Flu vaccines have a surprising health benefit: They could also prevent COVID-19, especially in its most severe forms¹.

A study of more than 30,000 healthcare workers in Qatar found those who received a flu shot had almost a 90% lower risk of contracting severe COVID-19 over the next few months than those who had not recently had the flu had been vaccinated.

The study, conducted in late 2020 before the introduction of COVID-19 vaccines, is consistent with previous work suggesting that boosting the immune system with influenza vaccines and other vaccinations could help the body fight off the SARS coronavirus . CoV-2.

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In the early months of the pandemic — while COVID-19 vaccines were still under development — researchers became intensely interested in the possibility that existing vaccines might provide some protection against SARS-CoV-2. However, it is difficult to gather strong evidence for such an effect because people who get vaccinated against diseases other than COVID-19 may also make other choices that reduce their risk of contracting SARS-CoV-2.

To minimize the impact of this “healthy user effect,” a team led by Laith Jamal Abu-Raddad, an infectious disease epidemiologist at Weill Cornell Medicine-Qatar in Doha, analyzed the medical records of 30,774 medical workers in the country. There are probably fewer differences in health-related behaviors among such workers than in the general population, which reduces – but probably does not eliminate – prejudices – says Abu-Raddad.

The researchers tracked 518 workers who tested positive for SARS-CoV-2 and matched them to more than 2,000 study participants who had tested negative for the virus. Those who received an influenza vaccine this season were 30% less likely to test positive for SARS-CoV-2 and 89% less likely to develop severe COVID-19 disease lower than among workers who had not (although the number of severe cases was 100% higher). small in both groups). The study was published on May 10 on the medRxiv.org preprint server.

Günther Fink, an epidemiologist at the University of Basel in Switzerland, says the Qatar analysis reduces the likelihood that other studies uncovering the same link were a fluke. His team reported that flu vaccines were associated with a reduced risk of death in hospitalized COVID-19 patients in Brazil².

“This is important evidence,” says Mihai Netea, an infectious disease specialist at Radboud University Medical Center in Nijmegen, the Netherlands. The observation that influenza vaccines are linked not only to a reduction in SARS-CoV-2 infections but also in the severity of the disease strongly suggests the protection is real, he adds.

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How long this protection lasts is unclear. Among those in the Qatar study who had the flu shot and later contracted COVID-19, Abu-Raddad’s team recorded SARS-CoV-2 infections averaging about six weeks after vaccination. “I don’t expect this effect to last long,” he says. Netea estimates the benefits will last between six months and two years.

It’s not entirely clear why flu vaccines – which consist of killed influenza viruses – would also protect against COVID-19. Vaccines train the immune system to recognize specific pathogens, but they also activate broad-spectrum antiviral defenses, says Netea, which has found evidence of such responses in flu vaccine recipients³.

Netea’s team is also working to better quantify the benefits of vaccines against influenza and other diseases against COVID-19. To completely rule out the effects of healthy users, his team launched a randomized, placebo-controlled trial in Brazil testing whether influenza and measles, mumps, and rubella vaccines can protect against COVID-19.

Knowing that vaccines for influenza and other diseases can provide protection against COVID-19, albeit only partially and for a limited time, could potentially limit the damage caused by a future pandemic before a vaccine for that disease is developed, Netea argues . “If you have something early on, you can save millions of lives.”