New York, March 1 (IANS): Pfizer's Covid vaccine for children aged 5 to 11 has proved to be significantly less effective than in older adolescents or adults against the infectious virus, according to a study.

The yet to be peer-reviewed study, led by The New York State Department of Health, found that during the Omicron era, the effectiveness of the Pfizer-BioNTech Covid vaccine declined rapidly -- from 68 per cent to 12 per cent for children aged 5-11 years.

Vaccine efficacy also declined from 66 per cent to 51 per cent for those between 12 and 17 years.

Although vaccination was found protective against severe disease the efficacy against hospitalisation also declined from 85 per cent to 73 per cent for children aged between 12 and 17 years, and from 100 per cent to 48 per cent for those aged between 5 and 11 years.

The difference in effectiveness may be explained by "lower vaccine dose", said the researchers.

Compared to children 12-17 years, who received two 30g doses,children 5-11 years, received two 10g doses.

The study examined Pfizer vaccine effectiveness from December 13, 2021 to January 30, 2022, among 852,384 fully-vaccinated children 12-17 years and 365,502 children 5-11 years.

Among children 12-17, protection declined substantially, albeit more slowly than observed among younger children.

Within just two weeks of full vaccination the efficacy for children aged 5-11 dropped to 65 per cent and by 28-34 days it was 12 per cent

For children between 12-17 years, effectiveness declined from 76 per cent to 56 per cent by 28-34 days.

"Our data support vaccine protection against severe disease among children 5-11 years, but suggest rapid loss of protection against infection, in the Omicron variant era," said Eli S. Rosenberg, from University at Albany School of Public Health, State University of New York, in the paper.

"These results highlight the potential need to study alternative vaccine dosing for children and the continued importance of layered protections, including mask wearing, to prevent infection and transmission," she added.

The study comes as several states and the US Centers for Disease Control and Prevention have eased the mask restrictions.