“The study has been amended to give a third dose to everybody who’s less than five at least eight weeks after their last vaccination,” Dr Alejandra Gurtman, a Pfizer vaccine researcher said at a meeting of the US Centers for Disease Control and Prevention’s Advisory Committee on Immunization Practices (Acip).

She said the company aims to have data for the age group by the end of March or the beginning of April.

In December, Pfizer said it was changing the design of the trial because children between the ages of two and four who were given two 3 microgram doses of the vaccine did not have the same immune response that a larger dose of the vaccine generated in older children.

Gurtman also said the company was studying a third dose of its vaccine in children ages five to 11, six months after their second dose.

The Pfizer-BioNTech vaccine is authorised in the US for people age five and older.

Yesterday, Acip backed booster shots of the vaccine for ages 12 to 15.