Getting a flu shot is not the same as getting a COVID-19 vaccine. If so, the world would be in a very different place right now.
However, a new study published by health researchers in Michigan has come to an intriguing finding and is one that scientists cannot yet fully explain.
In an analysis of medical records of more than 27,000 Michigan patients who underwent COVID-19 testing in July 2020, patients who had received a flu vaccine during the previous year were significantly less likely to test positive for coronavirus than those who had not. .
Significantly, yes, but not by a lot.
In total, of the 27,201 patients in the study who underwent COVID-19 testing, 1,218 tested positive, representing 4.5 percent of the cohort. It is worth noting that it is an average figure, which takes into account both patients who had and had not had a flu vaccine.
When the figures are further broken down, a small but significant contrast appears in the data, in terms of the possibility of obtaining a positive COVID-19 test, and this after monitoring variables such as ethnicity, race, gender, l 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%.
This doesn’t sound like much, but the researchers also summarize the data like this: the chances of testing positive for COVID-19 were reduced in patients who received a flu vaccine by about 24% compared to those who did not. flu vaccine. the previous year.
This seems remarkable, even if the overall effect is relatively small compared to the amount of protection that a real COVID-19 vaccine provides.
Still, why does it happen? It may not reflect a mechanism of the flu vaccine, the researchers say, as much as a data bias effect, due to the behavior of people who choose to get vaccinated. But we don’t really know for sure.
“Patients receiving the flu vaccine may also be 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 we have seen this protective effect against COVID-19 in retrospective data. Several studies around the world have found evidence of the same link and the effect seems to go beyond whether people test positive or not.
In the Michigan study, patients with the flu vaccine were also less likely to require hospitalization and ventilator care. In other studies, having or not having a flu vaccine appears to also affect the risk of mortality, although it was not seen here.
If a real mechanism of the flu vaccine somehow protects people (and, again, there is no evidence here), what could it be?
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 arrogates 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, stressing that more research is needed to discern whether this phenomenon occurs here.
In any case, while we still don’t quite understand why this is happening (and we need to keep examining it), this is nonetheless another good thing about flu vaccines, especially in times of pandemic, no more and no more. less.
“While the greatest health benefit of the influenza vaccine comes from influenza prevention, the potential auxiliary benefit of COVID-19 protection may provide a sufficient impetus for hesitant patients to be vaccinated.” write the authors.
“Even if the direct link between COVID-19 prevention and influenza vaccine is minimal, through an overall reduction in the number of patients presenting … or requiring hospitalization for complications of influenza, vaccination will preserve health resources for those with COVID-19. “
The findings are reported in American Journal of Infection Control.