Pfizer's mRNA flu vaccine outperformed the standard flu shot in a phase 3 clinical trial, according to results published Wednesday in Pfizer's journal. New England Journal of Medicine.
The vaccine uses the same messenger RNA technology like Pfizer's Covid shot. Ever since mRNA Covid vaccines were approved in 2020, experts have looked to influenza as one of the next logical uses of the technology. No country has approved an mRNA flu vaccine.
Dr. Buddy Creech, a pediatric infectious disease physician at Vanderbilt University Medical Center in Nashville, Tennessee, said Pfizer's mRNA flu vaccine could be a “game changer,” potentially providing a closer match to strains in any flu season. One of the main reasons flu shots aren't as effective as they could be is because they don't target the circulating virus.
Because a traditional flu shot takes months to make, Creech says, scientists must select the strains to include in the vaccine about six to eight months before the flu season begins. Developing an mRNA sample typically happens more quickly, which could allow those decisions to be made later in the year—and give scientists more flexibility to make a decision if the circulating strain changes.
“There remains a significant need for improved approaches to influenza prevention,” Creech said. The trial results, he said, are “proof” that the mRNA shot has an advantage over traditional flu vaccines.
Pfizer, however, could face an uphill battle to get the new vaccine approved, said Dr. Peter Hotez, dean of the National School of Tropical Medicine at Baylor College of Medicine in Texas and co-director of the Vaccine Development Center at Texas Children's Hospital.
Secretary of Health Robert F. Kennedy Jr., an anti-vaccine activist, downplayed the benefits of vaccines and called vaccines made with mRNA especially dangerous, name of mRNA Covid vaccine “The deadliest vaccine ever created.” Several states have passed laws banning mRNA vaccines.
In August, the Department of Health and Human Services nearly $500 million canceled in funding mRNA vaccine research.
In May, Moderna, which also makes the mRNA Covid vaccine, withdrew its application for approval for a combination mRNA flu and Covid shot after the Food and Drug Administration (FDA) withdrew its application. requested additional information.
“Despite all the benefits of mRNA, I am concerned that our current DHHS has little or no interest in the science,” Hotez said.
HHS did not immediately respond to a request for comment.
Dr. Kelly Lindert, Pfizer's vice president of clinical research and development, said in an email that the company is “discussing with health authorities the path to licensure of mRNA influenza vaccines.” She declined to comment on the timing of filing or approval.
The new Pfizer-funded study compared the company's mRNA shot with Fluzone, one of the most widely used seasonal flu vaccines in the United States. More than 18,000 adults, most of them in the U.S., were randomly assigned to receive one vaccine or another during the 2022-2023 flu season. Other test locations included South Africa and the Philippines.
The study found that the Pfizer shot reduced the incidence of influenza-like illness by 34.5% compared with Fluzone – 57 cases in the mRNA group versus 87 cases in the Fluzone group. Almost all infections were caused by two common strains of influenza A: H3N2 and H1N1. The Pfizer vaccine also protected against two strains of influenza B: the Victoria strain and the Yamagata strain. The Yamagata strain went out of circulation during the Covid pandemic.
Both flu shots caused side effects, including sore arms and fatigue, although they were more common in people who received the mRNA flu shot compared with the traditional vaccine. A small percentage — 5.6% — of people who received the mRNA vaccine reported a fever, compared with 1.7% in the group who received the traditional flu shot. Most side effects were mild or moderate and resolved within 1 to 2 days.
Creech said it was not surprising that the mRNA vaccine caused more side effects, given that it produced a stronger immune response.
“In some ways, it’s like training for a race,” Creech said. “We may experience some muscle soreness or fatigue after a training run, but by experiencing a little discomfort we can avoid becoming too tired or sore during the race.”
Lindert said it is likely that people are reporting more side effects due to a “cascade of events associated with the activation of the immune response.”
“The relationship between side effects and immune responses is a source of ongoing debate in the scientific community,” Lindert said.
Moderna is also still working on a separate mRNA flu vaccine.
In June, the company reported results from a late-stage study in older adults that found its mRNA flu shot was about 27% more effective than GSK's approved flu shot. The company said people who received the shot experienced side effects similar to those seen with other flu vaccines.
Creech said that “not all mRNA vaccines are created equal,” adding that it is not surprising that the Pfizer and Moderna vaccines may have different efficacy levels and side effect profiles.
In an emailed statement, a Moderna spokesman said the company plans to file for approval of its standalone flu vaccine by January 2026.
