In the spring, the collective hope was that summer weather might stem the spread of COVID-19 — after all, the flu and other "cold" coronaviruses die down in the summer, and lab studies found that the pandemic virus died more quickly in higher temperatures and humidity.
That relief never came, at least not in the U.S.
Now, with the first full pandemic winter on the horizon, hope has turned to worry: What's going to happen with COVID-19 this winter? Will the cold weather make the pandemic worse?
The short answer is that if COVID-19 spreads more this winter, the weather won't be directly to blame.
COVID-19 During Winter
A new study led by researchers at the University of Texas at Austin has found that outdoor temperature and humidity plays only a minor role in the spread of COVID-19.
They studied data on coronavirus cases from March to July 2020 in different areas, ranging from the entire world down to specific counties in the U.S.
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When they compared new infections to an area's weather during that time period (specifically, an index of its temperature and humidity), they found that the weather alone had almost no influence on the spread of COVID-19.
They also determined that no particular combination of temperature and humidity appeared to have a larger impact on COVID-19's spread than another.
So we're not doomed to a huge winter wave. That doesn't necessarily mean we won't see an increase in cases of COVID-19 this winter, though.
While colder weather alone doesn't appear to affect the spread of the coronavirus, it does affect human behavior.
When it's cold out, people spend more time indoors, where the coronavirus is far more likely to spread. They're also less likely to walk places, meaning more people crowded onto public transportation.
Those are some of the reasons we could see an increase in COVID-19 this winter.
They're also some theories researchers have for why the flu tends to spread more in the winter — but why did COVID-19 persist this summer while the flu slowed down (virtually disappearing in some places this year)?
Researchers aren't entirely sure why the flu is seasonal — but another study, this one led by researchers at UC Santa Barbara, might explain why COVID-19 was able to continue spreading so effectively year-round.
It suggests that the coronavirus might not spread any better based on temperature and humidity — but it does spread differently, adjusting to its environment in a way that lets it continue infecting people under various conditions.
Using computer modeling, they determined that in colder indoor spaces with more humidity, the coronavirus spreads primarily through respiratory droplets that can carry it nearly 20 feet.
This could help us stem the spread of COVID-19 during winter and beyond.
In hotter, drier indoor areas, though, those droplets evaporate more quickly.
In those cases, fragments of the virus latch onto other aerosols from speaking, coughing, and breathing. They can then hang around in the air for hours before infecting a new person.
"These findings suggest that adaptive public health measures should be taken in accordance with seasonal weather variations and local environments," the authors write in their study.
Of course, in many places, whether an indoor room is hot or cold might have more to do with the person controlling the thermostat than the season, but this is still information we might be able to use to help stem the spread of COVID-19 during winter and beyond.
If a room is on the colder end, a person might want to increase their distance from others to minimize the chances of being infected by respiratory droplets. In hotter rooms, a mask made of a finer material might help prevent aerosol infection.
Combatting COVID-19 This Winter
Ultimately, as with most things relating to the coronavirus, exactly what will happen with COVID-19 this winter is impossible to predict.
"We don't know where (the pandemic) is going to end up — we haven't had a year with it yet," Akiko Iwasaki, an immunologist at the Yale School of Medicine, told STAT in October. "I think we have to be extra cautious going into the winter."
"I think we have to be extra cautious going into the winter."
Even if COVID-19 doesn't spread faster this winter, there is a chance that infections might become more severe, possibly due to seasonal changes in our immune systems.
The key thing to remember is that, unlike the weather, we do have some control over what happens these next several months.
Wearing a mask, sanitizing our hands regularly, and social distancing as much as possible can help keep the cases of COVID-19 this winter at a minimum — and help buy researchers the time they need to develop effective treatments and vaccines.
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