SINGAPORE: A Singapore study that tested the household close contacts of COVID-19 patients has found that an estimated 23 per cent of those infected were asymptomatic.
The seroepidemiological study conducted by the National Centre for Infectious Diseases (NCID) traced about 2,500 household close contacts who were placed under quarantine.
Seroepidemiology uses data from antibody-based tests to identify which population segments have been exposed to an infectious disease, and in what proportion. It can give insight into under-diagnosed mild cases and how they may contribute to the spread of COVID-19, NCID had said.
Available studies of viral concentrations in swab tests suggest that asymptomatic COVID-19 patients have similar viral concentrations compared with symptomatic patients at comparable stages of illness, said Associate Professor Ng Oon Tek, NCID senior consultant.
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"Multiple studies including an analysis from Singapore reported in our local media have demonstrated onward transmission from pre-symptomatic patients while they were still asymptomatic," he said.
The earlier study published in April had found that three people in Singapore had caught the virus after visiting the same church as an infected couple from Wuhan.
The couple, who were Chinese tourists, showed no symptoms at the time.
One woman who fell ill sat in the same seat in the church that the couple had occupied earlier that day.
For the NCID study on close household contacts, researchers did not perform repeated swab tests among asymptomatic subjects, and were not able to estimate the proportion of presymptomatic subjects, said Assoc Prof Ng.
Asymptomatic COVID-19 cases are labelled presymptomatic if they develop COVID-19 symptoms later. A COVID-19 positive person can only be classified presymptomatic after observing the individual, usually for at least a week, for the development of symptoms, said Assoc Prof Ng.
ASYMPTOMATIC CASES IN SINGAPORE, ASIA
Singapore has not disclosed national figures for asymptomatic cases but authorities have said that most of the cases among migrant workers are mild or show no symptoms.
On Thursday, the Ministry of Health reported 15 COVID-19 cases in the community, all of which were asymptomatic. Of the 99 cases reported in the community since May 18, 57 were asymptomatic, according to MOH data.
South Korea has said 20 to 30 per cent of its cases were asymptomatic, Reuters reported. In India, around 28 per cent of 40,184 people who tested positive between Jan 22 and Apr 30 were asymptomatic, according to a study.
In Vietnam, which has kept the number of COVID-19 cases to just slightly more than 300, almost 37 per cent did not display any symptoms, Reuters has reported, citing health ministry data.
A study on the coronavirus cases on board the Diamond Princess cruise ship had found that about 18 per cent of the 634 cases were asymptomatic.
The proportion of asymptomatic cases and whether they can transmit the disease to others is an indicator of the transmission potential of the disease, said the study authors. It can range from 8 per cent for measles and 32 per cent for norovirus infections, and up to 90 to 95 per cent for polio.
An editorial in the New England Journal of Medicine on May 28 called asymptomatic transmission the "Achilles’ heel" of the US' strategies to control COVID-19.
For the virus, having a large number of asymptomatic and mild infections means it is better able to spread and persist in the community. But COVID-19 can also be severe enough to put about 5 per cent of those it infects in intensive care.
While the death rate in Singapore is low, at less than 1 per cent, about 6 per cent of confirmed COVID-19 cases in the US have died. This combination of surreptitious transmission and lethality has made the virus harder to deal with compared with outbreaks such as SARS.
"Multiple clinical studies ... have demonstrated that COVID-19 infected persons with mild or even no symptoms are still be able to spread the disease," said Assoc Prof Ng.
Authorities here have repeatedly warned that re-opening too quickly after the "circuit breaker" will result in a resurgence of cases. COVID-19 has infected more than 6.5 million people worldwide and the number of reported deaths from the disease has crossed 385,000.