Flu vaccines have surprising health benefits: They may also prevent COVID-19, especially in its most severe form1.
A study of more than 30,000 healthcare workers in Qatar found that those who had contracted the flu were 90% less likely to develop severe COVID-19 over the next few months, compared to those who had not recently been vaccinated against the flu. .
The study, conducted in late 2020 before COVID-19 vaccines were introduced, is in line with previous work suggesting that ramping up the immune system with flu vaccines and other hits can help the body ward off the SARS-CoV-2 coronavirus.
In the early months of the pandemic – while vaccines for COVID-19 were still being developed – researchers were keenly interested in the possibility that current vaccines could provide some protection against SARS-CoV-2. But gathering solid evidence of such an effect is difficult, because people who seek vaccination against diseases other than COVID-19 may also make other choices that reduce the risk of contracting SARS-CoV-2.
To reduce the impact of the “health user effect,” a team led by Laith Jamal Abu Raddad, an infectious disease epidemiologist at Weill Cornell Medicine-Qatar in Doha, analyzed the health records of 30,774 medical workers in the country. Abu-Raddad says there is likely to be less variance in health-related behavior among these workers than in the general population, reducing — but perhaps not eliminating — prejudice.
The researchers tracked 518 workers who tested positive for SARS-CoV-2 and compared them to more than 2,000 study participants who tested positive for the virus. Those who received the flu shot that season were 30% less likely to test positive for SARS-CoV-2, and 89% less likely to have severe COVID-19, compared to workers who did not (although the number of severe cases was small in both groups). The study was published on the preprint server medRxiv.org on May 10.
Gunther Fink, an epidemiologist at the University of Basel in Switzerland, says the Qatar analysis reduces the chances that other studies that have revealed the same link were just a fluke. His team reports that flu vaccinations are associated with a lower risk of death in hospitalized COVID-19 patients in Brazil2.
“This is important evidence,” says Mihai Nitya, an infectious disease specialist at Radboud University Medical Center in Nijmegen, the Netherlands. And he adds that the observation that influenza vaccines are associated with reducing not only SARS-CoV-2 infection, but also disease severity, strongly suggests that protection is real.
How long this protection lasts is not clear. Among those participants in the Qatar study who contracted the flu and later contracted the coronavirus, Abu Raddad’s team recorded SARS-CoV-2 infections, on average, about six weeks after vaccination. “I wouldn’t expect to see this effect lasting long at all,” he says. Netea estimates that the benefits last between six months and two years.
It’s not entirely clear why influenza vaccines – which consist of killed influenza viruses – also protect against COVID-19. Vaccines train the immune system to recognize specific pathogens, but they also activate broad-spectrum antiviral defenses, says Nitya, who has found signs of such responses in flu vaccine recipients.3.
The Netea team is also working to better determine the benefits of vaccinations for influenza and other diseases against COVID-19. To completely rule out user health effects, his team launched a randomized, placebo-controlled trial in Brazil that will test whether influenza, measles, mumps and rubella vaccines can protect against COVID-19.
Nitya argues that knowing that vaccines for influenza and other diseases can provide protection against COVID-19, even if only partially and for a limited time, could potentially limit the damage from a future pandemic before a vaccine for that disease is developed. “If you have something to start with, you can save millions of lives.”