U.S. regulators on Monday authorized the emergency use of Pfizer Inc.'s coronavirus vaccine for adolescents as young as 12 years old, lowering the minimum age from 16.

The move will help protect junior and senior high school students from COVID-19, the respiratory illness caused by the novel coronavirus, as the administration of President Joe Biden continues its mass vaccination campaign to bring an end to the pandemic.

The regimen of two doses administered three weeks apart of the vaccine developed by the U.S. pharmaceutical giant and its German partner BioNTech SE will be retained for the 12-15 age group, according to the U.S. Food and Drug Administration.

Safety data has been collected through a clinical trial in the United States that involved more than 2,000 participants aged between 12 and 15.

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Among participants without evidence of prior infection with the virus, no cases of COVID-19 occurred among 1,005 vaccine recipients and 16 infection cases occurred among 978 individuals who received dummy shots.

"The vaccine was 100 percent effective in preventing COVID-19," the FDA said in its press release, adding that the known and potential benefits of the vaccine in individuals 12 years of age and older outweigh the known and potential risks.

The most commonly reported side effects in the clinical trial participants, typically lasting one to three days, were pain at the injection site, tiredness, headache, chills, muscle pain, fever and joint pain. With the exception of injection site pain, more adolescents reported the side effects after the second dose than after the first dose, the FDA said.

From March 1, 2020, through April 30, 2021, approximately 1.5 million cases in individuals 11 to 17 years of age have been reported to the Centers for Disease Control and Prevention. Children and adolescents generally have a milder COVID-19 disease course as compared to adults, according to the FDA.

The Pfizer-BioNTech vaccine was initially granted an emergency use authorization in December for administration in individuals 16 years of age and older.

Pfizer and BioNTech have also proceeded with a study targeting children 6 months to 11 years of age. They are expected to seek an authorization for administering children aged 2 to 11 as early as September.

In the United States, 34.8 percent of the total population, or about 116 million people, have been fully vaccinated. Around half of them received the Pfizer-BioNTech vaccine, according to data provided by the CDC.