The U.S. Food and Drug Administration on Friday authorized booster doses of COVID-19 vaccines for all adults six months after having been fully vaccinated with the shots from either Pfizer Inc (PFE.N) and partner BioNTech SE or Moderna Inc (MRNA.O).

The agency's decision authorizes booster doses for all U.S. adults. It had previously allowed the additional shot for all recipients of Johnson & Johnson's (JNJ.N) vaccine.

The move paves the way for millions of Americans to get additional protection and is also aimed at fighting the Delta variant of the coronavirus, which has driven breakthrough infections among the fully vaccinated who can then transmit the virus.

After about two months of declining infections, the United States has reported daily increases for the past two weeks, driven by the Delta variant and people spending more time indoors due to colder weather. The nation's top infectious disease expert Anthony Fauci said this week boosters can help reduce that spread. 

The FDA said its decision was supported by data showing that a third round of shots increased the immune response to the virus in studies of both the Moderna and the Pfizer/BioNTech vaccines.

About 31 million Americans have already received boosters, which had been approved for people in several categories in the United States. Some states in recent days had opened them to all adults ahead of FDA authorization, creating a patchwork of eligibility. 

"Streamlining the eligibility criteria and making booster doses available to all individuals 18 years of age and older will also help to eliminate confusion about who may receive a booster dose," FDA official Peter Marks said in a statement.

The Biden Administration first proposed boosters for everyone in August, but has made them available in stages as health experts argued there was not enough scientific data to support the need for further vaccination in all groups.

Dr. Paul Offit, an infectious disease expert at the University of Pennsylvania, said on Friday that boosting will restore protection against and mild and moderate illness but that he is concerned about the risk of rare cases of heart inflammation in young men. The original round of shots appear to still offer protection against severe disease and death.

"Is giving a third dose to people who are already vaccinated better than doubling down our efforts to get people who were unvaccinated vaccinated?" Offit said.

Final regulatory review moves to a meeting of an advisory committee to the U.S. Centers for Disease Control and Prevention (CDC) later on Friday, and then a recommendation from the agency's director, who has publicly supported boosters for all.

Moderna shares were trading more than 3% higher, while Pfizer was up slightly.

Pfizer reported data from a large clinical study that showed a booster dose of its vaccine was 95.6% effective against the coronavirus when compared with a vaccinated group that did not get the third shot.

Israeli data showed that administering Pfizer/BioNTech booster shots widely slowed virus transmission.

BOOSTERS FOR ALL

The FDA did not have similar datasets to consider for the Moderna booster. Dr. Walid Gellad, a professor at University of Pittsburgh's medical school, said he was surprised it was authorizing Moderna's booster shots broadly with so little data, particularly for heart inflammation.

There has been "a ground swell of interest in boosters-for-all," said Dr. William Schaffner, a leading infectious disease expert from the Vanderbilt University School of Medicine.

Booster doses of messenger RNA (mRNA) vaccines from Pfizer/BioNTech or Moderna have been available for people who are immunocompromised, those aged 65 and above, and for individuals at high-risk of severe disease or who are regularly exposed to the virus through work or living conditions.

Anecdotal reports suggest that Americans have been largely able to get boosters, whether or not they are eligible.

Nearly 60% of adult Americans - some 195.6 million people - are considered fully vaccinated, having received two doses of either the Pfizer/BioNTech or Moderna vaccines, or one shot of J&J's vaccine.

If the CDC signs off in the expansion, about 130 million people in the United States would be eligible to receive the additional shots. Around 32.5 million people have received a booster shot so far, according to CDC data.