WASHINGTON - The Pfizer-BioNTech COVID vaccine is safe and effective when given as 3 doses to children under 5 years, the companies said Monday, welcome news for parents of the only age group not yet eligible for immunization in most countries.

The announcement comes as the US Food and Drug Administration (FDA) is planning highly-anticipated meetings in the coming weeks to weigh authorization.

In a clinical trial, Pfizer-BioNTech evaluated 3 doses, given at 3 micrograms, and found the vaccine evoked a strong immune response in children aged from 6 months through 4 years.

Side effects were similar in the vaccine and placebo groups.

Vaccine efficacy was 80.3 percent, according to a preliminary estimate -- though this was based on very low numbers and may change significantly by the time of the final estimate.

"We are pleased that our formulation for the youngest children, which we carefully selected to be one-tenth of the dose strength for adults, was well tolerated and produced a strong immune response," said Pfizer CEO Albert Bourla in a statement.

"We look forward to soon completing our submissions to regulators globally with the hope of making this vaccine available to younger children as quickly as possible, subject to regulatory authorization," he added.

The FDA has tentatively scheduled 3 dates in June where experts will meet and likely decide whether to authorize the Pfizer COVID vaccine for under-fives and the Moderna vaccine for under-sixes, which is given as 2 shots of 25 micrograms.

• Pfizer eyes COVID vaccine for all variants before 2023

The agency was originally set to evaluate the Pfizer vaccine given as 2 doses in February, but data showed it did not provoke a strong enough immune response in children aged 2 to 4. The FDA then asked to see data for a third shot.

• Pfizer to seek US authorization for 3rd COVID shot in children

RESULTS WELCOMED

In the latest study, 1,678 children received a third dose at a time when omicron was the predominant variant.

An analysis of a subset of participants showed antibody levels were similar to 16- to 25-year-olds who were given the full strength vaccine at 2 doses.

No new adverse events were identified, and the majority of side effects were mild or moderate.

Three doses of the vaccine "appear to be very safe and highly effective in preventing not only severe disease, hospitalization, and death from COVID, but even symptomatic COVID at a time when omicron was the dominant variant," Celine Gounder, editor-at-large for public health at Kaiser Health News told AFP.

"However, we know that protection against SARS-CoV-2 infection and milder symptomatic disease wanes over time," added Gounder, an infectious disease specialist and epidemiologist.

"Pfizer is reporting follow-up data only out to 7 days after the third dose of vaccine. It's too early to say how the 3-dose series would perform out to several months or a year."

Jeremy Faust, of Brigham and Women's Hospital Department of Emergency Medicine, told AFP: "My first impression is very positive. These numbers are exactly the kinds of signals we wanted to see."

"I wish the 2-dose series had worked for Pfizer-BioNTech. It didn't. But the 3-dose series appears to have given these very young children the protection we want them to have," the doctor added.

If and when both Pfizer's and Moderna's vaccines are authorized, parents will have to consider whether they want their children to receive Moderna's two dose vaccine -- which will offer faster protection -- or Pfizer's 3 doses -- which will take longer to be effective but may ultimately be more protective.

As with adults, the first 2 doses of Pfizer's vaccine are given to under-fives three weeks apart, but then the third is given 2 months after the second.

Severe disease from COVID is very rare among under-fives but can occur, with 477 US deaths in this age group since the start of the pandemic, or about 0.1 percent of all deaths.

Children can also contract a rare post-viral condition called multisystem inflammatory syndrome in children (MIS-C), which has affected some 8,210 US children and killed 68.

Like adults, some children who get COVID may go on to develop long COVID, with new, ongoing or returning symptoms, including brain fog and fatigue.

© Agence France-Presse

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