Study Examines Role of Flu Vaccine in COVID-19 Infection Rates, Severity

A recent study looked at the role of the influenza vaccine in COVID-19 susceptibility and severity.¹

The findings, which were published in the American Journal of Infection Control, showed that individuals who received the flu vaccine had a lower chance of testing positive for COVID-19 compared with those who did not receive the vaccine.¹

For the study, the investigators reviewed medical charts for more than 27,000 patients who were tested for COVID-19 at Michigan Medicine between March and mid-July of 2020. According to the data, the odds of testing positive for COVID-19 was reduced by 24% in those who were vaccinated against flu versus those who were not (odds ratio 0.76, 95% CI 0.68-0.86; P<.001). Additionally, vaccinated patients were also less likely to require hospitalization (odds ratio, 0.58, 95% CI 0.46-0.73; P<.001) or mechanical ventilation (odds ratio, 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).¹

However, the investigators did not find a significant difference in mortality between the 2 groups. The results also did not identify an underlying mechanism behind the association.

“It is possible that patients who receive their flu vaccine are also people who are practicing more social distancing and following CDC guidelines,” senior study author Marion Hofmann Bowman, MD, said.2 “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.”

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,” co-first author Anna Conlon, PhD, U-M Medical School student, said.²

The investigators urged for continued patient education and widespread promotion of the influenza vaccine to increase uptake.¹

Reference

1. Conlon A, Ashur C, Washer L, Eagle K, Hoffman Bowman MA. Impact of the influenza vaccine on COVID-19 infection rates and severity. American Journal of Infection Control. 2021. Doi: https://doi.org/10.1016/j.ajic.2021.02.01

2. Otman H. Flu Shot Associated With Fewer, Less Severe COVID Cases. University of Michigan Health. March 12, 2021. Accessed March 24, 2021. https://labblog.uofmhealth.org/lab-report/flu-shot-associated-fewer-less-severe-covid-cases