If you are infected with COVID-19, having had a flu shot makes it less likely you will suffer severe body-wide infection, blood clots, have a stroke or be treated in an intensive care unit, according to the study.
"Our work is important," said study co-author Dr. Devinder Singh, noting limited resources around the world continue to constrain access to the COVID vaccine.
"The global population may benefit from influenza vaccination, as it can dually act to prevent a coronavirus and influenza 'twindemic,' which could potentially overwhelm health care resources," said Singh, chief of plastic surgery at the University of Miami Miller School of Medicine.
Why a flu shot would protect against some severe effects of COVID-19 isn't clear, but it's possible that it primes the immune system to reduce the odds of some system-wide harms also seen with flu, the researchers say.
They caution, however, that the flu vaccine is not a substitute for the COVID-19 vaccine. Also, the study can't prove that a flu shot is protective when it comes to COVID-19, only that it might be.
Dr. Eric Cioe-Pena is director of global health at Northwell Health, New Hyde Park, N.Y., and was not part of the study. "While the study shows a clear association between those who get their flu shot and lower morbidities of COVID infection, we must be clear that this study does not show causation and does not even suggest a clear causal link on how flu vaccination would help with COVID," he said.
"Regardless, I fully support the flu vaccine and COVID vaccine as prudent public health measures, and if this happens to be a secondary benefit, great," Cioe-Pena said.
For the study, Singh and his colleagues used the TriNetX research database to collect data on two groups, each with more than 37,000 patients.
People in the first group got a flu shot two weeks to six months before being diagnosed with COVID-19. Those in the second group also had COVID-19, but had not been vaccinated against flu.
The researchers compared the incidence of severe effects between the two groups, looking at sepsis, stroke, a blot clot known as deep vein thrombosis (DVT), pulmonary embolism, respiratory failure, respiratory distress syndrome and joint pain. They also assessed rates of kidney failure, loss of appetite, heart attack, pneumonia, emergency department visits, hospital admission, ICU admission and death.
They found that people who had not had the flu shot were up to 20% more likely to be admitted to the ICU, up to 58% more likely to visit an emergency room, and up to 45% more likely to develop sepsis. They were also as much as 58% more likely to have a stroke, and up to 53% more likely to have a DVT. No effect on the risk of death was seen.
The findings were presented Sunday at the virtual annual meeting of the European Congress of Clinical Microbiology and Infectious Diseases. Research presented at medical meetings is usually considered preliminary until published in a peer-reviewed journal.
Dr. Marc Siegel is a professor of medicine at NYU Langone Medical Center in New York City, and was not part of the study. "It's possible that the flu shot primes the immune system in a way that undercuts the dysfunctional inflammatory response of COVID that causes blood clotting and other serious problems," he said.
But Siegel pointed out that you can't rely on a flu shot to protect you from COVID-19. He urged everyone to get the COVID-19 vaccine. "It's the greatest vaccine ever invented. It's amazing, amazing," he said.
For more on COVID-19 vaccination, see the U.S. Centers for Disease Control and Prevention.