A nurse, Cindy Mendez, wearing a protective mask, holds a syringe containing a dose of Pfizer-BioNTech COVID-19 vaccine during the coronavirus disease (COVID-19) pandemic at NYC Health + Hospitals Harlem Hospital in the Manhattan neighborhood of New York City. New York, February 25, 2021.

Jeenah Moon | Reuters

Pfizer said on Wednesday that its Covid-19 vaccine was 100% effective in a study in adolescents ages 12-15.

Albert Bourla, CEO of Pfizer, said the company hoped to submit the new data on the vaccine, which was being developed in partnership with German drug maker BioNTech, “as soon as possible” to the Food and Drug Administration and other regulatory agencies that children in the age group can get vaccinated before the next school year.

“We share the urgency to expand our vaccine approval to younger populations and are encouraged by clinical trial data from adolescents 12-15 years of age,” Bourla said in a press release.

The study enrolled 2,260 participants in the United States. 18 confirmed Covid-19 infections were observed in the placebo group and no confirmed infections were observed in the group that received the vaccine, the company said. This resulted in a vaccine effectiveness of 100%. The shot is also well tolerated, with side effects generally the same as in adults.

The company also said the vaccine produced a “robust” antibody response in children that outperformed that in a previous study of 16-25 year olds.

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

According to the government, children make up around 20% of the US population. According to experts, between 70% and 85% of the US population must be vaccinated against Covid to achieve herd immunity and some adults may refuse to get the shots.

Dr. Scott Gottlieb, former FDA commissioner and board member of Pfizer, reckons it will take the US agency about a month to review the new data. If the FDA process goes well, the vaccine could be available to children between the ages of 12 and 15 by the fall, he told CNBC’s Squawk Box.

Isaac Bogoch, an infectious disease specialist who served on various data and security monitoring bodies, described the results as “wonderful news”. This is a “big step forward” in protecting more people from the virus and making schools safer for children.

“We are talking about improving the safety of youth activities such as youth sports and art, as well as after-school activities for youth,” he said.

Pfizer’s vaccine has already been approved for use in the United States in people aged 16 and over. Clinical studies testing the vaccine in children whose immune systems may react differently than adults had yet to be completed.

The Chief Medical Officer of the White House, Dr. Anthony Fauci, speaking to a House committee earlier this month, said the U.S. could vaccinate older children against Covid-19 starting this fall, while elementary school-age children may get their shots early next year.

Moderna, which also has a US-approved vaccine, announced on March 16 that it has started testing its shot in children under the age of 12. Moderna started a study in December testing children ages 12-17.

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

Pfizer began testing its vaccine in children aged 12 years old in October. It announced last week that it had started a clinical trial testing its Covid-19 vaccine on healthy 6-month-old to 11-year-old children.

In the first phase of this study, the company will determine the preferred dosage level for three age groups – between 6 months and 2 years, 2 and 5 years, and between 5 and 11 years. The children will initially receive a dose of 10 micrograms of the vaccine before gradually moving to higher doses, according to the company. Participants also have the option of ingesting 3 micrograms doses.

Bogoch said it is difficult to predict whether the vaccine will work as well in young children as it does in teenagers, despite being “optimistic”.

Pfizer said Wednesday it plans to apply to the FDA for an amendment to its current emergency approval to include adolescents ages 12-15. All participants in the study will be monitored for two more years after their second dose, the company said.

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

Disclosure: Scott Gottlieb is a CNBC employee and a member of the boards of directors of Pfizer, the genetic testing startup Tempus, and the biotech company Illumina. Gottlieb is also co-chair of Norwegian Cruise Line Holdings and Royal Caribbean’s Healthy Sail Panel.