• Pfizer-BioNTech's COVID-19 vaccine was highly effective in late stage trial of more than 2000 participants.
• Pfizer said that the vaccine was "well tolerated" in this age group.
• Pfizer's CEO said Wednesday that he hopes to get kids vaccines by September.
• See more stories on Insider's business page.

Pfizer-BioNTech's COVID-19 vaccine was highly effective at preventing COVID-19 infection in kids aged 12 to 15 in late-stage trial of more than 2000 participants, the company announced Wednesday.

"In participants aged 12-15 years old, [Pfizer's COVID-19 vaccine] demonstrated 100% efficacy and robust antibody responses, exceeding those reported in trial of vaccinated 16-25 year old participants in an earlier analysis and it was well tolerated," Pfizer said in a press release.

To calculate efficacy, the company compared the number of people who caught COVID-19 in the vaccinated group with the number of people who got infected with the virus in the placebo group.

A relatively small number of cases lead to the 100% efficacy – 18 people out of 1,129 in the placebo group caught COVID-19 compared to no-one of the vaccinated group of 1,123. The end point of the trial was COVID-19 infection generally, so we don't know the severity of disease that Pfizer's vaccine prevented. In adult trials, Pfizer looked at whether its vaccine prevented COVID-19 with symptoms to get the 95% topline results.

Pfizer said side effects were similar for 12 to 15 year olds as for those aged 16 to 25, but did not detail what they were. Generally, side effects of Pfizer's vaccine last for a couple of days and includes: fevers, headache, tiredness and pain, redness or swelling around the injection site.

As it stands, Pfizer's two-dose vaccine is authorized in 66 countries worldwide for those over 16 years old, including the US.

So far, more than 26.7 million adult Americans have been fully immunized with two doses of Pfizer's vaccine, according to the Centers for Disease Control and Prevention (CDC).

Pfizer plans to submit the data to the U.S. Food and Drug Administration (FDA) and the European Medicines Agency (EMA) in order to request for emergency authorization of the vaccine.

Albert Bourla, Pfizer's chief executive officer, said: "We plan to submit these data to FDA as a proposed amendment to our Emergency Use Authorization in the coming weeks and to other regulators around the world, with the hope of starting to vaccinate this age group before the start of the next school year."

Pfizer started trials in 5 to 12 year olds on March 28. The company said Wednesday that it plans to initiate trials in 2-5 year old next week.

Read more: COVID-19 Vaccine Tracker: AstraZeneca's shot proves safe and effective, and is headed to the FDA

Vaccines in kids

COVID-19 vaccines available in the US from Moderna, AstraZeneca-Oxford University and Johnson & Johnson are authorized for over 18 year olds only.

Moderna's announced March 16 that it has started testing its COVID-19 vaccine in kids aged 6 months to 12 years in the US and Canada.

AstraZeneca is testing its vaccine in kids aged 6 to 17, the company announced February 15.

Alex Gorsky, Johnson & Johnson chief executive officer said March 3 that the company was likely to have a Covid-19 vaccine available for children under 18 by September, per Washington Post.

Dr. Anthony Fauci, the US top infectious disease doctor, said in February getting children vaccinated is crucial to reaching herd immunity, per ProPublica.

Dr. Scott Gottlieb, the former head of the US Food and Drug Administration, told CNBC Wednesday, that the infection rates under 16s in Israel - where more than 50% of its population is fully immunized with Pfizer's shot - "dropped dramatically" once got adults vaccinated as infection rates in adults fell.

"The best way to protect children is to get adults vaccinated," he said.