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The U.S. Food and Drug Administration is asking pharmaceutical companies to expand the COVID-19 vaccine trials for 5 to 12-year-olds to see whether a rare side effect of heart inflammation is likely to occur in younger children who get the shot.

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The request from federal regulators is aimed at getting enough research to determine if rare side effects that have been seen in some in their teens, 20s and 30s who had gotten an mRNA vaccine would be seen in younger children..

The makers of two of the vaccines — Moderna and Pfizer-BioNTech — are conducting trials on shots for 5 to 12-year-olds and both have said they are aiming to finish those trials by early fall.

The FDA’s request could delay those timelines. The agency has asked that both companies have at least 3,000 children in the 5-to-11-year-old trial group. Someone with knowledge of the request, ganted anonymity to speak freely by The New York Times, described that figure as double the original number of study participants, the Times reported.

Ray Jordan, a spokesman for Moderna, told The Washington Post that the company was “actively discussing a proposal” with federal regulators to expand its trial which currently has in it nearly 7,000 children ages 6 months to 12 years.

He said the company has not settled on the size of the expanded trial yet. Jordan predicted authorization could occur in “winter 2021/early 2022.”

“The objective is to enroll a larger safety database which increases the likelihood of detecting rarer events,” Jordan said in an email to the Post.

According to Pfizer spokeswoman Jerica Pitts, Pfizer has not made any changes to its trial program for youngsters.

The company had earlier said it expected to have results of its vaccine trials for children ages 5 to 11 by September. Trial results for children ages 2 to 5, were expected to follow that release within weeks.

According to the Post, “a federal official, who spoke on the condition of anonymity because they were not authorized to speak publicly, predicted that authorization of a coronavirus vaccine for children 5 through 11 might come by late October or early November.”

Rare cases of myocarditis and pericarditis, which is inflammation of the heart or the tissue around the heart, have been seen after both mRNA vaccines. Health officials have said the incidents of inflammation — around 1,200 cases out of 300 million shots according to the Post — are likely caused by the vaccine.

Most people who develop the heart inflammation conditions have now recovered completely.