SINGAPORE — A medical study on Singapore COVID-19 patients found that at least 10 of them got the coronavirus from those without symptoms.
The study, which was co-authored by Ministry of Health’s director of communicable diseases Vernon Lee, was published on Wednesday (1 April) on the United States’ Centers for Disease Control and Prevention website.
It said that the possibility of COVID-19 transmission without symptoms raises the challenges of containment measures, and emphasises the importance of social distancing such as avoiding crowded places.
“Public health officials conducting contact tracing should strongly consider including a period before symptom onset to account for the possibility of pre-symptomatic transmission,” the study advised.
“The potential for pre-symptomatic transmission underscores the importance of social distancing, including the avoidance of congregate settings, to reduce COVID-19 spread.”
Seven clusters of pre-symptomatic transmission
The study was conducted on all 243 COVID-19 cases in Singapore discovered between 23 January and 16 March. It found that, among the 157 non-import cases in this period, there were 10 cases among seven clusters which can be attributed to transmission without symptoms:
Three of the 10 cases were from the Grace Assembly of God and Life Church and Missions Singapore cluster. All three attended a church service on 19 January with a pair of tourists from Wuhan, both of whom did not show any symptoms of COVID-19 at the time. One of the three cases, a 52-year-old woman, sat on the same seat that the tourists occupied earlier in the day.
A 54-year-old woman attended the Safra Jurong dinner event on 15 February, and came into contact with a patient with confirmed COVID-19. Showing no symptoms yet, the woman subsequently attended a singing class with a 63-year-old woman on 24 February. Both women developed COVID-19 symptoms a few days later.
A 53-year-old woman was exposed to a COVID-19 patient on 26 February, and likely passed the infection to her 59-year-old husband. Both developed symptoms only on 5 March.
A 37-year-old man travelled to the Philippines from 23 February to 2 March, and was in contact with a patient with pneumonia who later died. He likely transmitted the coronavirus to his 35-year-old wife while not showing symptoms. Both developed COVID-19 symptoms on 8 March.
A 32-year-old man travelled to Japan from 29 February to 8 March, and was likely infected there. He subsequently transmitted the infection to his housemate, a 27-year-old woman, before he developed symptoms. Both developed symptoms on 11 March.
A 58-year-old woman attended a singing class on 27 February, and was exposed to a patient with confirmed COVID-19. She attended a church service on 1 March without symptoms, and likely infected a 26-year-old woman and a 29-year-old man, both of whom sat one row behind her. All developed COVID-19 symptoms days after the church service.
A 63-year-old man travelled to Indonesia from 3 to 7 March. He met a 36-year-old woman on 8 March and likely transmitted COVID-19 to her. The man developed symptoms on 9 March, while the woman developed symptoms on 12 March.
Contact tracing should include period before onset of symptoms
Transmission without symptoms has been of great concern in the current pandemic, as many measures to curb its spread – such as temperature screening – rely on the infected individual showing symptoms.
The Singapore study advises that containment measures such as contact tracing should include the period before symptoms are discovered by the patients, in order to account for pre-symptomatic transmission.
It also said that, to control the pandemic, it might not be enough for only people with symptoms to limit their contact with others. Social distancing for everyone will therefore be important in such controlling measures.
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