Your flu shot may protect against severe COVID, study finds — but it’s not clear why

Getting a flu shot may come with an added benefit: protection against severe coronavirus infection.

That’s the conclusion researchers made from a review of nearly 75 million COVID-19 patient records from around the globe, which found those who received a flu vaccine up to six months before getting infected were less likely to have a stroke, experience sepsis or develop blood clots caused by the disease.

Coronavirus patients vaccinated against influenza were also less likely to visit the emergency department and be admitted to the intensive care unit, according to new research presented online during the European Congress of Clinical Microbiology & Infectious Diseases.

The data showed that COVID-19 patients who did not get their flu shots were more likely to be admitted to the ICU (up to 20%), visit the emergency department (up to 58%), have a stroke (up to 58%), develop sepsis (up to 45%) and develop blood clots (up to 40%).

The risk of death, however, was not reduced among those who were recently vaccinated against the flu.

The reason behind the association remains unclear. Some scientists speculate the flu shot boosts the immune system’s general defenses used to fight any foreign invader.

University of Miami researchers say their findings, which align with other studies on the topic, suggest flu shots can be used to offer protection against severe disease in countries where coronavirus vaccines are in short supply.

Flu shots could also benefit people who are hesitant to get COVID-19 vaccines, the team says. Not to mention they will prevent thousands of serious influenza cases while “conserving scarce medical resources for the care of people with COVID-19,” according to the Centers for Disease Control and Prevention.

“Only a small fraction of the world has been fully vaccinated against COVID-19 to date and, with all the devastation that has occurred due to the pandemic, the global community still needs to find solutions to reduce morbidity and mortality,” study senior author Dr. Devinder Singh, a professor of plastic surgery at the University of Miami Miller School of Medicine, said in a statement. “This finding is particularly significant because the pandemic is straining resources in many parts of the world. Therefore, our research — if validated by prospective randomized clinical trials — has the potential to reduce the worldwide burden of disease.”

Why may flu shots protect against severe COVID-19?

In the absence of clinical trials testing the association between influenza vaccination and COVID-19 severity, scientists speculate flu shots could trigger general immune responses that can help fight any illness, including the coronavirus.

Evidence of this kind of reaction has been seen with a specific tuberculosis vaccine, which appears effective against yellow fever and malaria.

The idea is that flu shots boost activity of natural killer cells that work to control coronavirus infections by limiting their spread in the body, while other parts of the immune system work on destroying viral particles.

“If the natural killer cell levels increase because of vaccination, they could presumably mount an attack on infections other than influenza,” Dr. Ming-Jim Yang, author of a similar study published in October and a resident in family medicine at the University of Florida Health, said in a statement.

Other studies have shown similar connections

The Miami researchers analyzed medical records of two groups of 37,377 COVID-19 patients — one of those which received the flu shot and the other of those which did not — from several countries, including Germany, Italy, Israel, Singapore, the U.S. and the U.K.

The groups were matched according to their age, gender, ethnicity, smoking activity and health conditions that could affect their risk of severe COVID-19.

After comparing the frequency of 15 different health outcomes between the groups, including heart attack, pneumonia, hospital admission and respiratory failure, the team learned those who had been vaccinated against the flu faced much better odds during their bout with COVID-19.

Other studies have shown similar connections.

A paper published February in the American Journal of Infection Control found that people jabbed with influenza vaccines were 24% less likely to get infected with the coronavirus than unvaccinated people. They also had reduced chances of needing hospitalization (58%) and mechanical ventilation (45%), as well as shorter hospital stays (76%).

This study also found no “significant differences” for risk of coronavirus-related death among those who received a flu shot.

A separate study published in October showed people who did not get a flu vaccine within the year were 2.4 times more likely to be hospitalized with COVID-19 and 3.3 times more likely to be admitted to the ICU.