NEW YORK, May 23 — Drugmakers Pfizer Inc and BioNTech SE said today that three doses of their Covid-19 vaccine generated a strong immune response in children under age 5 and was safe and well-tolerated in their clinical trial.

The companies said they plan to soon ask global regulators to authorise the shot for the age group, children for whom no vaccine is currently approved in most of the world. They said they expect to complete their submission of data to the US Food and Drug Administration (FDA) this week.

The clinical trial involved giving 1,678 children ages 6 months to under 5 years smaller doses of the vaccine than given to older children and adults. Pfizer and BioNTech said that three shots of a 3 microgram formulation of their vaccine generated a similar immune response in that age group as in 16 to 25-year-olds who had received two doses of the 30 microgram formulation of the vaccine in an earlier clinical trial.

"The study suggests that a low 3-microgram dose of our vaccine, carefully selected based on tolerability data, provides young children with a high level of protection against the recent Covid-19 strains," BioNTech Chief Executive Ugur Sahin said in a statement.

Rival vaccine maker Moderna Inc released trial data in March showing that its vaccine was safe and generated a similar immune response in young children as for adults.

New York-based Pfizer Inc and Germany-based BioNTech said that an early analysis of 10 symptomatic Covid-19 cases identified through April 29, while the Omicron variant was dominant, suggested a vaccine efficacy of 80.3 per cent in the under-5 age group. This analysis was not conclusive, as the trial's protocol specified that efficacy should be calculated based on at least 21 cases.

Pfizer and BioNTech said final efficacy data in the age group will be made public when it is available.

The drugmakers had previously tested two doses of the 3 microgram vaccine in the children. But the two-dose trial failed to meet its primary endpoint after results for children ages 2 to 4 showed a weaker immune response than in adults.

The trial participants received their third dose at least two months after their second shot. Pfizer said the vaccine was well tolerated, with most adverse side effects being mild or moderate.

The Pfizer/BioNTech shot is currently authorised for use in all Americans ages 5 and older. Children ages 5 to 11 receive two 10 microgram doses as their primary course, while people aged 12 and older receive two 30 microgram doses.

It was not clear how many American parents will vaccinate their children in the age group. Only 29% of American children ages 5 to 11 are fully vaccinated, and Covid-19 is generally more mild in children than adults.

US regulators are already preparing to review the data in younger children as well as data submitted by Moderna for children under age 6. The FDA said today that a committee of its external advisors will meet on June 15 in order to consider the use of the two vaccines in the youngest children.

Moderna has said two 25 microgram shots of its vaccine were around 37% effective in preventing infections in children ages 2 to 5 and 51 per cent effective for children ages 6 months to 2 years. — Reuters