Being vaccinated against the flu is not the same as vaccinating COVID-19. If it were, the world would be somewhere else right now.
However, a new study released by Michigan health researchers has reached an interesting finding and is one that scientists cannot yet fully explain.
In an analysis of medical records from 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 coronavirus. than those who did not have.
Significantly, yes – but not in large quantities.
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 is worth noting that this is an average figure, which takes into account both patients who have had and have not received a flu vaccine.
When you break down the numbers further, however, there is a small but significant contrast in the data, in terms of the chance of getting a positive COVID-19 test, after checking for variables such as ethnicity, race, sex, age and other factors. related to health.
In the Michigan cohort, only 4 percent of those who received a flu vaccine tested positive for COVID-19; Meanwhile, among those who did not receive a vaccine against influenza, the share of positive COVID-19 cases was 4.9%.
This does not sound like much, but the researchers summarize the data as follows: the chances of testing positive for COVID-19 were reduced in patients who received a flu vaccine by about 24%, compared to those who were not vaccinated against the flu. in the previous year.
This seems remarkable, even if the overall effect is relatively small compared to the amount of protection that a COVID-19 vaccine actually provides.
However, why is it not happening at all? It may not actually reflect a mechanism of the flu vaccine, the researchers say, both as an effect of data bias, due to the behavior of people who choose to get vaccinated. But we really don’t know for sure.
“Patients receiving the flu vaccine may also be people who practice greater social distance 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 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. A number of studies around the world have found evidence of the same connection, and the effect seems to go beyond just whether or not people are positive.
In the Michigan study, patients who were vaccinated against the flu were also less likely to require hospitalization and ventilator care. In other studies, influenza vaccination may or may not appear to affect the risk of mortality, although this has not been seen here.
If a real flu vaccine mechanism somehow protects people – and again, there is no evidence to that effect – what could it be?
Researchers speculate that a plausible immune mechanism could be a process called entrained immunity, in which exposure to pathogens (in this case, in the form of a vaccine) hypothetically establishes the immune system to respond to other threats.
“This ‘heterologous immunity’ could explain the nonspecific cross-reactivity of vaccines against unaffiliated pathogens,” the researchers said, stressing that further research is needed to determine whether such a phenomenon occurs here.
In any case, although we do not fully understand why this is happening (and we must continue to look at it), this is still another good thing about flu vaccines – especially in a time of pandemic, no less.
While the greatest health benefit from the flu vaccine comes from flu prevention, the potential ancillary benefit of COVID-19 protection may provide enough impetus for hesitant patients to get vaccinated, the authors write.
“Even if the direct link between COVID-19 prevention and influenza vaccine is minimal, through a general reduction in the number of patients presenting … or requiring hospitalization for influenza complications, vaccination will preserve the medical resources for those with COVID-19 . “
The findings are reported in American Journal of Infection Control.