Some viruses display seasonal behavior. Influenza cases, for example, peak during the fall and winter, commonly known as the flu season. There are various reasons for this, including biological factors related to the virus, as well as social and environmental factors.
The seasonality of the flu virus has raised hopes that the same may be true for the novel coronavirus, SARS-CoV-2, and that cases of the virus may, therefore, decrease over the summer months.
Indeed, laboratory data suggest that SARS-CoV-2 survives for a shorter period in higher temperatures. However, the effect of temperature on the real-world transmission of the virus is currently unclear.
As well as temperature, UV light also has a powerful effect on viral agents. People routinely use specific wavelengths of UV light, known as UV-C, to disinfect surfaces in laboratories and hospitals.
It is possible that the summer weather, with its increased sunlight, could also affect the transmission on the ground.
Researchers at Mount Auburn Hospital in Cambridge, MA, have now analyzed the impact of these factors on real-life transmission rates. As well as UV index and temperature, the researchers considered the effect of precipitation levels on the rate of disease, using real-world data on the number of COVID-19 cases occurring in the United States.
They found that temperature and UV both have a small effect on transmission, but precipitation seemingly has none.
The findings appear in full in the journal Clinical Infectious Diseases.
The researchers assessed the number of cases of SARS-CoV-2 infection reported per day across the U.S. from January 22 to April 3, 2020, using Johns Hopkins University’s COVID-19 Dashboard.
They combined this with state-level data on temperature, precipitation, and UV index from the National Centers for Environmental Information.
The team then evaluated the associations between the sets of data using regression analysis, a statistical methodology that estimates the relationships between variables.
Although previous studies have looked at the effect of temperature and UV light on SARS-CoV-2 in the lab, this is the first study to assess the impact of these factors on virus transmission in the community.
“[T]his is probably one of the first peer-reviewed studies that examine the influence that temperature, precipitation, and UV light have in terms of virus transmission in the general population across the U.S.”
– Dr. Shiv T. Sehra, first author and director of the Internal Medicine Residency Program at Mount Auburn Hospital
The analysis revealed that lower rates of COVID-19 were associated with warmer temperatures, but only up to 52°F (11°C). Beyond this, warmer temperatures were not associated with a significant decrease in disease transmission.
“While the rate of virus transmission may slow down as the maximum daily temperature rises to around 50 degrees, the effects of temperature rise beyond that don’t seem to be significant,” says Dr. Sehra.
UV index also had a significant, but modest, association with the number of cases, while precipitation showed no impact on viral transmission.
The researchers also modeled what would happen to transmission rates at five different temperature ranges: below 30°F, 30–40°F, 40–50°F, 50–60°F, and over 60°F.
They found that the lowest rate of new cases occurred when the temperature 5 days earlier was higher than 50°F. The model produced the highest infection rates when this temperature dropped below 30°F.
These results suggest that there could be an increase in the transmission of the disease in the U.S. during the fall. In their paper, the authors propose that countries in the southern hemisphere could also see an increase in cases as temperatures start to decrease.
Overall, the findings suggest that the effect of temperature on the transmission of the virus is small and unlikely to have a significant effect on the number of cases.
“Based on our analysis, the modest association suggests that it is unlikely that disease transmission will slow dramatically in the summer months from the increase in temperature alone,” summarizes Dr. Sehra.
With this in mind, the authors warn against the easing of containment measures over the summer months, explaining that the effect of temperature on transmission is small and unlikely to slow down disease spread if containment measures are relaxed.
It is important to note that the collection of the study data took place when much of the U.S. had daily temperatures below 70°F. In the summer, temperatures will likely far exceed that in most of the country. Therefore, the study cannot evaluate the effect that temperatures over 70°F may have on virus transmission.
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