Flu Vaccination Associated With Fewer COVID-19 Cases, Less Severe Symptoms

Patients who received a flu shot during the last flu season were significantly less likely to test positive for a COVID-19 infection at the start of the pandemic, according to a study published in the American Journal of Infection Control. Additionally, those who tested positive for COVID-19 had fewer complications if they had been vaccinated for the flu.

Researchers reviewed medical charts for more than 27,000 patients who were tested for a COVID-19 infection at Michigan Medicine between March and July 2020. Of the nearly 13,000 individuals who got a flu shot in the previous year, 4% tested positive for COVID-19. Among the 14,000 who hadn't received a flu shot, nearly 5% tested positive for COVID-19. The association remained significant after controlling for other variables including ethnicity, race, gender, age, body mass index, smoking status, and many comorbid conditions.

The study also found that those who had received the flu vaccine were less likely to require hospitalization if they did test positive for COVID-19, although there was no significant difference in mortality between the two groups. The mechanism behind this correlation is currently unclear.

"It is possible that patients who receive their flu vaccine are also people who are practicing more social distancing and following CDC guidelines. 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," said Marion Hofmann Bowman, MD, the study’s senior author, in a press release.

Prospective longitudinal studies to examine the effect of the flu vaccine on respiratory illness are ongoing, including the Household Influenza Vaccine Evaluation study through the University of Michigan's School of Public Health.

REFERENCE

Flu shot associated with fewer, less severe COVID cases [news release]. EurekAlert; March 23, 2021. Accessed March 31, 2021. https://www.eurekalert.org/pub_releases/2021-03/mm-u-fsa032321.php