As a result, Moderna (NSDQ:MRNA) and Pfizer (NYSE:PFE) have agreed to expand clinical trials of their vaccines in children aged five to 11 before seeking emergency use authorization.

In April, Israel first reported suspected cases of heart inflammation in recipients of the COVID-19 vaccine from Pfizer and its partner BioNTech.

Since then, FDA has substantiated the risk of myocarditis, or heart inflammation, from mRNA vaccines, which is rare and tends to be minor.

According to The New York Times, federal regulators concluded that the proposed clinical trials to study the Pfizer and Moderna vaccines in school-aged children were insufficiently powered to detect rare side effects, including heart inflammation.

[Related: AstraZeneca, Pfizer Moderna vaccines fare well against Beta, Gamma and Delta variants in study]

Pfizer (NYSE:PFE) and BioNTech (NASDAQ: BNTX) announced in June that it was expanding its COVID-19 vaccine trial to include up to 4,500 children. The companies announced they would test a 10-µg dose in children aged 5 to 11 and administer a 3-µg dose to those aged six months to five years old.

Moderna had initially aimed to enroll 6,795 participants in its trial that would focus on children 11 or younger.

As Moderna and Pfizer take steps to expand the clinical trials, the uncertainty grows about the near-term availability of COVID-19 vaccines for children aged five to 11.

The changes to ongoing pediatric trials could potentially delay the availability of COVID-19 vaccines in that age group.

The Washington Post, citing an anonymous official, reported that U.S. regulators could authorize the Pfizer and Moderna vaccines in October or November.

Moderna expects authorization for the 5–11-year-old demographic in winter 2021 or early 2022.