Those who are vaccinated against influenza contract COVID-19 less often and less severely. Is this because vaccinated people are more cautious or are there medical reasons?
Does a flu vaccination protect against COVID-19? And if so, why? These are the questions medical professionals are asking, because a team of doctors led by Anna Colon from the University of Michigan came to some startling conclusions in a study published in the American Journal of Infection Control.
The physicians had looked at patient data from 27,201 Michigan people who had taken a COVID-19 test by 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%. In addition, the flu-vaccinated patients were also less likely to require hospitalization or ventilation for coronavirus infection. In addition, 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: Is there a medical and a microbiological explanation for this? This could be, for example, the innate immune defense, which is possibly activated by the vaccination. It functions independently of 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. These react rather non-specifically to infections.
This standing army of our immune defenses includes, for example, phyagocytes and dendritic cells (cells ingesting harmful foreign particles), but also various cytokines (proteins that play a role in immune reactions and inflammatory processes) as well as T and B leukocytes (white blood cells).
Some vaccinations are generally good for the immune defense, as can be seen in those vaccinated for 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 vaccination.
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.
In the US, for example, many seniors and retirees had already voluntarily isolated themselves early last year, while others were still working.
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.
And there were no seniors over 70 years of age in either of these groups. All of those studied were of working age and had a correspondingly high number of contact encounters.