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.
Innate immune defense
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. In contrast, the innate immune defense, which may be stimulated by vaccination, consists of a number of different elements that react to infections in general, not specific viruses.
Some vaccinations are generally good for the immune defense, as can be seen in people vaccinated against measles, for example. Epidemiological studies showed years ago that vaccinated children still had a higher immunity to a variety of pathogens than non-vaccinated children, even a very long time after the vaccination took place.
Or is it just correlation?
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 pre-existing conditions, typically get vaccinated against influenza than young and healthy people.