• Pfizer said its coronavirus vaccine was 100% effective in preventing Covid-19 in children ages 12 to 15.
• Pfizer plans to submit the new vaccine data to the FDA "as soon as possible," CEO Albert Bourla said.
• Kids in the age group could be eligible for the vaccine before the new school year.

Pfizer said Wednesday its Covid-19 vaccine was 100% effective in a study of adolescents ages 12 to 15, encouraging results that could clear the shots for use in middle school students before school starts this fall.

Pfizer CEO Albert Bourla said the company plans to submit the new data on the vaccine, which is developed in partnership with German drugmaker BioNTech, to the Food and Drug Administration and other regulators "as soon as possible," with the hope that kids in the age group will be able to get vaccinated before the next school year.

"We share the urgency to expand the authorization of our vaccine to use in younger populations and are encouraged by the clinical trial data from adolescents between the ages of 12 and 15," Bourla said in a press release.

The trial enrolled 2,260 participants in the United States. There were 18 confirmed Covid-19 infections observed in the placebo group and no confirmed infections in the group that received the vaccine, the company said. That resulted in a vaccine efficacy of 100%, it said, adding that the shot was also well-tolerated, with side effects generally consistent with those seen in adults.

The company also said the vaccine elicited a "robust" antibody response in children, exceeding those in an earlier trial of 16- to 25-year-olds.

Vaccinating children is crucial to ending the pandemic, public health officials and infectious disease experts say. 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, experts say.

Children make up around 20% of the U.S. population, according to government data. Between 70% and 85% of the U.S. population needs to be vaccinated against Covid to achieve herd immunity, experts say, and some adults may refuse to get the shots.

Dr. Scott Gottlieb, a former commissioner at the FDA and a member of the Pfizer board, said he expects it will take the U.S. agency about a month to review the new data. If the FDA process goes well, the vaccine could be available for children as young as 12 by the fall, he told CNBC's "Squawk Box."

Isaac Bogoch, an infectious disease specialist who has sat on several data and safety monitoring boards, called the results "wonderful news," saying it is a "huge step forward" in protecting more people against the virus and making schools safer for kids.

"We're talking about improving the safety of youth activity like youth sports and youth art and youth extracurricular activities," he said.

The Pfizer-BioNTech vaccine has already been authorized for use in the U.S. in people 16 and older. Clinical trial studies testing the vaccine in kids, whose immune systems can respond differently than adults, still needed to be completed.

The White House's chief medical advisor, Dr. Anthony Fauci, told a House committee earlier this month that the U.S. could begin vaccinating older kids against Covid this fall while elementary-age children may start getting their shots early next year.

Moderna, which also has a vaccine authorized in the U.S., said on March 16 that it had begun testing its shot in children under age 12. Moderna in December began a study testing kids ages 12 to 17.

Johnson & Johnson plans to test its single-shot vaccine in infants and even in newborns, after testing it first in older children, according to The New York Times.

Pfizer began testing its vaccine in kids as young as 12 in October. It announced last week it started a clinical trial testing its vaccine on healthy 6-month to 11-year-old children.

For the first phase of that trial, the company will identify the preferred dosing level for three groups — ages 6 months to 2 years, 2 to 5, and from 5 through 11. The kids will begin by receiving a 10-microgram dose of the vaccine before progressively moving to higher doses, according to the company. Participants also have the option to take 3-microgram doses.

Bogoch said it is difficult to predict if the vaccine will work as well in young kids as it does in adolescents, though he added he was "optimistic."

Pfizer said Wednesday it plans to request an amendment to its current emergency use authorization with the FDA to include adolescents 12 to 15 years of age. All participants in the trial will continue to be monitored for an additional two years after their second dose, the company said.

Pfizer and BioNTech said they plan to submit the data for scientific peer review.

Disclosure: Scott Gottlieb is a CNBC contributor and is a member of the boards of Pfizer, genetic testing start-up Tempus and biotech company Illumina. Gottlieb also serves as co-chair of Norwegian Cruise Line Holdings′ and Royal Caribbean's "Healthy Sail Panel."