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Is coronavirus (COVID-19) seasonal? Should infection rates slow during summer?

People over 20 years old more likely to get infected with COVID-19

DETROIT – Is the coronavirus (COVID-19) seasonal? We usually refer to the fall and winter months as “cold and flu season” because there’s a seasonal pattern to the spread of many viruses. Does research suggest that could be the same for COVID-19?

As the start of summer approaches, many of the states with the largest recent COVID-19 increases are in the hottest areas of the country -- Florida and Arizona among them.

That has led many people to wonder if there is seasonal variation to the virus if it’s getting even worse.

A paper published June 11 in the Journal of the American Medical Association used global temperature and humidity trends seen during the initial outbreak and spread of COVID-19 at the start of 2020 to estimate how likely it was to have a seasonal element.

Researchers found eight cities with substantial spread of COVID-19 between January and March -- Wuhan, China; Tokyo, Japan; Daegu, South Korea; Qom, Iran; Milan, Italy; Paris, France; Seattle, Washington; and Madrid, Spain -- were all in a narrow band with consistently similar temperatures between 40-50 degrees Fahrenheit and low humidity.

Experts suggested weather modeling might help predict areas with a higher risk of spreading the virus.

Another study recently published in Clinical Infections Diseases drilled down more specifically on the United States.

Researchers looked at cases of COVID-19 between Jan. 22 and April 3, using models to see if there was a relationship between temperature, precipitation or sunlight and the spread of the virus.

They found warmer temperatures lined up with a slower transmission of the virus, but only up to 52 degrees Fahrenheit. Beyond that, warmer temperatures didn’t appear to help.

The model also predicted that the highest infection rates would occur below 30 degrees.

Sunlight, or the UV index, was also associated with a lower rate of new infections, but precipitation had no relationship.

The bottom line is that there’s growing evidence that weather modestly affects the spread of the virus, but only to a degree.

Once the weather warms up everywhere in the summer, the spread will become less dependent on the weather and more dependent on community behaviors such as distancing, mask wearing and hygiene.

Based on both studies, it’s a good bet that as temperatures and humidity drop in the fall and winter, we’ll lose any help from the weather in terms of slowing the spread. But if everyone maintains good protocols, the change in weather might not be a setback.


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