In the new study, Mihai Netea, an infectious disease immunologist at Radboud University Medical Center in the Netherlands, and his colleagues combed through their hospital’s databases to see if employees who got a flu shot during the 2019–2020 season were more or less likely to get infected by SARS-CoV-2, the virus behind COVID-19. Workers who received a flu vaccine, the researchers found, were 39 percent less likely to test positive for the coronavirus as of June 1, 2020. While 2.23 percent of nonvaccinated employees tested positive, only 1.33 percent of vaccinated ones did. Netea and his team posted their findings on the preprint server MedRxiv on October 16.

These findings do not prove that flu vaccines prevent COVID-19, however. “This is an intriguing study, but it doesn’t provide definitive evidence,” says Ellen Foxman, an immunobiologist and clinical pathologist at the Yale School of Medicine. There could be other explanations for the association the Radboud scientists and their colleagues found. For instance, people who choose to receive a flu shot may be more health-conscious and more likely to follow COVID-19 prevention guidelines than individuals who do not get vaccinated. Netea agrees, noting that overall behavior, rather than the shot, might have made people in the former group less likely to get sick in his study.