Preliminary research out this month may provide you with a bit more incentive to get the flu vaccine. It found evidence that flu vaccination led to a lowered risk of getting covid-19 among health care workers in the Netherlands. Though not definitive, the study isn’t the first to suggest that certain vaccines can provide minor, nonspecific protection against infectious diseases not targeted by the vaccine, possibly including covid-19.
The research paper was released on the preprint website medRxiv in mid-October by scientists from the Netherlands and Germany. In it, they detail two separate investigations of the theory that the flu vaccine used in the Netherlands last winter—an inactivated vaccine meant to protect against the four main strains of the influenza virus in circulation—could lower the risk of covid-19. In many countries, including the U.S., this is the primary type of flu vaccine used annually.
The basic idea is that some vaccines can not only train the parts of the immune system meant to recognize a specific germ‚ known as the adaptive immune system, but also temporarily boost the effectiveness of our innate immune system. The innate immune system uses a wide array of weapons to repel all sorts of invading germs, making it one of the body’s first lines of defense against unfamiliar microbial threats. A vaccine, it’s thought, can briefly keep the innate immune system more alert and ready to stop the next germ that crosses its way. Scientists call this theoretical boost “trained immunity.”
In the lab, the researchers found that human immune cells first exposed to the flu vaccine seemed to form a trained immune response to the coronavirus behind covid-19, compared to a control group of cells. In Dutch hospitals, they found that hospital workers who were vaccinated with last winter’s flu vaccine had a lower risk of being diagnosed with covid-19 than workers who didn’t get vaccinated.
“In conclusion, a quadrivalent inactivated influenza vaccine can induce trained immunity responses against SARS-CoV-2, which may result in relative protection against covid-19,” they wrote.
The results have yet to go through the peer review process, meaning they should be viewed with some extra caution. And though this study’s authors do provide two lines of evidence for their conclusions, neither is clear proof that a flu vaccine will protect you from covid-19. There are other possible explanations for why Dutch hospital workers who took a flu shot may have been less likely to get covid-19, for instance. It could be that these workers are extra cautious about catching any contagious illness—a personality trait that would both nudge them to get vaccinated for the flu and to avoid situations where covid-19 is likelier to spread.
This isn’t the first study to find evidence that existing vaccines can meaningfully protect people from covid-19. But not all of the data has been encouraging, and some experts remain skeptical of the theory. A study published in JAMA this May, for instance, found no evidence that people in Israel who took a BCG vaccine, used to prevent tuberculosis, in childhood were less likely to contract covid-19 as adults in their 30s and 40s. It’s possible a trained immunity effect created by the BCG vaccine could still exist, just not for that long, but those findings should be a reminder that this theory is not a sure thing. If even trained immunity is the real deal, it almost certainly won’t provide complete protection from the pandemic.
There are ongoing clinical trials testing out whether the BCG vaccine and others can provide some short-term protection from covid-19—trials that will hopefully give us a clear answer one way or the other. In the meantime, though, getting vaccinated against the flu is already one of the most helpful things you can do for yourself and your community this winter. If it turns out that it lowers the risk of covid-19 too, so much the better.