In a move that seemed out of reach just six months ago, the Food and Drug Administration approved Moderna’s influenza vaccine for older adults on Wednesday, the first licensed flu shot made using messenger RNA technology.

In February, Vinay Prasad, the FDA’s head of biologics at the time, overturned the recommendation of career staff and issued a rare “refusal-to-file” notice to the company, saying the agency wouldn’t review the vaccine based on the available data package. A week later — after Moderna disclosed the FDA’s decision as well as its own displeasure over the agency’s position — Prasad’s decision was reversed. He left the FDA at the end of April.

The vaccine will be marketed as mFlusiva. It is Moderna’s fourth vaccine to enter the U.S. market.

“Flu remains a significant public health challenge, and mFLUSIVA provides an important new option for America’s seniors. This approval also reflects the ongoing potential of our mRNA platform to help address important public health challenges through continued scientific innovation,” Moderna CEO Stéphane Bancel said in a statement.

Moderna said it expects to have some doses of the vaccine available for administration in advance of the coming flu season.

The FDA issued a standard approval for use of the vaccine in adults ages 50 to 64, and an accelerated approval for adults 65 years and older. In the case of the latter group, the company has agreed to do a Phase 4 (post-licensure) trial to gather more effectiveness data, looking in particular at how the vaccine compares to existing high-dose or adjuvanted vaccines recommended for that older age group.

While the U.S. flu vaccine market is already crowded, the Moderna product offers some short- and longer-term benefits that aren’t seen with most traditional influenza vaccines.

Most existing flu vaccines are produced in eggs; a portion is produced in cells. Both production methods are lengthy, requiring the World Health Organization committee that advises on the strains each year’s vaccine should target to do so months in advance. That long lead time can result in vaccines that aren’t well targeted to the circulating viruses. But mRNA vaccines can be made much quicker, which could theoretically allow for the updating of targeted strains closer to flu season.

That manufacturing speed would also be of benefit in the event of a flu pandemic. Having a licensed mRNA seasonal flu vaccine could speed up the licensure pathway for a pandemic flu vaccine, if the need arose, experts have said.

Prasad’s objection to reviewing the licensure application for the vaccine last winter stemmed from the fact that the company hadn’t used either an adjuvanted or high-dose vaccine as its comparator in the older age group; those types of vaccines are preferentially recommended for older adults in the U.S.

In his letter, Prasad noted that the FDA had raised the issue with Moderna before the trial began. The company released documents showing that while the issue had been raised, it had not been cast as a deal-breaker.

Moderna also argued that some of the 11 countries in which it ran its Phase 3 clinical trial do not use adjuvanted or high-dose vaccines in older adults.

At a meeting in June, an expert panel that advises the FDA on vaccines voted unanimously that the benefits of the vaccine outweighed any risks.

Data presented to the meeting of the Vaccines and Related Biological Products Advisory Committee showed that in adults ages 50 to 64, trial participants who received Moderna’s vaccine were 27% less likely to contract test-confirmed influenza as compared to those who received the standard-dose comparator. Though the safety profiles of both vaccines were similar, there were substantially more side effects reported in the mRNA vaccine arm. Most were mild and lasted one to two days.

The vaccine has been accepted for regulatory review in the European Union, Canada, and Australia, with submissions to additional countries planned for later this year.