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Finding and isolating people with symptoms, and tracing their contacts, was a cornerstone of early COVID-19 containment. But if people could spread SARS-CoV-2 before they had symptoms, that strategy would miss some transmission. Early reports from China suggested it could happen, and an analysis of cases there estimated that 12.6% of transmission occurred before symptoms began. Singapore's Ministry of Health looked for evidence in its own cases, and its findings were published in CDC's MMWR on April 1, 2020.
How Singapore looked
Singapore's first COVID-19 case was detected on January 23, 2020. By March 16 there were 243 confirmed cases, 157 of them acquired locally. Doctors were legally required to report suspected and confirmed cases; every case in this review was confirmed by PCR. Patients were interviewed about symptoms and activities in the two weeks before they fell ill, and their contacts were traced and monitored daily.
Investigators counted transmission as presymptomatic when a source patient infected someone before developing symptoms — judged by exposure and onset dates — with no sign the second person had been exposed to anyone else with COVID-19. They also required that the source patient's own infection be clearly traceable, to reduce the chance an unknown source was involved.
Seven clusters with likely presymptomatic transmission, Singapore, January 19–March 12, 2020 (MMWR).
Seven clusters
Investigators found seven clusters, each involving two to five people, between January 19 and March 12. Ten cases in them — 6.4% of the 157 locally acquired cases — were attributed to presymptomatic transmission.
| Cluster | What happened |
|---|---|
| A | Two tourists from Wuhan attended a Singapore church on January 19, the day they arrived, and fell ill on January 22 and 24. Three others who attended that day fell ill on January 23 and 30 and February 3. One sat in the same seat the tourists had used earlier that day, as seen on closed-circuit camera. |
| B | A woman exposed at a dinner on February 15 attended a singing class on February 24 with another woman. She fell ill two days later; the classmate fell ill on February 29. |
| C | A woman exposed on February 26 likely infected her husband before she had symptoms; both fell ill on March 5. |
| D | A man who had contact with a pneumonia patient (who later died) while in the Philippines likely infected his wife before his symptoms; both fell ill on March 8. |
| E | A man likely infected while traveling in Japan passed the virus to his housemate before his symptoms; both fell ill on March 11. |
| F | A woman exposed at a singing class on February 27 attended church on March 1 and likely infected two people sitting one row behind her. She fell ill on March 3; they did on March 3 and 5. |
| G | A man back from Indonesia met a woman on March 8 and likely infected her; he fell ill on March 9, she on March 12. |
No other possible sources were found for those infected. In clusters A, B, F and G, the transmission happened 1 to 3 days before the source patient's symptoms began. In C, D and E, the people lived together, so exposure was continuous and exact timing couldn't be pinned down.
How it might happen
Presymptomatic spread may happen through respiratory droplets or indirectly. Talking and singing release particles into the air, more so the louder the voice; news outlets reported that at a choir practice in Washington on March 10, presymptomatic spread likely played a role in infecting about 40 of 60 members. The virus has also been found contaminating surfaces, so objects touched by a presymptomatic person's droplets or hands could spread it where hygiene is lax. Such transmission is known in other respiratory viruses, such as influenza, but how long and how strongly people are infectious before symptoms still needed study.
Limitations
- An unknown source could have started some clusters, though with little community spread in Singapore at the time and extensive contact tracing, presymptomatic transmission was judged more likely.
- Patients may have misremembered when mild symptoms began.
- Because testing focused on people with symptoms, asymptomatic infections were likely missed, and recall or interviewer bias could have played a part.
What it means
Together with other studies, the findings support the likelihood that people can shed the virus before symptoms appear. The authors concluded that:
- contact tracing should include a period before symptom onset;
- it may not be enough for only people with symptoms to limit contact with others;
- social distancing, including avoiding crowded gatherings, is critical to controlling the pandemic.
Sources
- Wei WE, Li Z, Chiew CJ, Yong SE, Toh MP, Lee VJ. "Presymptomatic Transmission of SARS-CoV-2 — Singapore, January 23–March 16, 2020," MMWR Vol. 69, No. 14, CDC; published by CDC and in the public domain.
Licencia: CC0 1.0 (dominio público) · Adaptado de www.cdc.gov
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