Children can spread coronavirus among themselves efficiently, but young adults are the primary source of coronavirus spread, according to a study published Wednesday.

The study, based on a giant contact tracing effort involving more than 3 million people in India, shows most COVID-19 patients never infected anyone else. The researchers found that 70 per cent of infected people did not infect any of their contacts, while 8 per cent of patients accounted for 60 per cent of observed new infections.

The study also contradicts the widely held belief that children are unlikely to catch the coronavirus.

"We find otherwise. They are getting infected in significant numbers," study leader Ramanan Laxminarayan of the Center for Disease Dynamics, Economics and Policy in New Delhi, and also of Princeton University, told CNN.

"While the role of children in transmission has been debated, we identify high prevalence of infection among children who were contacts of cases around their own age," the team wrote in their report, published in the journal Science.

And that's even though schools have been closed in India since March, Laxminarayan noted.

The study involved the universal contact tracing efforts undertaken in two large states in the south of India. Authorities tracked down and tested more than 575,000 people exposed to nearly 85,000 confirmed coronavirus cases from March until August. "This is the largest contact tracing study in the world, and by a long shot," Laxminarayan said.

"We were surprised to find that just 8 per cent of the infected primary cases were responsible for 60 per cent of the contacts that were infected," he said. "That's a hugely disproportionate effect. Superspreading has been suspected, but not really documented." Most of the index cases -- the first patient in a chain of transmission -- were adults aged 20 to 45, the team found.

"The young to middle adult age group is the one that is coming into contact with people. They are the people most likely to be outside the household. They are the ones taking the disease from one place to the other," Laxminarayan said.

The study also showed lockdowns worked. Restrictions lowered the rate of transmission, the team found. Case fatality ratios, one measure of the death rate, also fell over time. "Those who tested positive in May and June were 13 per cent less likely to die than those tested in March and April," Laxminarayan said.