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At least 10 Covid-19 patients in Singapore found to have contracted virus from people with no symptoms: Study

SINGAPORE — A new study by Singapore researchers has found that at least 10 Covid-19 patients here were infected by the coronavirus after being exposed to cases who had no symptoms.

A new study examined 243 cases of Covid-19 reported in Singapore between Jan 23 and March 16, 2020.

A new study examined 243 cases of Covid-19 reported in Singapore between Jan 23 and March 16, 2020.

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SINGAPORE — A new study by Singapore researchers has found that at least 10 Covid-19 patients here were infected by the coronavirus after being exposed to cases who had no symptoms.

These pre-symptomatic cases accounted for 6.4 per cent of the 157 locally transmitted cases reported in Singapore between Jan 23 and March 16.

This suggests a possibility that pre-symptomatic individuals who are carrying the coronavirus but do not have symptoms can pass on the virus to others, the researchers said.

As a result, public health officials should consider including the few days before the onset of symptoms when conducting contact tracing, they added.

Right now, when identifying close contacts, infected patients are asked to provide details of their activities from the time after they show symptoms to when they are successfully isolated.

The study, published on Wednesday (April 1), examined 243 cases of Covid-19 reported in Singapore between Jan 23 and March 16.

It is co-authored by Dr Vernon Lee, director of communicable diseases at the Ministry of Health (MOH). Others involved in the study were researchers Wycliffe Wei, Li Zongbin, Calvin Chiew, Sarah Yong and Matthias Toh.

They said: “These findings suggest that to control the pandemic, it might not be enough for only persons with symptoms to limit their contact with others because persons without symptoms might transmit infection.

“(They also) underscore the importance of social distancing in the public health response to the Covid-19 pandemic, including the avoidance of congregate settings.”


Among the 243 cases, 157 were infected locally, and of these, the authors identified cases grouped into seven research-specific clusters where they believed pre-symptomatic transmission had taken place and led to the occurrence of secondary cases.

Pre-symptomatic transmission was defined as the transmission of the coronavirus from an infected person — or source patient — to a secondary patient before the source patient developed symptoms, and with evidence that the secondary patients did not get the disease from elsewhere.

The seven clusters do not correlate with the clusters announced earlier by MOH but refer to cases that were linked in terms of how they spread among groups of people. They were specifically identified for review based on the criteria of the study.

One of the seven clusters identified in the study include cases from the Life Church and Missions Singapore, where there were two imported cases from Wuhan, China — the epicentre of the first outbreak.

The couple from Wuhan had visited the church in Paya Lebar on Jan 19, but reported symptoms only a few days later on Jan 22 and 24. Three members of the church congregation attended church service on the same day as them and were later confirmed to have the virus. One of these three church members had sat on the same seat that the Wuhan couple occupied earlier that day.

Another example of pre-symptomatic transmission was from the Safra Jurong cluster. A woman attended a dinner event there on Feb 15 where she was exposed to a patient with confirmed Covid-19. On Feb 24, she and another woman attended the same singing class and on Feb 26, the first woman developed symptoms, followed by the second woman on Feb 29.

For the cases where the date of exposure could be determined, the researchers also found that pre-symptomatic transmission occurred up to three days before the patient reported symptoms.

Such pre-symptomatic transmission exposure has also been observed in other respiratory viruses such as influenza, the authors noted. 

However, they added that more research is needed to determine how contagious an infected person is during the pre-symptomatic period.


Separately, the researchers said that during the pre-symptomatic phase, the virus can also be spread through the generation of droplets — such as through speech and other vocal activities such as singing — and indirectly through contact with contaminated surfaces.

This is why aside from appealing to the authorities to take into account pre-symptomatic transmission when contact tracing, the authors stressed the importance of safe distancing to curb the spread of the virus.

Approached by TODAY to comment on the findings, infectious disease specialist Leong Hoe Nam from Mount Elizabeth Novena Hospital emphasised the need for the public to comply with the existing safe-distancing measures to arrest the transmission.

He said: “Our compliance with safe-distancing measures needs to be ramped up. If you look at the people who are getting sick, they are younger people who are less symptomatic and who may inadvertently and unknowingly pass the infection to someone.

“That is why those who show mild symptoms should not go out.”


However, the researchers acknowledged that the findings of the report are limited by several factors.

For one thing, it is possible that an unknown source might have initiated the clusters that were highlighted, though they added that this is less likely given Singapore’s strong surveillance systems and that there was no widespread community transmission of Covid-19 at the time.

Separately, the patient’s reporting of symptoms may not be entirely accurate, especially if the symptoms they experienced were mild, and this would therefore cast doubt on the duration of the pre-symptomatic period.

Lastly, because detection and surveillance measures have focused on testing symptomatic individuals, it is to be expected that some asymptomatic individuals have flown under the radar.

Related topics

Covid-19 coronavirus coronavirus transmission symptoms contact tracing

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