NEW YORK (REUTERS) - Children aged five to 11 who received the Pfizer-BioNTech Covid-19 vaccine were 68 per cent less likely to be hospitalised during the Omicron wave in the United States than unvaccinated children, according to a study published on Wednesday (March 30).

Adolescents aged 12 to 18 who received two shots of the vaccine were around 40 per cent less likely to be hospitalised with the Omicron variant of the virus, the study led by scientists from the US Centres for Disease Control and Prevention (CDC) and Boston Children's Hospital found. It was published in the New England Journal Of Medicine.

The risk of more serious outcomes, including the need for mechanical breathing assistance or death, was nearly 80 per cent lower for those who received the shots in that age group.

"Infections like Covid-19 and respiratory infections have a range of severity," said CDC researcher Manish Patel. "But action against severe disease can still be maintained."

The study looked at vaccinated and unvaccinated patients with and without Covid-19 at 31 hospitals in 23 states.

The vaccine performed better against the Delta variant of the coronavirus, which was circulating last year. Vaccine effectiveness against hospitalisation in adolescents when that variant was dominant was around 93 per cent, the study found.

The results of the CDC study were better for the younger age group than a study that New York state researchers published in February.

That study found that the vaccine was around 48 per cent effective in keeping children out of the hospital, with 73 per cent efficacy against hospitalisation among adolescents in January this year.

"Most kids that have critical illness were unvaccinated," Dr Patel said. "And we should be able to prevent that with a simple act of vaccination."