As the US government plans to soon roll out Covid-19 vaccines for children under 12, the US Food and Drug Administration (FDA) has requested vaccine makers Pfizer and Moderna to expand their trial size on coronavirus shots to include several thousand school-aged children before seeking authorisation, media reports said.

The move will help identify any potential safety signals.

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The increase in trial size will also help assess whether a rare inflammation of the heart muscle, seen in young adults shortly after vaccination, is common in younger age groups, the Washington Post reported.

US President Joe Biden, at a CNN town hall meeting last week, said that children under 12 might have access to a vaccine “soon”.

Currently, none of the three Covid-19 vaccines used in the US is available to children under the age of 12. Pfizer, Moderna, and Johnson and Johnson are still conducting clinical trials to evaluate the safety and efficacy of the vaccines in children under 12.

While results of Moderna and Pfizer vaccines for children aged 5 to 11 were expected in September, these changes may delay the availability of the vaccines, the report said.

Authorisation of a Covid vaccine in children aged 5-11 may come by late October or early November, the Post quoted a federal official, who spoke on the condition of anonymity.

The regulatory decision to increase the trial size means recruiting and vaccinating many more children. The government believes that since so many parents are eager to get their children vaccinated, enrolling more children will not be a problem, the official added.

Meanwhile, Moderna has said that the company was “actively discussing a proposal” with federal regulators to expand its trial, but had not settled on final numbers yet, Ray Jordan, a spokesman for the US pharma firm was quoted as saying.

He also predicted that the authorisation may happen in “winter 2021/early 2022”.

On the other hand, Pfizer hasn’t yet made any updates to its trial design or its timeline, according to Jerica Pitts, a spokeswoman for Pfizer.

The company had originally planned to report results in September for children ages 5 to 11, with results for children ages 2 to 5 expected to follow shortly and for children as young as 6 months in October or November.

The FDA is also likely to require two months of follow-up data for the 5-to-11 age group, as it did for adults and adolescents. For children under 5, the agency might require four to six months of follow-up data, meaning vaccines are unlikely to be available for several months for infants and young children, the Post said.