Explainer: Are asymptomatic Covid-19 patients safe or silent carriers?

China said 300 asymptomatic carriers of the novel coronavirus in Wuhan, the epicentre of the pandemic, had not been found to be infectious, in a bid to reassure people as countries around the world ease restrictions.

But some experts say asymptomatic infections are common, presenting a huge challenge in the control of the disease.

What is asymptomatic and pre-symptomatic?

The World Health Organisation (WHO) defines asymptomatic carriers as those who don’t show symptoms but have been confirmed infected through a lab test.

The incubation period, or the time a person takes to show symptoms after getting infected, is the pre-symptomatic phase, the WHO says. Carriers can infect others during this period.

Health experts are not yet sure whether asymptomatic or pre-symptomatic cases are infectious. Some say that data so far suggests those cases are probably equally likely to be able to spread infection.

The WHO agrees that pre-symptomatic carriers are infectious, and adds that there is also a possibility — although little evidence so far — that people who are asymptomatic may also transmit the virus.

The WHO had said in early April that there had been no documented asymptomatic transmissions.

What else does China say?

China has reported around 83,000 confirmed cases of Covid-19.

It does not include asymptomatic cases in that total official count, but began reporting them separately on a daily basis on March 31.

That has raised concerns about Beijing’s commitment to transparency and some experts say it could also paint a misleading picture of how the virus spreads.

“If you watch [such asymptomatic cases] really closely, you would see something that probably fits with a more realistic mild disease than a completely asymptomatic disease,” Ian Mackay, a virologist at the University of Queensland said.

“But the term is around now and it’s going to stick. It’s a nice piece of theatre, but I don’t think it’s going to give us useful information,” he added.

Asymptomatic cases under medical observation in China dropped to 357 as of Tuesday.

Wuhan has tested almost its entire population of 11 million and found no new Covid-19 cases.

Wuhan’s low rate of symptomless carriers is in line with China’s previous reporting, said Zhong Nanshan, the government’s senior medical adviser, adding that the result showed that the country didn’t cover up the epidemic as some US politicians claimed.

How about the rest of Asia?

Some countries in Asia include asymptomatic carriers in their total confirmed cases.

In Vietnam, which has just over 300 Covid-19 cases, almost 37 per cent were symptomless, according to health ministry data.

Researchers concluded that asymptomatic infection was common and found two asymptomatic patients had infected at least four other people.

South Korea, which had early success in taming the outbreak through aggressive testing, said 20pc to 30pc of its confirmed cases were asymptomatic.

A senior health official in South Korea said that the virus could be widely transmitted during the incubation period, but asymptomatic patients were less likely to transmit it.

Singapore, which has the highest number of cases in Southeast Asia, does not give data on asymptomatic cases but has said that an overwhelming majority of positive cases in its crowded migrant workers’ dormitories show mild or no symptoms.

The Philippines said about 13pc of its nearly 19,000 cases were asymptomatic.

In India, some 28pc of 40,184 people who tested positive between January 22 and April 30 were asymptomatic, according to a study.