With husband Stephen by her side, Erin Shih hugs her children Avery 6 and Aidan 11 after receiving their second Moderna COVID-19 vaccines at Kaiser Permanente Los Angeles Medical Center on Friday, June 25, 2021.

Sarah Reingewirtz | MediaNews Group | Getty Images

Moderna plans to expand the scope of its clinical trial testing its Covid-19 vaccine in children ages 5-11, the company confirmed to CNBC on Monday.

The U.S. drugmaker is expanding the study, which began in late March, to increase the likelihood of discovering potential rare side effects, the company said, declining to state how many children it will eventually take in. The Food and Drug Administration added a warning label to the Pfizer and Moderna Covid-19 vaccines last month to list a rare risk of heart inflammation that has been reported as a potentially rare side effect in young people.

“It is our intention to expand the study and we are actively discussing a proposal with the FDA,” the company said in a written statement to CNBC. “At this point in time, we expect a package that will support approval in winter 2021 / early 2022, should the FDA use the approval route.”

The New York Times reported Monday that the FDA had asked both Moderna and Pfizer to include 3,000 children in the 5 to 11-year studies, citing unnamed sources. A source described this as twice as many study participants, according to the Times.

In a statement to CNBC, Pfizer said it did not provide any updates to the aforementioned schedules or details for its study.

The update comes as parents in the United States wait patiently for their children to be vaccinated. In May, the FDA approved the use of the Pfizer-BioNTech Covid vaccine for children ages 12-15. Moderna’s vaccine is expected to be approved every day for children aged 12 and over.

Vaccinating children is seen as critical to ending the pandemic. The nation is unlikely to achieve herd immunity – if enough people in a given community have antibodies to a given disease – until children can be vaccinated, scientists say.

The federal health authorities must weigh the risk of potentially rare side effects of the shots against the risks of Covid infection.

In June there were more than 1,200 cases of myocarditis or pericarditis, according to health officials, mostly in those under the age of 30 who received the injections. Myocarditis is the inflammation of the heart muscle and pericarditis is the inflammation of the tissues surrounding the heart.

There were only 12.6 cases of heart infections per million doses for both vaccines combined, officials said at the time. They added that the benefits still outweigh the risks.