However they said the bad news was that human immunity may not last that long.

As many countries start to return to work after lockdowns imposed to slow the spread of COVID-19, world leaders are trying to work out when it is safe for children and students to get back to their studies.

London School of Hygiene and Tropical Medicine infectious disease modeller Dr Rosalind Eggo said she had seen some indications from research that children may not spread the novel coronavirus as much as adults.

"There are hints that children are less infectious but it is not certain," said Eggo, who sits on a panel that advises the British government about transmission of COVID-19 among children and within schools.

"We need more studies to really pin this down as it is so important," she told the science committee of the British parliament's upper house, the House of Lords.

Eggo said her research had shown there was a much lower level of symptomatic infection in those under 20 years-old - perhaps as little as 20 per cent of infections showing clinical symptoms.

"We think that children are less likely to get it so far but it is not certain," she said on Tuesday.

"We are very certain that children are less likely to have severe outcomes."

John Edmunds, a member of Britain's Scientific Advisory Group for Emergencies (SAGE), told the same science committee hearing it was striking how children did not seem to easily spread the novel coronavirus.

"It is unusual that children don't seem to play much of a role in transmission because for most respiratory viruses and bacteria they play a central role, but in this they don't seem to," said Edmunds, a professor at the London School of Hygiene and Tropical Medicine.

"There is only one documented outbreak associated with a school - which is amazing; you would normally expect most of the outbreaks to be associated with schools but yet in global literature there is only one documented study," Edmunds said, citing a study of a French secondary school. "It is pretty remarkable."

But he added there was potentially bad news - that human immunity to the novel coronavirus may not last long.

"Antibody responses decline over time from survivors of SARS so after a couple of years their antibodies have declined quite significantly," Edmunds said, referring to Severe Acute Respiratory Syndrome, which is also caused by a type of coronavirus.

"We can also see from other coronaviruses - the ones that cause coughs and colds - that individuals again do seem to not have particularly long-term immunity to many of those viruses and so allowing them to be infected later.

"So that's potentially bad news for us: that immunity may not last that long against this virus," he said.

Australian Associated Press