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A third of coronavirus cases asymptomatic

March 24, 2020

In South Korea, nationwide testing, comprehensive isolation of Covid-19 patients and strict rules on social interaction have shown results. The measures have proved effective in slowing the spread of the coronavirus.

A medical worker of hospital of Yeungnam University makes health screening at a drive-through clinic in Daegu, South Korea, March 4, 2020.
Am I infected although I feel fine? Rapid testing delivers quick answersImage: Imago Images/Xinhua/Wang Jingqiang

The strategies for handling the spread of coronavirus are as varied as countries themselves. National health authorities, for instance, have different interpretations of the infectiousness of asymptomatic patients.

As a rule, infected people start to show symptoms within five days. But in some cases, the incubation period can last as long as three weeks.

Information released by the Chinese government, and reported in the South China Morning Post newspaper, suggests that the number of "silent carriers" — that's people who have tested positive for the Covid-19 coronavirus but who show no symptoms, or delayed symptoms — could be as high as 30%. 

Those Chinese data have been confirmed by a group of Japanese experts, led by Hiroshi Nishiura, an epidemiologist at Hokkaido University.

Among the Japanese patients evacuated from Wuhan, the center of the coronavirus outbreak, 30.8% were asymptomatic.

In a letter to the International Journal of Infectious Diseases, the Japanese experts say there's "a gap" between reports from China and estimates based on cases diagnosed outside China and that that suggests a "substantial number of cases are underdiagnosed." 

Their numbers stand in stark contrast to estimates from the World Health Organization (WHO), which suggests transmission of the virus via asymptomatic patients is rare.

WHO data suggests asymptomatic infections in the European Union are between one and 3%.

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Polymerase chain reaction device at the Fraunhofer Institute for Molecular Biology in Schmallenberg
The coronavirus is most often detected with a process called polymerase chain reaction (PCR)Image: DW/F. Schmidt

Blanket testing without symptoms

In most European Union countries and the USA, where only people with symptoms are tested for coronavirus, the number of infections is rising rapidly.

Germany's Federal Ministry of Health says people will only get tested if they present with flu-like symptoms AND were in a region with known cases of coronavirus cases in the past 14 days OR if they had contact with a person who had tested positive for Covid-19 in the past 14 days. 

If, however, you've had contact with an infected person but show no symptoms yourself, you are sent into self-isolation. But self-isolation is seldom monitored in Germany.

Meanwhile, in China and South Korea, where the earliest cases of SARS-CoV-2 were detected, the number of new infections is in decline. In both countries, people are tested if they have had any close contact with an infected person, regardless of whether they show symptoms or not.

Read more: How does testing for the coronavirus work?

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People who test positive in China and South Korea are sent into quarantine and constantly monitored, even if they show no symptoms.

In South Korea, people who breach quarantine regulations can face a fine of up to 3 million South Korean Won (about $2500). New draft legislation could see that fine go up to 10 million South Korean Won and include a potential one-year prison sentence.

Uncomplicated test opportunities

The number of new infections in South Korea rose rapidly in February. But since then, the country has seen the number of new infections fall.

About 9000 of South Korea's 50 million-strong population are infected, and 120 South Koreans have died due to a Covid-19 infection. There are less than 100 new infections per day.

South Korean officials have put up numerous checkpoints and tents, where anyone and everyone can get tested for coronavirus free-of-charge. The country has tested at least 300,000 people.

Every day, a further 20,000 people can get tested, including at 40 "drive-through" coronavirus test sites. Those drive-throughs have been copied in other countries.

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Medical members wearing protective gear take samples from a driver with suspected symptoms of the COVID-19 coronavirus, at a "drive-through" virus test facility in Goyang, north of Seoul, on February 29, 2020.
Caption: Just a quick stop to provide a sample: How Korea has tested at least 300,000 residentsImage: AFP/Jung Yeon-je

As a result, South Korea has tested more people than other countries, namely 5.6 in every 1000 residents. Compare that to Germany, where a maximum of 160,000 tests can be conducted each week, according to the Robert-Koch-Institute, and you get a rate of 1.9 in every 1000 people. The USA has tested a mere 30,000 people.

Successful containment policy

Containment in South Korea seems to have worked without a full lockdown or any drastic travel restrictions. Public life has been reduced massively, and people are keeping their distance from one another, but the country wants to avoid a complete shutdown.

"South Korea is a democratic republic, we feel a lockdown is not a reasonable choice," said Kim Woo-Joo, an expert in infectious diseases at Korea University, quoted in the specialist journal, Science.

Instead of a nationwide lockdown, South Korea had introduced "the most expansive and well-organized testing program in the world, combined with extensive efforts to isolate infected people and trace and quarantine their contacts," writes Dennis Normile in the journal.

It's thought that the intensive testing regime has also possibly caught many "silent carriers" in areas around infected people.

"Korea currently has a significantly higher rate of asymptomatic cases than other countries, perhaps due to our extensive testing," said Jeong Eun-kyeong, director of Korea Centers for Disease Control and Prevention (KCDC), at a press briefing on March 16.

Far-reaching powers

On top of that, South Korea has dug deep into the personal rights of its population.

In an attempt to minimize infections, residents receive personalized information about the risks of the coronavirus in their immediate areas. Local authorities have access to detailed information about coronavirus patients, including their age, biological sex, and a profile of their movements.

Since the MERS crisis in 2015, the South Korea government has legal powers to collect cell phone and credit card information, enabling it to reconstruct the movements of Covid-19 positive people. 

The data are then anonymized and forwarded to apps, which can help everyone see whether they have had contact with an infected person.

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