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Getting the flu shot is not the same as getting the COVID-19 shot. If it did, the world would be in a very different place right now.
Still, a new study published by Michigan health researchers has come up with an intriguing discovery, and it’s a study scientists can’t yet 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 shot in the previous year were significantly less likely to be tested positive for the coronavirus than those who had not. .
Significantly, yes – but not significantly.
In total, of the 27,201 patients in the study who were tested for COVID-19, 1,218 tested positive, representing 4.5% of the cohort. It should be borne in mind that this is an average figure, which represents both patients who have received and who have not been vaccinated against influenza.
However, when you break down the numbers further, a small but significant contrast appears in the data, in terms of the chance of getting a positive COVID-19 test, and that is 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 had received a flu shot tested positive for COVID-19; Meanwhile, among those who had not received a flu shot, the share of COVID-19 positive cases was 4.9%.
That doesn’t sound like much, but the researchers also summarize the data this way: The chances of testing positive for COVID-19 were reduced by about 24% in patients who received a flu shot compared to those who did. have not been vaccinated against influenza. the previous year.
This seems noticeable, even though the overall effect is relatively small compared to the amount of protection offered by an actual COVID-19 vaccine.
Yet why does this happen at all? This might not actually reflect a mechanism of the flu vaccine, the researchers say, as much as a bias in the data, due to the behavior of people who choose to be vaccinated. But the truth is, we don’t know for sure.
“It is possible that patients who receive their flu shot are also people who practice more 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 influenza vaccine on the immune system relevant to the fight against the SARS-CoV-2 virus.”
What is certain is that this is not the first time that we have seen this apparent protective effect against COVID-19 in historical data. A number of 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 got the flu shot were also less likely to require hospitalization and the help of ventilators. In other studies, whether or not you have a flu shot also appears to affect the risk of death, although this was not seen here.
If an actual mechanism of the flu vaccine somehow protects people – and again, there’s no evidence for that here – what could it be?
Researchers believe that a plausible immunological mechanism could be a process called induced immunity, in which exposure to pathogens (in this case, in the form of a vaccine) hypothetically triggers the immune system to respond to other threats.
“This ‘heterologous immunity’ could explain the nonspecific cross-reactivity of vaccines against unrelated pathogens,” the researchers explain, noting that more research is needed to discern whether such a phenomenon occurs here.
Either way, while we don’t yet fully understand why this is happening (and need to keep thinking about it), it’s nonetheless another good thing about the flu shot – especially in times of a pandemic, nothing less.
“While the greatest health benefit of the influenza vaccine comes from the prevention of influenza, the potential ancillary benefit of protection against COVID-19 may provide sufficient impetus for patients reluctant to get the vaccine,” the authors write. .
“Even though the direct link between COVID-19 prevention and the influenza vaccine is minimal, thanks to an overall reduction in the number of patients presenting … or requiring hospitalization for influenza complications, the vaccination will conserve health resources for people with COVID-19. “
The results are reported in the American Journal of Infection Control.
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