Flu vaccinated people are more resistant to COVID-19, but why? | Knowledge & Environment | DW


Does a flu shot protect against COVID-19? And if so, why? These are the questions that occupy doctors, because a team of doctors led by Anna Colon from the University of Michigan came to astonishing results in a study in the American Journal of Infection Control.

The medics looked at patient data from 27,201 people from the US state of Michigan who had taken a COVID-19 test by July 15, 2020. Of these, 12,997 had previously been vaccinated against flu. It turned out that the proportion of those vaccinated against the flu who were infected with the coronavirus was slightly lower than among those not vaccinated, namely 4.0 instead of 4.9 percent.

In addition, the flu-protected patients with a coronavirus infection also had to be treated or ventilated less frequently in hospital. In addition, hospital stays were shorter on average. In terms of mortality, however, there were no significant differences between the two comparison groups.

What influence does the innate immune defense have?

The key question for professionals is: is there a medical and a microbiological explanation for this? This could be the innate immune defense, for example, which the vaccination may activate. It works independently of the learned antibody immunity, which in the fight against COVID-19 mainly attacks the characteristic spike protein and thus makes the virus harmless.

The innate immune defense, which may be stimulated by the vaccination, on the other hand, consists of a number of different elements. These react rather unspecifically to intruders or foreign bodies. This standing army of our immune defense includes phyagocytes and dendritic cells (scavenger cells), 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).

It is known from measles vaccination, for example, that various vaccinations are generally good for the immune system. Epidemiological studies have shown years ago that vaccinated children still had a higher immunity to a large number of pathogens than non-vaccinated children long after vaccination.

Or is it just correlation?

However, it is also conceivable that fewer people with the flu vaccination have contracted COVID-19 because they behaved more cautiously than those who were not vaccinated. Usually, more people from risk groups (senior citizens and people with previous illnesses) are vaccinated against the flu than young and healthy people. In the USA, for example, many senior citizens and retirees had already voluntarily isolated themselves last year while other people still had to work.

Two pieces of evidence speak against such a correlation: Seniors typically also show more severe COVID-19 disease courses, which was not the case in the Michigan study. In addition, there is a non-peer-reviewed preliminary study from last year that suggests an immunological explanation: COVID-19 was significantly less common among Dutch hospital employees who had received a flu vaccination in winter 2019/2020 than among those who who were not vaccinated.

And in these two groups there were no seniors over 70 years of age, all of the examined were employed and had correspondingly many contact encounters.

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