Getting a flu shot is not the same thing as getting a COVID-19 vaccination. If it were, the world would be in a very different place right now.

Still, a new study released by health researchers in Michigan has arrived at an intriguing finding, and it's one the scientists can't as yet fully explain.

In an analysis of medical records from over 27,000 patients in Michigan who got tested for COVID-19 by July 2020, patients who had received an influenza vaccine within the previous year were significantly less likely to test positive for the coronavirus than those who hadn't.

Significantly, yes – but not by a large amount.

All up, of the 27,201 patients in the study who got tested for COVID-19, 1,218 tested positive, representing 4.5% of the cohort. It's worth bearing in mind that's an average figure, accounting for both the patients who had and hadn't gotten a flu shot.

When you break the numbers down further, though, a small but significant contrast emerges in the data, in terms of the chance of getting a positive COVID-19 test, and that's after controlling for variables like ethnicity, race, gender, age, and other health-related factors.

In the Michigan cohort, only 4% of those who had received a flu vaccine tested positive for COVID-19; meanwhile, amongst those who hadn't received a flu shot, the share of positive COVID-19 cases was 4.9%.

That doesn't sound like much, but the researchers also summarize the data thus: the odds of testing positive for COVID-19 were reduced in patients who received a flu shot by about 24% compared to those who didn't get vaccinated against influenza in the previous year.

That sounds notable, even if the overall effect is relatively small compared to the amount of protection an actual COVID-19 vaccine delivers.

Still, why does it happen at all? It might not actually reflect a mechanism of the flu vaccine, researchers say, so much as an effect of bias in the data, due to the behavior of people who choose to get vaccinated. But in truth we just don't know for sure.

"It is possible that patients who receive their flu vaccine are also people who are practicing more social distancing and following CDC guidelines," says cardiologist Marion Hofmann Bowman from 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 relevant for the fight against SARS-CoV-2 virus."

What is certain is that this isn't the first time we've seen this apparent protective effect against COVID-19 in retrospective data. A number of studies worldwide have found evidence of the same link, and the effect seems to go beyond just whether or not people test positive.

In the Michigan study, patients who had a flu shot were also less likely to require hospitalization and the assistance of ventilators. In other studies, having or not having a flu vaccination appears to affect mortality risk too, although that wasn't seen here.

If an actual mechanism of the flu shot is somehow protecting people – and again, there's no evidence of that here – what might it be?

The researchers speculate a plausible immunologic mechanism could be a process called trained immunity, in which exposure to pathogens (in this case, in vaccine form) hypothetically primes 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 discern whether such a phenomenon is occurring here.

In any case, while we don't yet fully understand why this is happening (and need to keep looking into it), this is nonetheless another good thing about flu shots – especially in a time of pandemic, no less.

"While the largest benefit to health from the influenza vaccine comes from prevention of influenza, the ancillary potential benefit of COVID-19 protection may provide enough impetus for hesitant patients to get vaccinated," the authors write.

"Even if the direct link between the prevention of COVID-19 and the influenza vaccine is minimal, through an overall reduction in the number of patients presenting … or requiring hospitalization for complications of influenza, vaccination will preserve healthcare resources for those with COVID-19."

The findings are reported in the American Journal of Infection Control.