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Moderna said Tuesday that two shots of its coronavirus vaccine prevented 100 percent of cases in volunteers 12 to 17 years old in a late-stage clinical trial and that the Cambridge biotech will ask federal regulators in early June to authorize use of it in that age group.

If regulators grant the request, the vaccine would become the second available in the United States for adolescents. The Food and Drug Administration cleared a similar two-shot vaccine from Pfizer-BioNTech for those 12 to 15 on May 10, a major step in the country’s steady recovery from the pandemic.

Moderna’s vaccine, which was cleared for use in people 18 and older in December, was tested on 3,732 adolescent volunteers. Two-thirds received two doses of the vaccine four weeks apart, while one-third got injections of a placebo. Participants didn’t know what they received.

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No cases of COVID-19 were detected in anyone who had received the second dose of the vaccine at least 14 days earlier, while four cases were diagnosed in recipients of the placebo. That suggested vaccine efficacy of 100 percent, the company said. There were no significant side effects.

“We are encouraged that mRNA-1273 was highly effective at preventing COVID-19 in adolescents,” said Stephane Bancel, chief executive of Moderna, using the clinical trial designation for the vaccine. “We will submit these results to the US FDA and regulators globally in early June and request authorization.”

Moderna also said the vaccine prevented 93 percent of cases in volunteers who received only the first dose. The company used a secondary case definition from the Centers for Disease Control and Prevention for this part of the study, diagnosing someone as infected with COVID-19 if they had only one symptom and a positive nasal swab or saliva sample.

Overall, the results closely resemble those that Pfizer announced in late March for its two-dose vaccine, which, like Moderna’s, relies on messenger RNA technology. Pfizer’s trial included 2,260 participants, half of whom received the vaccine three weeks apart, and half of whom were injected with a placebo.

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Eighteen people in Pfizer’s placebo group developed COVID-19, while none of the people in the vaccinated group did, the company said.

Dr. Eric Rubin, an immunologist at the Harvard T.H. Chan School of Public Health and member of the FDA advisory committee that recommended clearing the Pfizer, Moderna, and Johnson & Johnson coronavirus vaccines, said Moderna’s announcement was “another piece of good news, both for adolescents and their families who can now be protected against disease.”

Nonetheless, Rubin, editor in chief of the New England Journal of Medicine, noted that the news came in “a press release, with all the downsides of a press release.”

It was hard to draw statistically significant conclusions based on four cases of COVID-19 in the placebo group versus none in the group that got the vaccine, he said. And he wanted to see the actual coronavirus case numbers that Moderna relied on to measure efficacy after only one dose.

Still, he said, he wasn’t surprised that Moderna’s data closely resembled Pfizer’s, given that they both use the same vaccine technology. “These vaccines seem both safe and effective,” he said.

More than 600,000 adolescents 12 to 15 years old in the United States received their first shot of the Pfizer vaccine in the week after it was cleared for public distribution, Dr. Rochelle Walensky, director of the CDC, said last Tuesday.

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Recipients included tens of thousands of youngsters from Massachusetts, according to data shared Friday by the state Department of Public Health. Overall, the state said, 16 percent of an estimated 322,219 people in the age group, or about 51,000 adolescents, had received a shot of the Pfizer vaccine as of May 18.

But the data suggested uneven uptake across the state. Northampton was among a handful of municipalities reporting that at least half of their children ages 12 to 15 received their first shot. But the percentage was in the single digits in many other places that have lower overall vaccination rates among all age groups.

Just 4 percent of young teens in New Bedford had received the first shot, for example, according to the health department. In Lawrence, it was 3 percent. A health department spokeswoman said the data will be updated later this week.

Some parents have been wary about getting adolescents vaccinated given that COVID-19 is far less common and milder in children and teenagers. From March 1 of last year through April 30 of this year, the CDC received reports of about 1.5 million cases in youngsters 11 to 17 across the country. As of May 10, the agency said, there were 127 coronavirus-associated deaths among children and adolescents between the ages of 12 and 17.

Dr. Rick Malley, an infectious diseases specialist at Boston Children’s Hospital, said children over the age of 10 are more likely to spread COVID-19 and have serious cases of the illness than younger kids who have less contact with people outside their families, so he supports vaccinating them.

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But he wants to take a more cautious approach about vaccinating the youngest children, particularly given the limited supply of vaccines in poorer countries.

“Maybe it’s better to use these vaccines in the elderly in other countries than to give it to 2-year-olds in this one,” he said.

Pfizer and its German biotech partner, BioNTech, and Moderna are both testing their vaccines in children from 6 months to 11 years old. Pfizer has said it hopes to seek emergency use authorization for that age group in September, which means the shots could be available later this fall.

The Pfizer and Moderna vaccines are the first deployed that rely on synthetic messenger RNA, an ingenious variation on the natural substance that directs cells to produce proteins.

While traditional vaccines carry a dead or weakened virus into the body to stimulate an immune response, mRNA vaccines use custom-made messenger molecules that tell cells to create a viral protein. The two vaccines instruct cells to create the distinctive spike protein on the coronavirus. Once that happens, the body’s immune system generates antibodies to fend off the disease if the recipient is exposed to the virus.





Jonathan Saltzman can be reached at jonathan.saltzman@globe.com.