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

By Cate Cadell and Roxanne Liu

BEIJING (Reuters) – China said 300 symptomless carriers of the novel coronavirus in Wuhan, the epicenter of the pandemic, had not been found to be infectious, in a bid to reassure people as countries 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 Organization (WHO) defines https://www.who.int/docs/default-source/coronaviruse/situation-reports/20200402-sitrep-73-covid-19.pdf asymptomatic cases as those who don’t show symptoms but have been confirmed infected through a lab test. WHO notes there are few reports of truly asymptomatic cases.

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 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 cases of COVID-19, the disease caused by the new coronavirus. 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 complete asymptomatic,” 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 useful information.”

Asymptomatic cases under medical observation in China dropped to 357 as of Tuesday from 1,541 as of March 30.

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 U.S. politicians claimed.

HOW ABOUT 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% 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 20%-30% were asymptomatic. A senior health official said 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 an overwhelming majority of positive cases in its crowded migrant workers’ dormitories show mild or no symptoms.

The Philippines said about 13% of its nearly 19,000 cases were asymptomatic. In India, some 28% of 40,184 people who tested positive between Jan. 22 and April 30 were asymptomatic, according to a study.

(Reporting by Cate Cadell and Roxanne Liu in Beijing; Additional reporting by Kate Kelland in London, John Mair in Sydney, James Pearson in Hanoi, John Geddie in Singapore, Neil Jerome Morales in Manila, Sangmi Cha in Seoul, Rocky Swift in Tokyo and Miyoung Kim in Singapore; Writing by Sayantani Ghosh in Singapore; Editing by Kim Coghill)

Coronavirus clue? Most cases aboard U.S. aircraft carrier are symptom-free

By Phil Stewart and Idrees Ali

WASHINGTON (Reuters) – Sweeping testing of the entire crew of the coronavirus-stricken U.S. aircraft carrier Theodore Roosevelt may have revealed a clue about the pandemic: The majority of the positive cases so far are among sailors who are asymptomatic, officials say.

The possibility that the coronavirus spreads in a mostly stealthy mode among a population of largely young, healthy people showing no symptoms could have major implications for U.S. policy-makers, who are considering how and when to reopen the economy.

It also renews questions about the extent to which U.S. testing of just the people suspected of being infected is actually capturing the spread of the virus in the United States and around the world.

The Navy’s testing of the entire 4,800-member crew of the aircraft carrier – which is about 94% complete – was an extraordinary move in a headline-grabbing case that has already led to the firing of the carrier’s captain and the resignation of the Navy’s top civilian official.

Roughly 60 percent of the over 600 sailors who tested positive so far have not shown symptoms of COVID-19, the potentially lethal respiratory disease caused by the coronavirus, the Navy says. The service did not speculate about how many might later develop symptoms or remain asymptomatic.

“With regard to COVID-19, we’re learning that stealth in the form of asymptomatic transmission is this adversary’s secret power,” said Rear Admiral Bruce Gillingham, surgeon general of the Navy.

The figure is higher than the 25% to 50% range offered on April 5 by Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases and a member of President Donald Trump’s coronavirus task force.

‘DISCONCERTING’ DATA FOR PENTAGON

Defense Secretary Mark Esper, speaking in a television interview on Thursday, said the number of asymptomatic cases from the carrier was “disconcerting.”

“It has revealed a new dynamic of this virus: that it can be carried by normal, healthy people who have no idea whatsoever that they are carrying it,” Esper told NBC’s “Today” morning show.

Such data present challenges to the Pentagon, which is deployed around the world, sometimes in confined environments like submarines, ships and aircraft.

Testing the entire military is not yet feasible, given still-limited testing capacity, officials say, and detecting enough cases without tests is impossible if most cases are asymptomatic.

The U.S. coronavirus death toll – the highest in the world – surged past 31,000 on Thursday after doubling in a week.

It also claimed the life of a sailor from the Theodore Roosevelt this week. Five other members of the crew are hospitalized.

NUMBERS UNKNOWN

Still, the case of the Theodore Roosevelt offers a case study for researchers about how the virus spreads asymptomatically in a confined environment among mostly younger adults.

That cohort has been somewhat underrepresented in the epidemiological data so far, said William Schaffner, an infectious disease specialist at Vanderbilt University Medical Center.

“The findings are of enormous interest because the proportion of people who are asymptomatic is just simply not known,” Schaffner said, when asked about the Navy’s data.

Vice Admiral Phillip Sawyer, a deputy chief of naval operations at the center of the Navy’s coronavirus response efforts, presented the 60% figure in a call with a small group of reporters on Wednesday.

But he declined to speculate about the implications.

“I don’t know if we’re proving something different,” Sawyer said.

“I do agree that we are providing some data that some other organizations might not have.”

(Reporting by Phil Stewart and Idrees Ali; Editing by Mary Milliken and Jonathan Oatis)