Social distancing, lower population density, and daily weather temperature were associated with a decrease in novel severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) instantaneous reproduction number (Rt), according to data published in JAMA Network Open.
Investigators aimed to examine the associations of county-level factors with variation in SARS-CoV-2 reproduction number over time. The cohort study included 211 counties, representing state capitals and cities with at least 100,000 residents, in 46 states and the District of Columbia between February 25, 2020 and April 23, 2020. Social distancing was measured by percentage change in visits to nonessential businesses.
The 211 included counties contained 178,892,208 (54.8%) of United States residents. County-level median population density was 1022.7 people per square mile (interquartile range [IQR] 471.2-1846.0). The mean peak reduction in visits to nonessential businesses between April 6 and April 19 — the period where the country was sheltering in place — was 68.7%±7.9%. Median daily wet-bulb temperatures were 7.5°C (IQR 3.8-12.8).
In the top decile of densely populated counties, median case incidence and fatality rates per 100,000 people were approximately 10 times higher, with 1185.2 cases (IQR 313.2-1891.2) and 43.7 deaths (IQR 10.4-106.7) compared with counties in the lowest density quartile: 121.4 cases (IQR 87.8-175.4) and 4.2 deaths (IQR 1.9-8.0).
The mean Rt was 5.7±2.5 in the first 2 weeks in the top decile compared with 3.1±1.2 in the lowest quartile. From a relative Rt at 0°C of 2.13 (95% CI, 1.89-2.40), relative Rt decreased to a minimum as temperatures warmed to 11°C. It increased between 11°C and 20°C to 1.61 (95% CI, 1.42-1.84) and declined at temperatures greater than 20°C.
A 50% decrease in visits to nonessential businesses was associated with a 45% decrease in Rt (95% CI, 43%-49%), according to multivariable analysis. The results also revealed that with a 70% reduction in visits to nonessential businesses, an estimated 95.7% of counties fell below a threshold Rt of 1.0. This included 17 of 21 counties (81.0%) in the top density decile and 52 of 53 counties (98.1%) in the lowest density quartile.
Investigators noted several study limitations, including generalizability, that remain a concern for these types of observational studies. The 45% of residents not captured reside in smaller, rural counties, where the models used are not applicable. Observed temperature associations might also have been confounded by time period in the analysis, given that outbreaks occurred during spring in parallel with changing weather.
Additionally, increases in testing capacity might have biased the models by inflating the total cases reported in each county. It is possible that there are likely other unmeasured county factors that were not captured,
as additional variation was explained by the random county and metropolitan area intercepts. The use of cell phone records as a proxy for social distancing may not have captured all movement and gathering in a county. This proxy requires further validation, with other unmeasured factors, including commuter automobile traffic, public transportation usage, and domestic and international flights warranting further investigation as well.
“The results of this study suggest that social distancing, population density, and daily weather may account for variation in the Rt for SARS-COv-2 across the United States,” the investigators concluded, adding that these results might also guide policy decisions for managing the pandemic at a more selective and local level.
Disclosure: One study author declared affiliations with the pharmaceutical industry. Please see the original reference for a full list of authors’ disclosures.
Rubin D, Huang J, Fisher BT, et al. Association of social distancing, population density, and temperature with the instantaneous reproduction number of SARS-CoV-2 in counties across the United States. JAMA Netw Open. 2020;3(7):e2016099