A COVID-19 vaccine might not be ready yet, but the 2020 flu vaccine is, and health experts are urging everyone to get it.
If enough people get their flu shot this fall, we might be able to avoid a winter "twindemic" — a situation in which hospitals are swamped with patients battling COVID-19 and the flu (or even both at the same time).
But there might be another reason to get the shot: based on several studies, there's a chance that flu shots could prevent coronavirus infections, too.
Can Flu Vaccines Prevent COVID-19?
This research on the flu vaccine's effects on COVID-19 is still preliminary, but here's what scientists know so far.
One Dutch study (not yet peer-reviewed) found that hospital employees who had received their 2019-2020 flu shot were 39% less likely to test positive for COVID-19 by June 2020 than those who didn't.
Another preprint paper found that people who received at least one of eight vaccines (including the flu shot) in the past five years were less likely to test positive for COVID-19 than people who hadn't received any of the vaccines.
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Meanwhile, two peer-reviewed studies have found that parts of Italy where more seniors got their flu shots had lower COVID-19 rates.
A limitation with all of these studies is that they can't prove flu vaccines prevent COVID-19. It might just be that people who get their vaccinations are more health conscious in general and therefore more likely to take other preventative measures against COVID-19.
Additionally, vaccines train our immune systems to recognize and combat specific pathogens, so on the surface, it doesn't make much sense that a vaccine for influenza or another disease would have any effect on the body's ability to battle the coronavirus.
But this isn't the first time we've heard about existing vaccines potentially protecting people against COVID-19 — studies across the globe are testing to see whether vaccines for polio, tuberculosis, measles, and other diseases might prevent COVID-19 or reduce its severity.
As for why vaccines for other diseases might help, that comes down to a possible secondary effect of some vaccines, called "trained innate immunity."
Putting the Immune System on Alert
Our immune system is broken into two categories.
One category, the adaptive immune system, is specialized and trained through our experiences. When we get a flu shot, our body sees the weakened flu virus and creates antibodies against it — those antibodies are part of the adaptive immune system.
The other category, the innate immune system, is more broad. It protects us from all pathogens, regardless of whether we've encountered them before. It also jumps into action more quickly than the adaptive immune system.
If the adaptive immune system is a sniper patiently waiting on a hill with a photo of their target, the innate immune system is a troop of soldiers given machine guns and told to fire away at anything that looks fishy.
If flu vaccines prevent COVID-19, it might be due to trained innate immunity.
Past research has found that, while vaccines spur the adaptive immune system to fight a specific pathogen, they might also be training the innate immune system, improving its ability to battle a range of invaders.
Trained innate immunity could be why people who get other vaccines are less likely to catch COVID-19 — and a lab experiment by the Dutch researchers does seem to support the theory that flu shots could prevent coronavirus infections.
For that experiment, they started by exposing purified blood cells from healthy patients to a flu vaccine. After waiting six days, they then introduced the coronavirus to both those cells and ones that weren't exposed to the flu vaccine.
The cells primed by the flu vaccine produced more of a type of molecule that fights viruses, called cytokines, than the cells that weren't exposed to the flu vaccine. That could help those cells fight off infections from a range of different viruses.
Despite that, there's still not enough evidence yet to say whether flu vaccines prevent COVID-19 — and there might never be.
To find out for sure, researchers would need to conduct trials in which they purposely withheld the flu vaccine from some participants (or gave them placebos), but Mihai Netea, one of the researchers behind the Dutch study, told Scientific American that that would be unethical.
For now, all we know for certain is that people who do get the flu vaccine are less likely to contract COVID-19 — and even if it only prevents flu, it could still help avoid that twindemic experts fear may be on the horizon.
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