Getting a flu shot could reduce your risk of getting COVID-19, preliminary research suggests

• Health experts are encouraging people to get flu shots to avoid the possibility of getting the flu and COVID-19 at the same time.
• According to a preliminary study, flu shots may also reduce your risk of catching COVID-19.
• Researchers found that healthcare workers who got a flu shot in the 2019-2020 season were 39% less likely to have gotten the coronavirus by June.
• The study doesn't prove flu shots protect you from COVID-19, but additional research similarly suggests that vaccines can boost the body's innate immunity to other diseases.

As winter approaches, people are preparing for a looming "twindemic": the double threat of the coronavirus and the seasonal flu.

In the past month, coronavirus cases have surged worldwide. At the same time, the onset of cold, dry weather in the Northern Hemisphere heralds the start of flu season. So public-health officials in the US and Europe are pushing folks to get flu shots. The seasonal vaccinations reduce your risk of getting the flu by up to 60% and also reduce how severely you get sick if you do catch the flu.

But the shot might yield another important benefit: It may reduce your chances of getting COVID-19.

A new preliminary study, first reported by Scientific American, found that healthcare workers who got the flu vaccine ahead of the 2019-2020 season were 39% less likely to test positive for the coronavirus by June 1.

"Influenza vaccination may contribute not only to reduction of influenza but also to the COVID-19-related burden on the healthcare system," the study authors wrote.

Vaccines may train our immune systems to fight other viruses

For the new study, the researchers examined data from about 10,600 employees at the Radboud University Medical Center in the Netherlands. Of that group, they found, 184 healthcare workers had tested positive for the coronavirus by June 1. So they looked at who among the employees had gotten a flu shot during the winter.

The study authors found that 2.23% of healthcare workers who had not gotten a flu shot tested positive for COVID-19, whereas only 1.33% of vaccinated employees did.

Two other recent studies from Italy found a similar trend: Coronavirus rates turned out to be lower among Italians older than 65 who'd gotten a flu shot.

The findings also align with previous research suggesting that even vaccines targeting a specific virus, like measles or polio, can help train the body to fight other invaders, too.

When a person gets sick, the immune system makes antibody proteins that recognize and neutralize that virus and then remember that virus the next time it invades — what's known as an adaptive immune response. But the body also possesses an innate immune response: a speedier, more general defense of white blood cells brought to bear against any pathogen. Vaccines can help boost the ability of that innate immune response through a process called trained immunity.

Trained immunity is when the body's innate immune cells get reprogrammed to react faster and more efficiently to future invasions because of exposure to germs over time, like a pump being primed.

The authors of the new study also investigated what happened when human blood cells were exposed to a flu vaccine and then infected with the coronavirus. Cells exposed to the vaccine, they found, developed a better-trained immune response to the coronavirus than unexposed cells. This was primarily because of the production of immune proteins called cytokines that fight off infections.

The researchers also tried exposing the cells to both the flu vaccine and the BCG vaccine, which targets tuberculosis. Exposure to both vaccines created a synergistic effect, the results suggested, boosting the cells' cytokine production even more.

Scientists still need more evidence that flu shots help protect against COVID-19

Experts say more evidence is needed to determine whether there's a causal link between a flu shot and lower risk of getting the coronavirus.

"This is an intriguing study, but it doesn't provide definitive evidence," Ellen Foxman, an immunobiologist at the Yale School of Medicine who was not involved in the study, told Scientific American.

There could be other reasons healthcare workers who didn't get flu shots wound up with slightly higher rates of COVID-19. It's possible, for example, that people who got a flu shot were simply also more likely to take more precautions during the pandemic, Foxman said.

Healthcare workers in the study also had varying levels of direct contact with hospital patients, which could have affected the results. And the researchers did not collect information about whether workers had other health conditions that increased their risk, so that could be a factor as well.

"As far as telling people, 'You should go get a flu vaccine because it can protect you from COVID,' that's a little bit of a stretch at this point," Foxman said.

But she added people should get a flu shot anyway, since, of course, "it's going to protect you from the flu."

The flu can be deadly

About 0.1% of people who got the flu in the US last year died, according to the Centers for Disease Control and Prevention. That's one-fifth to one-tenth the estimated infection-mortality rate of COVID-19.

But influenza is no cakewalk. Every year, the seasonal flu kills tens of thousands of Americans. Last year, there were more than 18 million flu-related medical visits and 405,000 hospitalizations in the US.

According to the World Health Organization, there are 290,000 to 650,000 respiratory deaths related to influenza worldwide every year.

Unlike the new coronavirus, the influenza virus mutates quickly, leading to the appearance of new strains that undermine people's immunity. That's why we have to get a new flu shot every year, and it's also why flu vaccines aren't always 100% effective.