• Moderna said Tuesday its Covid-19 vaccine was 100% effective in a study of adolescents ages 12 to 17.
• The company said it plans to ask the FDA to expand the emergency use of its Covid vaccine for teens early next month.
• It is the second vaccine behind Pfizer's to demonstrate high efficacy in younger age groups.

Moderna said Tuesday its Covid-19 vaccine was 100% effective in a study of adolescents ages 12 to 17, making it the second shot behind Pfizer's to demonstrate high efficacy in younger age groups.

The company said it plans to ask the Food and Drug Administration to expand the emergency use of its Covid vaccine for teens early next month. If approved, it would likely dramatically expand the number of shots available to middle and high school students ahead of the next school year. Pfizer and German partner BioNTech were cleared to use their vaccine for 12- to 15-year-olds earlier this month.

"We are encouraged that mRNA-1273 was highly effective at preventing COVID-19 in adolescents," Moderna CEO Stephane Bancel said in a press release. "We remain committed to doing our part to help end the COVID-19 pandemic."

The two-dose vaccine, which is given four weeks apart, is already authorized for adults.

The phase 2/3 study the company is citing Tuesday included more than 3,700 adolescents. No cases of Covid were observed in participants who received two doses of the vaccine, while four cases were observed in the placebo group, according to the company.

No significant safety concerns have been identified, and side effects generally are consistent with those seen in an earlier trial of adults, the company said. The most common side effects after the second dose were headache, fatigue, muscle pain and chills, Moderna said.

The new data comes less than three weeks after the company disclosed in an earnings report that early data showed the shot was 96% effective at protecting against Covid in teens ages 12 to 17. That data was based on those who received at least one dose of the vaccine.

The company said Tuesday the shot was shown in the trial to be 93% effective after one dose. Because children are less likely to get seriously ill, Moderna used the Centers for Disease Control and Prevention's definition of Covid-19 to calculate that figure. It requires only one symptom and a positive Covid test.

U.S. regulators are expected to grant Moderna's request for use in teens. The approval process could take about a month, just in time for some summer activities and fall classes if Moderna submits the data by early June. Pfizer and BioNTech requested expanded use of their shot in adolescents on April 9, for example, and were authorized by the FDA on May 10.

Vaccinating children is seen as crucial to ending the pandemic. The nation is unlikely to achieve herd immunity — when enough people in a given community have antibodies against a specific disease — until children can get vaccinated, health officials and experts say.

Children make up around 20% of the total U.S. population, according to government data. Some 70% to 85% of the U.S. population needs to be vaccinated against Covid to achieve herd immunity, medical experts say, and some adults may refuse to get the shots. Though more experts now say herd immunity is looking increasingly unlikely as variants spread.

Vaccinating kids may also hasten the return of in-person learning and greenlight after-school extracurricular activities such as sports, art and other in-person activities, health experts say.