Flu vaccines are linked to a decline in COVID-19 infections, and scientists aren’t sure why

The flu vaccine is not the same as the COVID-19 vaccine. If it were, the world would be in a very different place right now.

Still, a new study published by health researchers in Michigan has come up with an intriguing finding, and it’s one that scientists still can’t fully explain.

In an analysis of the medical records of more than 27,000 Michigan patients who were tested for COVID-19 in July 2020, patients who had received a flu vaccine in the previous year were significantly less likely to test positive. by the coronavirus than those who had not. .

Significantly, yes, but not to a great extent.

In total, of the 27,201 patients in the study who were tested for COVID-19, 1,218 tested positive, representing 4.5 percent of the cohort. It is worth bearing in mind that this is an average figure, representing both patients who received the flu vaccine and those who did not.

However, when you break down the numbers further, a small but significant contrast emerges in the data, in terms of the chance of getting a positive COVID-19 test, and that’s after controlling for variables like ethnicity, race, gender, age, and other health-related factors.

In the Michigan cohort, only 4 percent of those who had received a flu vaccine tested positive for COVID-19; Meanwhile, among those who had not received a flu vaccine, the proportion of positive COVID-19 cases was 4.9 percent.

That doesn’t sound like much, but the researchers also summed up the data like this: The odds of testing positive for COVID-19 were reduced by 24 percent in patients who received a flu shot compared to those who didn’t get the flu shot. flu. in the previous year.

That sounds remarkable, even if the overall effect is relatively small compared to the amount of protection a real COVID-19 vaccine provides.

Still, why does it happen? It might not actually reflect a flu vaccine mechanism, the researchers say, but rather a bias effect in the data, due to the behavior of people who choose to get vaccinated. But the truth is that we don’t know for sure.

“It is possible that the patients who receive the flu vaccine are also people who practice greater social distancing and follow CDC guidelines,” says cardiologist Marion Hofmann Bowman of the University of Michigan.

“However, it is also plausible that there may be a direct biological effect of the flu vaccine on the immune system relevant to the fight against the SARS-CoV-2 virus.”

The truth is that this is not the first time that we have seen this apparent protective effect against COVID-19 in retrospective data. Several studies around the world have found evidence for the same link, and the effect seems to go beyond whether people test positive or not.

In the Michigan study, patients who received a flu vaccine were also less likely to require hospitalization and ventilator assistance. In other studies, whether or not to have a flu vaccine also appears to affect mortality risk, although that was not seen here.

If a real flu vaccine mechanism somehow protects people, and again, there is no evidence for that here, what could it be?

The researchers speculate that a plausible immune mechanism could be a process called trained immunity, in which exposure to pathogens (in this case, in the form of a vaccine) hypothetically primes the immune system to respond to other threats.

“This ‘heterologous immunity’ could explain the nonspecific cross-reactivity that vaccines have against unrelated pathogens,” the researchers explain, emphasizing that more research is needed to discern whether such a phenomenon is occurring here.

In any case, while we still don’t fully understand why this is happening (and we need to investigate further), this is another good thing about flu vaccines, especially in times of pandemic, no less.

“While the greatest health benefit of the influenza vaccine comes from preventing influenza, the ancillary potential benefit of COVID-19 protection may provide enough impetus for patients who are hesitant to get vaccinated,” the authors write. .

“Even if the direct link between COVID-19 prevention and the flu vaccine is minimal, through an overall reduction in the number of patients presenting … or requiring hospitalization for flu complications, vaccination will preserve health care resources for those with COVID-19. “

Findings are reported in the American Journal of Infection Control.


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