Asymptomatic cases have become the latest focus in China's continuing fight against the spread of the coronavirus, as fears grow over the emergence of such transmission in the past few days.
As travel restrictions ease around the country and people return to work, there are nagging worries over a second wave of infection. And pressure has been mounting for the authorities to be more transparent about these transmissions.
Yesterday, the National Health Commission said it would, from today, start including figures of asymptomatic cases in its daily report, in what it said was an effort to allay public concern.
It had previously excluded such cases in its official tally, confounding experts who point to a growing body of evidence that shows asymptomatic people are at risk of passing the virus to others.
"There is very good evidence that the majority of transmission events, even in China, arose from pre-or asymptomatic individuals," said Professor Babak Javid, an infectious disease expert at Beijing's Tsinghua University.
In one such study, researchers found that undetected cases, a majority of whom were mildly symptomatic or asymptomatic, were primarily responsible for the rapid spread of the coronavirus in China.
Based on computer modelling, the findings published in the journal Science show up to 86 per cent of infections in China had gone undetected in the weeks leading up to a lockdown in Wuhan, the epicentre of the outbreak, on Jan 23. These undocumented cases were half as contagious as the documented ones, yet were the source of two-thirds of documented infections.
A Wuhan-based virologist, Dr Yang Zhanqiu, told Global Times on Sunday that there may be as many as 200,000 silent carriers in China.
These people may have stronger immunity or carry a weaker virus, but can still pass it on in the early stage of infection, said Dr Yang.
Yesterday, the authorities said there are 1,541 asymptomatic people who have tested positive for the coronavirus and are under medical observation, the first time official numbers have been made public.
Of these, 205 were people who had entered the country.
200,000 Number of possible silent carriers in China, according to Wuhan-based virologist Yang Zhanqiu.
1,541 Number of people who have tested positive for the coronavirus but did not show symptoms; this is the first time official numbers have been made public.
205 Number of people from the asymptomatic group who entered the country.
There is very good evidence that the majority of transmission events, even in China, arose from pre-or asymptomatic individuals.
PROFESSOR BABAK JAVID, an infectious disease expert at Beijing's Tsinghua University.
China's tally stands at 81,518 infections and 3,305 deaths. The virus has sickened more than 800,000 people worldwide and killed more than 38,000.
The health commission's announcement comes after Premier Li Keqiang on Monday ordered health officials to scale up screening of symptom-free people and expand testing to close contacts of infected patients.
China has been putting asymptomatic cases under quarantine and will reclassify them as confirmed cases if they develop symptoms.
Local hospitals and clinics must now report asymptomatic cases within two hours, and all such cases will have to be centrally quarantined. Close contacts of these people will also have to be isolated.
Professor Javid said if asymptomatic cases were under lockdown, they would have limited opportunities to pass on their infection outside of their immediate household.
"So I don't think there is generally increased risk from asymptomatic individuals, per se."
On Sunday, Henan health officials sparked alarm after they said a female patient had caught the virus from a silent carrier. The woman had on March 21 shared a car ride and dined three times with a doctor friend, who was asymptomatic. Three days later, she developed symptoms. Two other doctors linked to this physician also tested positive for the virus despite displaying no symptoms; one of them had even finished a 14-day quarantine after returning from Wuhan.
Asymptomatic cases have also been reported in Dezhou in Shandong province, Guiyang in Guizhou, and Mianyang in Sichuan.
China's top respiratory disease expert Zhong Nanshan on Sunday said he did not think there were many such cases. "If we did, they would be spreading it to others and pushing the number higher. But now, it's actually going down."