Pfizer-BioNTech asked for authorization Friday to use their Covid-19 vaccine on 12 to 15-year-olds in the United States, which could mark a crucial next step toward achieving herd immunity.

The mass vaccination of teenagers would also relieve an enormous strain on parents who are juggling the demands of homeschooling their children while keeping up with jobs.

The companies said in a statement that they plan to make similar requests of other regulatory authorities worldwide in coming days.

Their request to the US Food and Drug Administration comes after Phase 3 clinical trials of the Pfizer vaccine in 12 to 15-year-olds showed it was 100 percent effective in warding off the disease, according to the companies.

In late March they published the results of trials carried out with 2,260 adolescents in the US whom the companies said showed "robust antibody responses."

The vaccine was "well tolerated with side effects generally consistent with those observed in participants 16 to 25 years of age," the companies said Friday.

For now the vaccine has emergency authorization for use in people aged 16 and over.

Children are less likely to develop severe Covid so vaccinating them has been less of a priority than getting older people inoculated.

But they do make up a large part of the community that will need to be inoculated in order to achieve herd immunity, when the proportion of people with antibodies largely prevents the spread of the virus.

Experts do not know precisely what percentage of the population needs to be vaccinated to get to that point, but one of the leading US immunologists, Anthony Fauci, has put it anywhere between 70 and 85 percent.

In February, he said that US children aged 12 or under could very likely be vaccinated by the start of 2022.

The BioNTech/Pfizer shot is based on new mRNA technology and was the first Covid-19 vaccine to be approved in the West late last year.

In March, the US biotech company Moderna said it had started trials on children aged between six months and 11 years. Johnson & Johnson, whose vaccine was the third approved for US use, has also started trials on the 12 to 17 age group.