US manufacturer Moderna on Tuesday said it has started Covid-19 vaccine trials for children aged from 6 months to under 12 years old, with plans to enroll 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 CEO Stephane Bancel in a statement.

"This pediatric 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 to adults, but they can be infected and can 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 United States have received its vaccine, as the country seeks to step up its innoculation program against the coronavirus pandemic that has killed over 535,000 people in America.

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

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