Getting a flu shot is not the same as getting a COVID-19 vaccination. If it were, the world would be in a very different place now.
Still, a new study released by Michigan health researchers has come to an intriguing finding, and it’s one the scientists can’t fully explain yet.
In an analysis of medical records of more than 27,000 Michigan patients who were tested for COVID-19 by July 2020, patients who received a flu vaccine in the previous year were significantly less likely to test positive for the coronavirus than those who did not. had done. .
Significantly, yes – but not to a great extent.
All told, of the 27,201 patients in the study who were tested for COVID-19, 1,218 were positive, representing 4.5 percent of the cohort. It’s worth bearing in mind that this is an average figure, taking into account both the patients who may or may not have received a flu shot.
However, when you break down the numbers further, there is a small but significant contrast in the data, in terms of the probability of a positive COVID-19 test, and that’s after controlling for variables such as ethnicity, race, gender, age and other health-related factors.
In the Michigan cohort, only 4 percent of those who received a flu vaccine tested positive for COVID-19; Among those who had not received a flu shot, the proportion of COVID-19 positive cases was 4.9 percent.
That doesn’t sound like much, but the researchers also summarized the data as follows: The chances of testing positive for COVID-19 were reduced by about 24 percent in patients who received a flu shot compared to those who had not been vaccinated against the flu. in the previous year.
That sounds remarkable, even if the overall effect is relatively small compared to the amount of protection an actual COVID-19 vaccine provides.
But why does it happen at all? It may not really reflect a mechanism of the flu vaccine, researchers say, but it does reflect an effect of bias in the data due to the behavior of people choosing to get vaccinated. But in reality, we just don’t know for sure.
“It is possible that patients who receive their flu vaccine may also be people who distance themselves more and follow CDC guidelines,” said cardiologist Marion Hofmann Bowman of the University of Michigan.
“However, it is also plausible that there could be a direct biological effect of the flu vaccine on the immune system that is relevant to the fight against the SARS-CoV-2 virus.”
What is certain is that this is not the first time we have seen this apparent protective effect against COVID-19 in retrospective data. A number of studies around the world have found evidence for the same link, and the effect seems to extend beyond whether people test positive or not.
In the Michigan study, patients with a flu shot were also less likely to be hospitalized and have the help of ventilators. In other studies, whether or not having a flu vaccination also appears to affect mortality risk, although that was not seen here.
If an actual mechanism of the flu shot is somehow protecting people – and again, there’s no evidence for that here – what could it be?
The researchers speculate that a plausible immunological mechanism could be a process called trained immunity, in which exposure to pathogens (in this case in vaccine form) hypothetically prepares 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 further research is needed to determine whether such a phenomenon occurs here.
In any case, while we don’t yet fully understand why this is happening (and should keep looking at it), this is still another good thing about flu shots – certainly in a time of pandemic, no less.
“While the greatest health benefit of the flu vaccine comes from influenza prevention, the additional potential benefit of COVID-19 protection may provide enough incentive for reluctant patients 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 who report … or who require hospitalization for complications from flu, vaccination will maintain health care for those with COVID-19. “
The findings are reported in the American Journal of Infection Control