Again and again, experts come across an unusual connection during the pandemic: Those who have been vaccinated against the flu also seem to contract Sars-CoV-2 less often. A team led by doctor Anna Conlon from the University of Michigan has now confirmed this finding in an analysis of 27,000 patients. In the “American Journal of Infection Control”, the working group also reports that those who have been vaccinated have to go to hospital less often, are ventilated less often and get out of the clinic more quickly. Mind you: It’s about the flu vaccination, not Covid-19.
The working group found 27,000 people tested for Sars-CoV-2 by July 15, 2020 in the Michigan state health system database. 13,000 had received the flu vaccine in the previous 12 months, and of these, only 4.0 percent had contracted the coronavirus, compared to 4.9 percent in the group not vaccinated against the flu. In addition, they came to the hospital a third more often when they were infected, stayed there longer on average and had to be ventilated about twice as often as the group vaccinated against the flu, reports Conlon’s team. This is all the more remarkable as the vaccinated tended to belong to a risk group more often – and should therefore actually have more severe courses. The team also evaluated mortality, but the difference between the groups is not statistically significant.
Although the phenomenon has now been described several times, experts are still puzzling over the reasons. It is possible that social and economic differences play a role; however, the effect has also been demonstrated in situations where this is unlikely. Therefore, some experts suspect that the vaccination, in addition to the learned immune response to the flu virus, also puts the innate immune system into an activated state, which then makes it more difficult for Sars-CoV-2. The protection provided by this “trained immune response” effect presumably also occurs with other vaccinations. It is easier to detect with the flu vaccination than with other vaccinations, because it is renewed annually and the trained immunity presumably decreases after a while.