A preprint study published last week in medRxiv found that flu shots reduced the risk of Covid-19 infection by nearly 30% in the two weeks following vaccination and cut the risk of severe or fatal Covid-19 by almost 90%—adding to a growing body of evidence indicating that flu shots may be an effective way to prevent infection and improve outcomes for Covid-19 patients.
Study details and key findings
For the study, researchers studied 30,774 health care workers in Qatar who were vaccinated against influenza between Sept. 17, 2020 and Dec. 31, 2020, prior to the release of Covid-19 vaccines. Study participants had a median age of 36 years, and the control group had a median age of 35.
About a month and a half after receiving the influenza vaccine, participants were tested for the coronavirus with a PCR test. Among study participants, more than 12,000 were tested. In total, there were 518 positive results and 10,000 negative results. Roughly two-thirds of participants were tested after experiencing potential Covid-19 symptoms.
Overall, the flu shots appeared to reduce the risk of Covid-19 infection by 29.7% in the two weeks following vaccination. In addition, researchers found that the flu shot reduced the risk of severe or fatal Covid-19 by 88.9%.
Notably, of the nearly 130 participants who tested positive for the coronavirus after receiving the flu shot, just one had to be hospitalized after developing severe Covid-19, and none of the participants developed critical or fatal infections.
However, of the nearly 400 unvaccinated patients who tested positive for the coronavirus, there were 17 severe infections, 2 critical cases, and no deaths.
While the study's authors concluded that "[r]ecent influenza vaccination is associated with an appreciable reduction in the risk of SARS-CoV-2 infection and COVID-19 severity," the study only included a small number of severe Covid-19 cases, Nature notes.
Still, the study provided evidence that supports the claim that the influenza vaccine could be an effective way to fend off Covid-19 and resulting severe illness.
According to Günther Fink, an epidemiologist at the University of Basel, the Qatar study decreases the likelihood that similar studies finding the same relationship were a coincidence. Notably, Fink's team reported that flu shots were tied to a reduced risk of death in hospitalized Covid-19 patients in Brazil.
"This is an important piece of evidence," said Mihai Netea, an infectious disease specialist at Radboud University Medical Center. According to Netea, the observation that flu shots are tied to a reduction in both Covid-19 infections and disease severity strongly suggests that the protection is legitimate.
Scientists do not know why flu shots—composed of killed influenza viruses—would also offer protection from the coronavirus. While vaccines train the immune system to detect specific pathogens, they also increase broad-acting antiviral defenses, according to Netea, who has also found signs of these responses in flu-shot recipients.
If vaccines for flu and other diseases can offer protection against the coronavirus, they could help limit the damage caused by a future pandemic before a vaccine for that disease can be developed, Netea argued. "If you have something in the beginning, you could save millions of lives." (Callaway, Nature, 5/16; Thomas, News Medical, 5/12; Tayar et al., medRxiv, 5/10)