BERLIN (BLOOMBERG) - Children with the new coronavirus may be as infectious as adults, according to a study from Germany that recommends caution against an unlimited reopening of schools and kindergartens.

While children have a lower risk of developing severe illness from Covid-19, they may be no less capable of spreading it. Levels of virus in the respiratory tract - the main route via which the pathogen is transmitted - don't appear significantly different across age groups, according to Christian Drosten, director of the Institute of Virology at Berlin's Charite hospital, and his colleagues.

Children contract the coronavirus less often and with less severity, and there don't appear to be cases of a child passing Covid-19 to an adult - observations that may be "misunderstood as an indication of children being less infectious", they said.

"All we really know at this point is that with a small number of exceptions, children are mildly affected by this infection," said Adam Finn, a professor of pediatrics at the University of Bristol and chairman of the World Health Organisation's European Technical Advisory Group of Experts on Immunisation.

"What is much less clear is how often they get infection and how infectious they are to each other and to other people in their families."

The WHO says more research is needed on the topic. For now, household transmission studies indicate that children are less likely to transmit Covid-19 to adults than the reverse, WHO epidemiologist Maria Van Kerkhove told reporters in Geneva on Wednesday (April 29).

The most detailed paediatric data on Covid-19 from China showed 13 per cent of confirmed cases had no symptoms, and when confirmed and suspected cases were combined, almost a third of children aged six to 10 years were asymptomatic.

It's possible that because children typically get milder cases of Covid-19, they are less likely to spread the virus via coughing and sneezing, said Mike Ryan, head of the WHO's health emergencies programme.

In this uncertain context, "we have to caution against an unlimited reopening of schools and kindergartens in the present situation, with a widely susceptible population and the necessity to keep transmission rates low", Drosten and other study authors said.

While scientists have speculated about why few children get severely ill from Covid-19, no studies have explained the exact mechanism of this protective effect.