Moderna said Monday it may expand the size of its COVID-19 vaccine trial for younger children, as both Pfizer and Moderna plan to seek authorization from the Food and Drug Administration to vaccinate some of the youngest Americans later this year.

Both drug makers based their COVID-19 vaccines on a new platform technology, mRNA, and there have been reports of rare cases of heart inflammation in people under 30 who were vaccinated with mRNA vaccines. So, health officials and FDA advisers say they hope expanding the size of the trials could help detect this and other possible rare side effects in younger children.

“It is our intent to expand the trial and we are actively discussing a proposal with the FDA. The objective is to enroll a larger safety database which increases the likelihood of detecting rarer events,” a spokesperson for Moderna said in a statement.

The FDA has said that in the past it has looked for up to some 3,000 trial participants before licensing other vaccines for children, and because of the use of mRNA, the FDA suggested in June that “an appropriate overall pediatric safety database would approach the upper end of this range.” 

Both Moderna and Pfizer have announced clinical trials of doses of their COVID-19 vaccine shots to be tested in children as young as 6 months old.

Moderna says it could be in position to seek FDA emergency authorization by the winter for these groups, which could allow its doses to be given to younger American children as early as next year. The drug maker announced last month that it had filed for emergency use authorization to vaccinate adolescents as young as 12. 

A spokesperson for Pfizer declined to comment on whether it also plans to expand the size of its pediatric vaccine studies, which plan to enroll up to 4,500 participants from the U.S. and a handful of European countries.

Pfizer, currently the only COVID-19 vaccine authorized in the U.S. for adolescents as young as 12,  expects “initial results” from its trials in children 5 through 11 years old by September. Results for other groups as young as six months old could come “in October or November,” perhaps enabling Pfizer to obtain emergency authorization to vaccinate all school-age children by the end of the year.

“We are projecting that we’ll have enough data by the end of the fall, early winter, as to whether or not these vaccines — which we totally anticipate they will — be safe and projected to be effective in children of all ages,” Dr. Anthony Fauci, President Biden’s chief medical adviser, told the Arkansas Democrat-Gazette last month.

The expanded trial was first reported by The New York Times.

A spokesperson for the FDA declined to comment, aside from saying that the regulator typically works with drug companies to ensure trials “are of adequate size to detect safety signals.”

While younger children generally suffer less severe cases of COVID-19 than adults, the CDC has warned that studies suggest rates of the disease’s spread through children “can be comparable, and in some settings higher, than in adults.” 

With federal and local health authorities urging schools to fully reopen for in-person teaching this fall, some officials have warned that the vast swaths of adults who remain unvaccinated could be susceptible to new surges when school returns.

“Best case, it’s a tough summer and we get more and more people vaccinated. And more realistic case is that the fall gets worse again, because people will be indoors more and kids will, hopefully, be back in school. And we have pockets of the country where there’s a lot of people unvaccinated,” Dr. Anne Schuchat, the CDC’s outgoing second-in-command, told the INFORMS Healthcare Conference 2021 last week.

“We need to really scale up vaccination big time,” Schuchat added later.