Flu-vaccinated people more resistant to COVID-19, studies explore why

Does a flu vaccination protect against COVID-19? And if so, why? These are the questions medical professionals are asking after a team of doctors led by Anna Conlon from the University of Michigan came to some startling conclusions in a recent study published in the American Journal of Infection Control. 

The physicians had looked at patient data from 27,201 Michigan residents who had taken a COVID-19 test before July 15, 2020. Of those, 12,997 had previously been vaccinated against the flu.

The study found that the proportion of flu-vaccinated people who contracted the coronavirus was slightly lower than among those who had not been vaccinated: 4% instead of 4.9%. That may not look like much at first glance, but it means that people who'd had a flu shot had a 24% lower chance of contracting COVID-19.

In addition, the flu-vaccinated patients were also less likely to require hospitalization or ventilation for coronavirus infection, and their hospital stays were shorter on average.

However, there were no significant differences in mortality between the two comparison groups.

The crucial question for the experts: Is there a medical and a microbiological explanation for these results? This could be, for example, the innate immune defense, which is possibly activated by the flu vaccination. The immune defense functions independently of specific learned antibody immunity, which primarily targets the characteristic spike protein when fighting COVID-19, thus rendering the virus harmless. 

It is also conceivable, however, that fewer people who had been vaccinated against the flu contracted COVID-19 simply because they were more cautious than non-vaccinated people. More people from high-risk groups, like seniors and people with preexisting conditions, typically get vaccinated against influenza than young and healthy people. ...

However, there are two indications against such a correlation: Seniors typically show more severe courses of COVID-19, which was not the case with those who got flu shots in the Michigan study.

A non peer-reviewed preprint study from last year points more to an immunologic explanation: Among Dutch hospital workers who had received the influenza vaccination ahead of the the 2019/2020 flu season, COVID-19 occurred significantly less often than among those who had not been vaccinated. ...