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Small Impact of Warmer Temperatures on SARS-CoV-2 Rate in US
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Mark Mascolini
Warmer temperatures tended to predict lower incidence (new-case rate) of SARS-CoV-2 infection in the United States, but statistical associations in this regression analysis were small and the authors believe "transmission is likely to remain high at warmer temperatures" [1].
Researchers from Harvard Medical School and collaborators at other centers noted that most general-population studies to date found reduced SARS-CoV-2 transmission with higher temperatures and humidity. Work in the United States and Spain determined that strong sunlight inactivates SARS-CoV-2 on nonporous surfaces [2] and that lower solar irradiance correlates with bigger COVID-19 outbreaks [3].
The Harvard team used negative binomial regression to estimate the impact of daily maximum temperature, precipitation, and UV Index on SARS-CoV-2 infection incidence 5 days later across the United States. The analysis focused on SARS-CoV-2 case data from each US state and Washington, DC, from January 22 to April 3, 2020 culled from the COVID-19 Global Cases dashboard at Johns Hopkins University (https://github.com/CSSEGISandData/COVID-19) and the COVID Tracking Project (https://covidtracking.com/api).
The researchers used National Centers for Environmental Information data (https://www.ncei.noaa.gov) for each state capital as a proxy for daily weather in the state. They evaluated the impact of a temperature threshold of 52° F (11° C) as well as temperatures stratified into five groups: below 30, 30-40, 40-50, 50-60, and 60° F or more.
When the research team modeled temperature as a continuous variable, the analysis linked each 1° F higher maximum daily temperature to a slightly but significantly lower rates of new reported SARS-CoV-2 cases per million people 5 days later (incidence rate ratio [IRR] 0.994, 95% confidence interval [CI] 0.989 to 0.999, P = 0.02). A daily maximum temperature above 52° F predicted a significantly lower case rate 5 days later (IRR 0.88, 95% CI 0.80 to 0.97, P = 0.009).
Considering observations with daily temperatures below 52° F, modeling indicated a significant inverse association between maximum daily temperature per 1° F higher and SARS-CoV-2 rate 5 days later (IRR 0.987, 95% CI 0.979 to 0.994, P = 0.001). When the investigators scrutinized the five temperature brackets, they found numerical trends toward higher SARS-CoV-2 rates in the lower temperature ranges. For example, case rates were nonsignificantly higher when maximum temperature lay below 30° F than above 60° F (IRR 1.59, 95% CI 0.83 to 3.06, P = 0.16).
Sensitivity analyses linked a maximum temperature above 52° F to fewer new cases 3, 7, and 9 days later. And a maximum temperature above 52° F meant fewer cases 5 days later when researchers limited the analysis to time before stay-at-home measures went into place (IRR 0.87, 95% CI 0.80 to 1.00, P = 0.051).
Each 1-unit higher UV Index predicted a lower SARS-CoV-2 rate 5 days later in adjusted models (IRR 0.97, 95% CI 0.95 to 0.99, P = 0.004). But precipitation on any given day did not correlate with the new-case rate 5 days later (IRR 0.98, 95% CI 0.90 to 1.06, P = 0.54). Predicted associations between temperature and new-case rate were independent of UV Index.
These results were not affected by an array of state-level nonclimate variables including population, population density, gross domestic product, median family income, obesity rate, graduation rate, and proportion of African Americans, uninsured, older adults, or children.
The researchers believe their findings suggest SARS-CoV-2 transmission rates will probably fall with warmer temperatures, but transmissions will probably occur "at meaningful rates" even as temperatures warm across the Northern Hemisphere. They suggest their UV Index findings support prior evidence that UV light "may disrupt the integrity of the virus."
The investigators propose that, "although there is an association between daily temperature and subsequent case volume across the US, this relationship is small and it is likely that if containment measures are discontinued, the disease may continue to spread in the United States even in periods of warmer weather."
References
1. Sehra ST, Salciccioli JD, Wiebe DJ, Fundin S, Baker JF. Maximum daily temperature, precipitation, ultra-violet light and rates of transmission of SARS-Cov-2 in the United States. Clin Infect Dis. 2020 May 30: ciaa681. doi: 10.1093/cid/ciaa681. https://academic.oup.com/cid/advance-article/doi/10.1093/cid/ciaa681/5849063
2. Ratnesar-Shumate S, Williams G, Green B, et al. Simulated sunlight rapidly inactivates SARS-CoV-2 on surfaces. J Infect Dis. 2020 May 20:jiaa274. doi: 10.1093/infdis/jiaa274. https://academic.oup.com/jid/advance-article/doi/10.1093/infdis/jiaa274/5841129
3. Guasp M, Laredo C, Urra X. Higher solar irradiance is associated with a lower incidence of COVID-19. Clin Infect Dis. 2020 May 19:ciaa575. doi: 10.1093/cid/ciaa575. https://academic.oup.com/cid/advance-article/doi/10.1093/cid/ciaa575/5840498
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