Flu Shot May Protect Against COVID-19, Reduce Severity, Study Finds

Individuals vaccinated against influenza were 24% less likely to be infected with COVID-19 than those who did not get the flu shot, according to Michigan Medicine researchers.

Individuals vaccinated against influenza had significantly reduced odds of testing positive for COIVD-19 versus those who did not receive the vaccine, according to a recent study in the American Journal of Infection Control.

In addition, those who did test positive were less likely to require hospitalization or mechanical ventilation and, if admitted, had a shorter hospital length of stay.

The findings are based on analysis of records of more than 25 000 patients who received a COVID-19 test in the Michigan Medicine healthcare system.

The study results are “... particularly relevant for vaccine hesitance, and maybe taking the flu shot this year can ease some angst about the new COVID-19 vaccine,” said senior author Marion Hofmann Bowman, MD, associate professor of internal medicine and a cardiologist at the Michigan Medicine Frankel Cardiovascular Center, in a blog post on MHealth Lab at Michigan Medicine.

For the retrospective cohort study, Bowman and colleagues reviewed medical charts for 27 201 patients who were tested for COVID-19 at Michigan Medicine between March and mid-July of 2020. The primary outcome was comparison of positive COVID-19 testing in those who received the influenza vaccine versus those who did not. Secondary end points in patients testing positive for COVID-19 included mortality, need for hospitalization, length of stay, need for intensive care, and mechanical ventilation.

RESULTS

Bowman et al found that the odds of testing positive for COVID-19 were reduced by 24% in those who were vaccinated against flu versus those who were not (odds ratio [OR] 0.76, 95% CI 0.68-0.86; P<.001). The association between flu vaccine and COVID-19 status remained significant after multivariable regression analysis that controlled for multiple variables including ethnicity, race, gender, age, BMI, smoking status, and a wide range of comorbidities.

Vaccinated patients were also less likely to require hospitalization (OR, 0.58, 95% CI 0.46-0.73; P<.001) or mechanical ventilation (OR, 0.45, 95% CI 0.27-0.78; P=.004), and had a shorter hospital length of stay (risk ratio, 0.76, 95% CI 0.65-0.89; P<.001).

There was a non-significant decrease in mortality in the influenza-vaccinated group, an outcome the authors note may be the result of small sample size and the low number of deaths observed. No association was found between timing of influenza vaccination and COVID-19 clinical outcomes.

The authors state that the mechanism underlying the association is not yet clear.

“It is possible that patients who receive their flu vaccine are also people who are practicing more social distancing and following CDC guidelines,” said senior author Bowman in the Michigan Health blog post. “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.”

The authors write, too, that while they were able in the analysis to control for a wide variety of variables, the protective association could also be confounded by socioeconomic disparities between vaccinated and unvaccinated groups.

Even if the direct link between the prevention of COVID-19 and the influenza vaccine is minimal, there is evidence that the flu vaccine has other benefits.

“There’s robust data that the flu shot prevents heart attack and hospitalizations for heart failure, which is an additional reason to get your vaccine every flu season,” noted co-first author Anna Conlon, PhD, U-M Medical School student, in the blog post.

Study limitations the authors identify include use of information from electronic health records which may not be updated for influenzas vaccinations administered out of state as well as the large single-center source of data, the latter limiting extrapolation of results to smaller communities.

The investigators called for continued promotion of influenza vaccine as well as patient education to help increase uptake.