US biotech firm Novavax said Monday it had started clinical trials of its proposed Covid-19 vaccine on children, in a program that will involve up to 3,000 adolescents aged 12-17.

Far fewer children have been sick with Covid-19 compared to adults, and most have mild to no symptoms, but they can be infected and spread the virus.

Novavax said the trials would test "the efficacy (and) safety" of the vaccine, with participants receiving either the vaccine candidate or placebo in two doses, 21 days apart.

Participants will be monitored for up to two years after their injections.

The Novavax vaccine has not yet been authorized in any country, including for adults, but the company plans to file for emergency authorization in Britain "in the second quarter of 2021", followed by in the United States.

Novavax announced at the end of January that clinical trials conducted in Britain involving 15,000 adults showed 89.3 percent efficacy.

The Novavax vaccine, which uses different technology from the doses already widely licensed around the world, is a protein-based vaccine engineered from the genetic sequence of the first strain of the coronavirus.

It can be stored at a temperature between 2 and 8 degrees Celsius (35 - 46.4 degrees Fahrenheit).

Other vaccine companies including Moderna, Johnson & Johnson and Pfizer are also conducting trials in adolescents.

Europe's medicines watchdog said Monday it had begun evaluating the use of Pfizer/BioNTech's vaccine for 12- to 15-year-olds, after a similar request in the United States.

Vaccine authorization for children is seen as a crucial step towards achieving herd immunity.