WASHINGTON (AFP) - United States manufacturer Moderna on Tuesday (March 16) said it has started Covid-19 vaccine trials for children aged from six months to under 12 years old, with plans to enrol about 6,750 participants.

"We are pleased to begin this Phase 2/3 study of mRNA-1273 in healthy children in the US and Canada," said chief executive Stephane Bancel in a statement.

"This paediatric study will help us assess the potential safety and immunogenicity of our Covid-19 vaccine candidate in this important younger age population."

US health authorities say that fewer children have been sick with Covid-19 compared with adults, but they can be infected and spread the virus.

Most infected children have mild symptoms or no symptoms at all.

School officials across the US are under pressure to fully reopen as soon as possible, but many say they need portable classrooms or shorter school days to meet social distancing rules.

Moderna said 17.8 million adults in the US have received its vaccine, as the country seeks to step up its inoculation programme against the coronavirus pandemic that has killed over 535,000 people in America.

The Pfizer, Moderna and Johnson & Johnson vaccines have all been authorised for emergency use, and the companies are set to deliver more than enough to cover the entire US adult population by mid-year.

The US is currently vaccinating around 2.2 million people per day, while almost 65 per cent of Americans 65 and older have had at least their first shot.