China’s exclusion of people without symptoms from its official count of confirmed coronavirus cases is renewing concerns over whether its outbreak has truly come under control.
Local authorities in Wuhan _ the city where the virus first emerged _ defended their practice of not counting such cases late Monday night, after Caixin reported that the city was still finding asymptomatic cases daily. China reported four domestic cases of the virus on Monday, the first time in five days that it’s discovered infections that haven’t come from overseas.
South Korea, Japan and Singapore are among the countries that count all positive tests in their official tallies of cases, regardless of whether or not the patients display symptoms such as cough, fever or sore throats. Countries with limited testing capacity are more likely to not track asymptomatic cases as tests are reserved for those with obvious symptoms.
The Wuhan Municipal Health Commission said in the Monday night statement that only a small number of asymptomatic people with the virus develop symptoms and most are “self-cured.” But the concern for health experts is that the people carrying the virus who never develop symptoms could be spreading the disease without anyone knowing.
The likely presence of undisclosed asymptomatic infections in China also threatens to undercut the government’s narrative that the epidemic has come under control after two months of harsh containment measures.
On Tuesday, Hubei province announced that it would allow the resumption of transportation in Wuhan on April 8, essentially lifting the mass quarantine that’s been in place since Jan. 23 in a show of confidence that the virus threat has been neutralized.
“It makes no sense to me that a person is virus positive but, unless they are sick, are not counted,” said Nigel McMillan, a professor of infectious diseases and immunology at Griffith University. “Covid-19 clearly has a number of new features and one is the ability to be infectious in people without symptoms.”
Such asymptomatic infections have been found in many countries now that the virus has spread to nearly every part of the world, killing more than 16,500 people and sickening over 382,000. Experts are still trying to figure out the extent to which these infected people have helped spread the virus, especially in countries where only the very ill are tested.
As many as a third of the people in China who test positive show delayed symptoms or none at all, the South China Morning Post reported Sunday, citing classified data from the Chinese government. Asymptomatic cases have been found among people who have been in close contact with those infected, in cluster infections and through contact tracing, according to the Wuhan statement.
Chinese officials on Tuesday attempted to allay public fears about the potential for asymptomatic carriers to reignite the outbreak.
“Asymptomatic people won’t cause the infection to spread because in China all close contacts are quarantined in isolated units,” Wu Zunyou, a researcher with the Chinese Center for Disease Control and Prevention, told reporters. “We are on high alert for this risk and closely monitoring it in our containment measures.”
Scott Gottlieb, the former head of the U.S. Food and Drug Administration, said in a Twitter post earlier this month said that asymptomatic or mildly symptomatic people “may be responsible for more transmission than previously thought.”
About 20% of those who tested positive for the virus in South Korea end up showing no symptoms at all during their hospital stays, according to the country’s CDC head Jung Eun-kyeong. The patients are usually kept under observation for 14 days and are tested twice before being discharged. China uses a similar process to handle asymptomatic cases.
Iceland, which says it’s tested a higher proportion of inhabitants than any other country, found that about half those who tested positive have no symptoms of Covid-19, Thorolfur Gudnason, the nation’s chief epidemiologist, told BuzzFeed News.
The virus, whose incubation period can be as long as 14 days, can go undetected in the early days of infection _ especially if people don’t develop fevers, a symptom commonly used to screen for potential cases. A growing number of those infected are also reporting loss of smell and taste as early symptoms in South Korea, China and Italy.
The virus reaches its “shedding peak” between the end of the incubation period and the onset of symptoms, according to Wu, the Chinese CDC researcher. If a healthy person comes into contact with someone at that stage, infection could occur, he said.
While acknowledging that asymptomatic people pose “certain transmission risks,” the Wuhan statement cited findings from the World Health Organization that the coronavirus is mostly spread by patients already showing symptoms and added that “asymptomatic people may not be the major source of transmission.”
Griffith University’s McMillan said people who are infected and display symptoms are more infectious than asymptomatic people, but these symptom-free virus carriers are likely the main drivers of the virus’ spread.
“They are difficult to detect,” said McMillan. “Why would you test them if they aren’t sick?”
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