Moderna said Thursday its Covid-19 vaccine is 96 percent effective among youths aged 12 to 17, according to the results of its first clinical trials.

Two-thirds of the 3,235 participants in trials in the United States received the vaccine and one-third were given a placebo.

The study "showed vaccine efficacy against COVID-19 of 96%; mRNA-1273 was generally well tolerated with no serious safety concerns identified to date," the company said.

Tests detected 12 cases of coronavirus 14 days after the first shot. For these intermediate results, participants were followed up on average 35 days after the second injection.

The pharmaceutical company said that any side effects had been "mild or moderate in severity," most commonly pain at the injection site. With the second shot, side effects included "headache, fatigue, myalgia and chills," similar to those observed in adults who had received the vaccine.

"No serious safety concerns have been identified to date," it said.

Moderna said it is currently "in discussions with regulators about a potential amendment to its regulatory filings" to authorize the vaccine for this age group. It is currently only certified for people aged 18 and over in countries where it has already been approved.

Pfizer and BioNTech have already applied for authorization of their own vaccine for 12-15 year olds in the United States and Europe. On Wednesday, Canada became the first country to authorize the Pfizer shot for this age group.

The vaccination of teens is the next step in the campaign to eventually contain the epidemic. Moderna also began trials of its vaccine in children aged six months to 11 years in March.

Pfizer and BioNTech announced on Tuesday that they hope to file an emergency authorization request for their vaccine for children aged two to 11 in September in the United States.

Pfizer CEO Albert Bourla said the company could apply for authorization to inoculate children aged between from 6 months and two years "in the fourth quarter."