Covid and flu shots can be safely given at the same time, and according to a small new study, doing so may even confer benefits.
Findings presented Monday at the Vaccines Summit Boston, an annual scientific conference, suggest that giving Covid and flu shots together could produce a stronger antibody response against the coronavirus than administering the vaccines separately.
The study measured the antibody levels of 42 health care workers in Massachusetts who were vaccinated last fall. Twelve of the participants received a bivalent Covid booster and seasonal influenza shot on the same day. Another 30 received the shots on different days within the span of a month.
Three to four weeks after people got their Covid boosters, those who had received the flu shot at the same time had higher levels of IgG1, an antibody involved in the body's front-line defense against Covid. The results held true six months later, suggesting that co-administering the vaccines could improve long-term protection.
“We showed that the Covid antibody responses were higher and more durable if the Covid and flu vaccines were given on the same day,” said Susanna Barouch, the study’s lead author and an intern at the Ragon Institute's Systems Serology Lab in Cambridge, Massachusetts.
The research is undergoing peer review and has not been published in a scientific journal. Ryan McNamara, the lab's director and senior author on the study, said the findings still need to be replicated in a larger group of volunteers, but he felt it was useful to share the results right away. In September, the findings were posted to the preprint server BioRxiv.
"We thought that these findings were very important for immediate public health decision making," McNamara said.
He expects the results to apply to this year’s updated Covid vaccines, he added, though his team plans to explore that further.
The Centers for Disease Control and Prevention recommends getting Covid and flu vaccines at the same appointment if people are eligible for both shots at once. But that recommendation is based on convenience rather than efficacy, said Dr. Judith O’Donnell, chief of infectious diseases at Penn Presbyterian Medical Center in Philadelphia, who wasn’t part of the study.
“We probably would want to see this kind of data replicated using the current 2023 vaccines to really know with 100% certainty how giving those together might enhance immunity or not,” she said.
How much of an immune response is too much?
McNamara said it's still unclear why getting Covid and flu shots together might result in stronger, more lasting protection.
One theory, he said, is that most people's bodies are already primed to produce Covid antibodies because they've been vaccinated or exposed to the virus before. Giving two vaccines at once could awaken this immune response to a greater extent than one shot alone, which may lead to more a robust antibody response, he said.
"Your immune system is a highly tuned army and it’s basically calling all the special forces at the same time," said Dr. Michelle Barron, senior medical director of infection prevention and control at UCHealth in Colorado, who was not involved in the research.
But that strong immune response could also induce greater side effects, she added. In a 2022 study, getting a Covid booster and flu shot together was associated with an 8% to 11% higher rate of self-reported side effects like fatigue, headache and muscle pain in the week after vaccination compared to getting a Covid booster alone.
An October analysis from the Food and Drug Administration, which has not been peer-reviewed, also identified a slightly elevated risk of stroke among people ages 85 and up who had received a high-dose flu shot at the same time as a bivalent Covid vaccine.
Dr. Scott Roberts, an infectious disease specialist at Yale Medicine, said it's possible that "we're starting to see the immune system revved up so much that we're seeing these inflammatory conditions like stroke in this select age group."
But Barron noted that many older people have underlying health conditions that independently increase their risk of stroke, and several other studies have not identified the same safety concern with co-administering Covid and flu shots.
"The benefit of getting the shots and knowing that you have that protection will outweigh any of these potential risks," she said.
Experts say to get vaccinated now
The CDC recommends getting a flu shot by the end of October. As of Monday, however, less than 33% of children and less than 35% of adults in the U.S. had gotten their flu shots, according to CDC data.
Uptake of Covid shots is even lower this year: 5% of children and 14% of adults have received the updated Covid vaccine.
O’Donnell said it’s reasonable for people to want to space out their shots to lower the risk of side effects. While the CDC does not recommend a certain waiting period between vaccines, O’Donnell suggested for people who prefer to space them out, separating the shots by a couple weeks.
But infectious disease experts widely agreed that co-administration may be the best option at this point, since the vaccines take two weeks to be fully effective and both Covid and flu are spreading in the U.S.
Covid hospital admissions fell 8% nationally the week ending Nov. 4, but they’re rising in many individual counties. Flu-related hospital admissions also rose 20% in that time, while the number of positive flu tests rose 50%.
"We've been seeing increases in Covid and now we're seeing increases in flu," Barron said. "There’s really no value in saying, ‘Oh, I’ll wait another week,’ because that’s another week where, whichever vaccine you didn’t get, you’re now potentially vulnerable."