The seasonal-COVID hypothesis: 6 things to know

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There's some evidence indicating that COVID-19 is, or may become, a seasonal disease, which means understanding the seasonal patterns could help scientists anticipate surges in advance, The Atlantic reported Nov. 20.

With two years' worth of data to analyze and the knowledge that COVID-19 won't likely be eradicated, some researchers are exploring the seasonality of the virus.

Six things to know about the possible seasonality of COVID-19, per The Atlantic:

1. The same disease can show different patterns in different areas. For example, the U.S. typically sees a winter flu season, but flu spikes in Bangladesh during the monsoon season — May to September — the warmest part of the year.  

2. A group of researchers analyzed how COVID-19 fared in weather conditions worldwide and found temperature and humidity didn't play much of a role. Instead, their findings suggested case rates would rise in a particular area during times of lower UV exposure. Since then, others have found that the virus dies when exposed to UV rays with the same wavelength as sunlight. 

3. A University of Pittsburgh study released in July projects a seasonal COVID-19 pattern in North America. The study has yet to be peer-reviewed. The researchers argue that COVID-19 in North America takes the form of three repeating waves like the ones of 2020: one starting in New England and eastern Canada in the spring, the second traveling north from Mexico in the summer, and the third traveling in all directions from the Dakotas in the fall. The researchers predicted a summer 2021 wave in the South, and a fall 2021 wave in the north-central states, which is similar to what happened.

4. If COVID-19 is driven more by seasonal changes than factors such as masking and vaccination rates, the disease would still behave like the flu at the local level, with each region experiencing one peak season per year — while the country overall had three.

5. This projected pattern may sharpen over the next few years, with patterning of past pandemics tending to follow a certain script, David Fisman, MD, epidemiologist at the University of Toronto, told The Atlantic. When a pandemic first hits, basically everyone is vulnerable. Then, as more people develop immunity, seasonal influences become more apparent. Finally, once the overwhelming majority of the population is immune, those same influences may become so subdued they're hardly visible.

6. Ben Zaitchik, PhD, a scientist at Baltimore-based Johns Hopkins University who co-chairs the World Meteorological Organization's COVID-19 Research Task Team, previously said researchers didn't have enough data to find strong patterns. Now, with more data available, Dr. Zaitchik told The Atlantic that he feels confident saying weather influences COVID transmission in a statistically significant way, though he added, "COVID-19 has proven beyond a doubt that it can create hugely deadly outbreaks anywhere in the world at any time of the year. And that's still true."

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