The Food and Drug Administration on Wednesday approved the U.S.’s first flu shot that uses mRNA technology: mFlusiva, from drugmaker Moderna.

The FDA granted approval for two age groups: adults 50 to 65 and adults 65 and up, on the condition that Moderna run an additional clinical trial in the older group, the company said in a release. In a late-stage trial, mFlusiva was found to be about 27% more effective than a standard flu shot.

Many scientists and public health experts have touted the idea of an mRNA-based flu vaccine, which uses the same messenger RNA platform as the Covid vaccines from Moderna and Pfizer. That’s because mRNA vaccines can be manufactured much faster than traditional vaccines, allowing scientists to better match circulating influenza strains.

Moderna said it takes two to three months from picking the strain to rolling out its flu shot, compared with about six months for traditional flu shots.

“If you’re choosing the strain half a year before, how good can we be at sharpshooting?” said Dr. Ofer Levy, director of the Precision Vaccines Program at Boston Children’s Hospital.

The flu puts hundreds of thousands of people in the hospital every year, according to the Infectious Diseases Society of America, and there can be tens of thousands of deaths. Young children and older adults are more likely to get very sick from the flu.

The traditional flu shot ranges in effectiveness from 22% to 56%, according to the Mayo Clinic. “We wish it were higher, quite frankly,” Levy said.

Even though mFlusiva outperformed the traditional flu shot in a late-stage clinical trial, its approval was not a sure thing.

Health and Human Services Secretary Robert F. Kennedy Jr. — an anti-vaccine activist — has repeatedly criticized mRNA technology, calling the Covid vaccines “the deadliest vaccine ever made.” Last year, HHS under Kennedy canceled nearly $500 million in funding for mRNA vaccine research.

The FDA initially refused this year to review Moderna’s application before it reversed course a week later. Agency officials said they wanted more data because Moderna had compared its vaccine with a standard flu shot in adults 65 and older, even though the Centers for Disease Control and Prevention recommends a high-dose flu vaccine for that age group. Moderna has said the FDA previously signed off on the trial design.

In June, the FDA’s independent vaccine advisory committee unanimously recommended approving the vaccine.

That decision was based on a late-stage clinical trial of more than 40,000 older adults across 11 countries, including the U.S. Rates of side effects — such as injection-site pain, fatigue and headaches — were more common in the mFlusiva group but still mild to moderate.

A combination flu and Covid vaccine from Moderna, which also uses mRNA, was approved by the European Union in April. Moderna withdrew its application for U.S. approval for that shot last year despite positive late-stage trial results.

Moderna said mFlusiva will be available for this fall’s flu season.

Vaccines, however, are typically formally recommended by the CDC’s vaccine advisory committee — called the Advisory Committee on Immunization Practices (ACIP) — before they are used. But a federal judge has blocked the committee from convening.

Levy said mFlusiva could still be legally prescribed in the U.S. without ACIP’s sign-off.

However, he said, patients most likely would not be able to get insurance coverage, as private insurers generally cover only ACIP-recommended shots, and the public distribution of the vaccine would be severely limited with no official federal guidance on who the shots should go to. Federal programs, such as Vaccines for Children and Medicaid, also rely on ACIP’s guidance.

“That is a serious concern,” he said.