Getting the flu vaccine is not the same as getting the COVID-19 vaccine. If it were, the world would be in a very different place now.
Still, a new study released by health researchers in Michigan has come to an intriguing discovery, which scientists are still unable to fully explain.
In an analysis of medical records of more than 27,000 patients in Michigan who were tested for COVID-19 in July 2020, patients who received a flu vaccine in the previous year were significantly less likely to have positive results for coronavirus than those who didn’t.
Significantly, yes – but not in large quantities.
In all, 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 remembering that it is an average number, taking into account the patients who received and those who did not receive the flu vaccine.
When you divide the numbers even more, however, a small but significant contrast appears in the data, in terms of the chance of getting a positive COVID-19 test, and this after controlling for variables such as ethnicity, race, sex, age and other health-related factors.
In the Michigan cohort, only 4% of those who received the flu vaccine tested positive for COVID-19; meanwhile, among those who did not receive the flu vaccine, the proportion of COVID-19 positive cases was 4.9 percent.
This does not seem like much, but the researchers also summarize the data thus: the chances of a positive test for COVID-19 were reduced in patients who received a flu vaccine by about 24 percent compared to those who were not vaccinated against the flu. flu in 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 offers.
Still, why does this happen? It may not really reflect a mechanism of the flu vaccine, the researchers say, as much as a bias effect on the data, due to the behavior of people who choose to be vaccinated. But in reality, we just don’t know for sure.
“It is possible that patients who received the flu vaccine are also practicing more social detachment and following CDC guidelines,” said University of Michigan cardiologist Marion Hofmann Bowman.
“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.”
What is certain 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 of the same link, and the effect seems to go beyond whether people test positive or not.
In the Michigan study, patients who received the flu vaccine were also less likely to require hospitalization and ventilator assistance. In other studies, whether or not you are vaccinated against the flu appears to affect the risk of mortality as well, although this has not been observed here.
If a real mechanism of the flu vaccine somehow protects people – and, again, there is no evidence of 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 prepares the immune system to respond to other threats.
“This ‘heterologous immunity’ could explain the nonspecific cross-reactivity that vaccines have against unrelated pathogens,” explain the researchers, 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 continue investigating), 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 influenza prevention, the potential auxiliary benefit of COVID-19 protection can provide sufficient momentum for hesitant patients to get vaccinated,” write the authors.
“Even if the direct link between COVID-19 prevention and the influenza vaccine is minimal, through a general reduction in the number of patients experiencing … or requiring hospitalization for influenza complications, vaccination will preserve the resources of health for those with COVID-19. “
The findings are reported in the American Journal of Infection Control.