(NEXSTAR) – At the urging of the Food and Drug Administration, Pfizer and Moderna will be expanding their COVID-19 vaccine trials in children ages 5 to 11 as a precautionary measure after rare side effects were detected in some people under the age of 30, the New York Times reports.

While most young people have experienced the same side effects as adults – a sore arm, fatigue or headache – in very rare cases patients developed myocarditis, or a swelling of the heart muscle, and pericarditis, a swelling of the heart’s lining.

The Centers for Disease Control and Prevention said in June that most cases were in male adolescents and young adults who were 16 or older. Most patients recovered quickly after receiving treatment.

Citing how rare the reports have been and the risk of serious symptoms from COVID-19, the CDC said it would continue to recommend the vaccine for anyone over the age of 12.

According to the New York Times, the FDA has requested that Moderna and Pfizer add 3,000 children between the ages of 5 and 11 to their ongoing studies of that age group, for whom results were expected to come first.

The studies of children under the age of 12 are more complex: Teens receive the same dose as adults, but researchers are testing smaller doses in younger children. The first results, expected in September, are from the 5 to 11 age group.

The precautionary measure of adding thousands of participants to the trials would increase the database and potentially allow researchers and public health officials to gain a better understanding of very rare side effects, but experts warn it could also delay the authorization of the vaccine’s use.

In the United States, children represent about 14% of the nation’s total coronavirus cases to date. And while the young are far less likely than the old to get seriously ill, at least 344 children have died of COVID-19 in the U.S. alone, according to a tally by the American Academy of Pediatrics.

So far in the U.S., just under half the population is fully vaccinated — with the highest rates, not surprisingly, among older adults. Just a quarter of 12- to 15-year-olds, who got access to Pfizer’s vaccine starting in May, have had their second dose, according to the Centers for Disease Control and Prevention. Among those 16 and 17, about 37% are fully vaccinated.

The Associated Press contributed to this report.