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May 25, 2021, 8:21 a.m. EDT

Moderna's COVID vaccine safe in adolescents

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By Peter Loftus

Moderna Inc. said its Covid-19 vaccine was effective in children aged 12 to 17 in a new study, a finding that could clear the way for a second shot for use in adolescents.

The Cambridge, Mass., company said the vaccine induced immune responses among children that were comparable to those seen in a study of adults last year.

Also, there were no cases of symptomatic Covid-19 among vaccine recipients, Moderna said, suggesting 100% vaccine efficacy in adolescents, though overall very few among the 3,700 children in the study got sick.

Based on the results, Moderna said it plans in early June to request that regulators in the U.S. and other countries authorize the use of its vaccine in children aged 12 to 17 years.

The U.S. Food and Drug Administration could make a decision within weeks of the request if it follows the same kind of timetable if took with Pfizer Inc.'s shot. The agency took about a month to clear Pfizer's request for use of its Covid-19 vaccine in adolescents.

U.S. regulatory clearance would add a second choice in Covid-19 vaccines for adolescents. Immunizing children is crucial, health specialists say, to developing the communitywide immunity needed to move fully past pandemic precautions.

In December, the Covid-19 vaccine from Pfizer and its partner BioNTech SE was cleared for 16- and 17-year-olds, along with adults. Earlier this month, the FDA authorized the Pfizer shot for use in children aged 12 to 15 years.

More than 5 million adolescents aged 12 to 17 years have received at least one dose of a Covid-19 vaccine so far, according to the Centers for Disease Control and Prevention Director Rochelle Walensky.

The FDA authorized use of Moderna's shot in people 18 years and older in December.

Expanded use of Moderna's shot could bolster plans to hold summer camps for children and ease the return of full-time, in-person instruction in middle and high schools in the fall.

"We are encouraged that mRNA-1273 was highly effective at preventing Covid-19 in adolescents," Moderna Chief Executive Stéphane Bancel said, referring to the vaccine's code name.

Moderna and Pfizer have also begun testing their vaccines in younger children. Results are expected later in the year.

Children generally have a lower risk of severe Covid-19 symptoms than older adults, but some have been hospitalized and a small number have died from it.

Moderna started its Phase 2/3 study in adolescents in December. About two-thirds of the children in the study were given two doses of its vaccine, four weeks apart, while the rest received a placebo.

About four weeks after the second dose, researchers took blood samples from study subjects and measured the levels of neutralizing antibodies, which are components of the immune system that can block the coronavirus from replicating.

The antibody levels among vaccinated adolescents were comparable with those from adults who had participated in a separate Phase 3 study of the vaccine in adults. The adult study had shown that the vaccine was 94.1% effective at preventing symptomatic Covid-19.

There were four cases of Covid-19 among adolescents who received the placebo, versus zero cases among those getting the vaccine, starting 14 days after the second dose, Moderna said.

The vaccine was 93% effective after the first dose, using a different set of criteria that included milder cases, Moderna said.

The company said the most common side effects include injection-site pain, headache and fatigue.

Write to Peter Loftus at peter.loftus@wsj.com

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