freezing temperatures (relative Rt, 2.13 at 0°C), lowest at 11°C (52°F, reference relative Rt, 1.0), somewhat higher at 20°C (68°F, relative Rt, 1.61), and lower at hot temperatures.
Conclusions:
Social distancing, population density, and weather are associated with SARS-CoV-2 transmission. Reduction in visits to nonessential businesses had a dramatic impact on transmission at the height of the pandemic. Transmission increased with cold weather and decreased with warm weather, though the relationship was nonlinear.
Perspective:
These findings are congruent with those of a previous study demonstrating increased SARS-CoV-2 transmission at northern latitudes (Sajadi MM, et al., JAMA Netw Open 2020;3:e2011834). This raises concern that January and February 2021 could be particularly problematic in the United States, especially if fastidious social distancing measures are not employed. Further study to estimate the relative impact of practices including universal masking and telecommuting, as compared with shelter-in-place orders, will be needed to inform policy.
Clinical Topics: Cardiovascular Care Team, COVID-19 Hub, Prevention
Keywords: Cold Temperature, Coronavirus, COVID-19, Diabetes Mellitus, Hot Temperature, Humidity, Population Density, Poverty, Primary Prevention, SARS Virus, severe acute respiratory syndrome coronavirus 2, Psychological Distance, Temperature, Weather
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