Asymptomatic Covid-19 patients are driving a surge in new cases in South Korea, frustrating efforts to control transmission by the Asian country which managed to keep infections under control in previous outbreaks.
South Korea reported 569 new cases in the 24 hours ending Thursday midnight, a level unseen in nearly nine months, as it grapples with the third wave of the pandemic that appears to be worsening despite tough new social distancing measures.
With young people at the centre of the surge, health authorities in South Korea estimate asymptomatic patients now account for 40% of total infections, up sharply from 20-30% in June.
That compares with research evidence suggesting about one in five infected people in general will experience no symptoms.
The rate is much lower in China where the state disease control centre said in February that around 1% of more than 70,000 cases it analysed were asymptomatic. In Tokyo, about 19 percent of patients are asymptomatic.
It's not clear why some patients who test positive for the virus do not display any symptoms, but health officials believe they pose less transmission risks. However, the people they infect may display symptoms.
Also, the officials are concerned about a rise in untraceable clusters since these asymptomatic infections are more difficult to identify.
Cold weather is further accelerating the spread as more meetings and activities are held indoors in places with poor ventilation, while the risk of unwitting infections by symptomless patients has increased.
That poses a major challenge in South Korea, which succeeded in keeping infections low in previous outbreaks through aggressive contact tracing.
It has introduced tougher social distancing measures this week to contain transmission and encouraged people to get tested.
"We should have maintained tough social distancing measures longer," said Kim Woo-joo, a professor of Infectious Diseases at Korea University Guro Hospital.
"In the wake of eased social distancing measures in early October, a lot of people, especially young people, let their guard down, and many of those who had very mild symptoms or no symptoms have gone unnoticed."
As young people drive the surge in new cases, the number of young patients in serious conditions and in need of ventilators has also shot up to 19 this week in South Korea to nearly one fourth of total patients who need ventilator support.