From the weekly morbidity and mortality report (CDC)
What is known about this topic? Preliminary evidence indicates the emergence of presymptomatic transmission of SARS-CoV-2, based on individual case reports in China. What does this report add? Investigation of the 243 COVID-19 cases reported in Singapore from January 23 to March 16 identified seven clusters of cases in which pre-symptomatic transmission is the most likely explanation for the emergence of secondary cases. What are the implications for public health practice? The possibility of presymptomatic transmission increases the challenges of containment measures. Public health officials conducting contact tracing should consider including a period before symptom onset to account for the possibility of presymptomatic transmission. The potential for presymptomatic transmission underscores the importance of social distancing, including avoiding crowded settings, to reduce the spread of COVID-19. |
Presymptomatic transmission of SARS-CoV-2, the virus that causes coronavirus disease 2019 (COVID-19), could pose challenges to disease control. The first case of COVID-19 in Singapore was detected on January 23, 2020, and by March 16, a total of 243 cases had been confirmed, including 157 locally acquired cases.
The clinical and epidemiological findings of all COVID-19 cases in Singapore up to March 16 were reviewed to determine whether pre-symptomatic transmission could have occurred. Presymptomatic transmission was defined as transmission of SARS-CoV-2 from an infected person (source patient) to a second patient before the source patient developed symptoms, as determined by exposure and dates of symptom onset. with no evidence that the secondary patient had been exposed to anyone else with COVID-19.
Seven epidemiological clusters of COVID-19 were identified in which presymptomatic transmission likely occurred, and 10 of these cases within these clusters represented 6.4% of the 157 locally acquired cases. In the four groups for which the date of exposure could be determined, presymptomatic transmission occurred 1-3 days before the onset of symptoms in the patient of presymptomatic origin.
To account for the possibility of presymptomatic transmission, officials developing contact tracing protocols should consider including a period before symptom onset. Evidence of presymptomatic transmission of SARS-CoV-2 underscores the critical role that social distancing plays in controlling the COVID-19 pandemic.
Early detection and isolation of symptomatic patients with COVID-19 and tracing close contacts is an important disease containment strategy; however, the existence of presymptomatic or asymptomatic transmission would present difficult challenges for contact tracing.
Such modes of transmission have not been definitively documented for COVID-19, although cases of presymptomatic and asymptomatic transmissions have been reported in China and possibly occurred at a nursing facility in King County, Washington.
COVID-19 cases in Singapore were reviewed to determine whether pre-symptomatic transmission occurred between COVID-19 clusters. The definition of a suspected case was based on the presence of respiratory symptoms and a history of exposure. Suspected cases were tested, and a confirmed case was defined as a positive test for SARS-CoV-2, using laboratory polymerase chain reaction or serological assays.
All cases in this report were confirmed by polymerase chain reaction. Asymptomatic people did not undergo routine testing, but such testing was performed for people in groups considered high risk for infection.
Patients with confirmed COVID-19 were interviewed to obtain information about their clinical symptoms and activity history during the 2 weeks prior to symptom onset to determine possible sources of infection.
Contact tracing examined the time from symptom onset to the time the patient successfully isolated to identify contacts who had interactions with the patient. All contacts were monitored daily, and those who developed symptoms were tested as part of active case finding.
The clinical and epidemiological data of the 243 reported cases of COVID-19 in Singapore during January 23 to March 16 were reviewed. Medical records were examined to identify symptoms before, during, and after the first positive SARS-CoV-2 test.
Records of cases that were epidemiologically linked (clusters) were reviewed to identify cases of probable presymptomatic transmission. Such groups had clear contact between a source patient and a patient infected by the source (a secondary patient), had no other likely explanations for the infection, and had the source patient’s symptom onset date after the date of exposure to the patient. secondary that was subsequently infected. Symptoms considered in the review included respiratory, gastrointestinal (e.g. diarrhea) and constitutional symptoms.
Seven clusters of COVID-19 cases that suggest presymptomatic transmission
Investigation of COVID-19 cases in Singapore identified seven clusters (AG clusters) in which pre-symptomatic transmission likely occurred. These groups occurred from January 19 to March 12 and involved two to five patients each. Ten of the cases within these groups were attributed to pre-symptomatic transmission and represented 6.4% of the 157 locally acquired cases reported as of March 16.
Group A. A 55-year-old woman (patient A1) and a 56-year-old man (patient A2) were tourists from Wuhan, China, who arrived in Singapore on 19 January. They visited a local church on the same day and then symptoms began: January 22 (patient A1) and January 24 (patient A2).
Three other people, a 53-year-old man (patient A3), a 39-year-old woman (patient A4), and a 52-year-old woman (patient A5), attended the same church that day and subsequently developed symptoms on January 23 and the January 30th. and February 3, respectively.
Patient A5 occupied the same seat in the church that Patients A1 and A2 had occupied earlier in the day (captured by closed-circuit camera). Investigations of other attendees did not reveal other symptomatic people who attended the church that day.
Group B. A 54-year-old woman (patient B1) attended a dinner party on February 15 where she was exposed to a confirmed COVID-19 patient. On February 24, patient B1 and a 63-year-old woman (patient B2) attended the same singing class. Two days later (February 26), patient B1 developed symptoms; Patient B2 developed symptoms on February 29.
Group C. A 53-year-old woman (patient C1) was exposed to a confirmed COVID-19 patient on February 26 and likely transmitted the infection to her 59-year-old husband (patient C2) during her pre-symptomatic period; both patients developed symptoms on March 5.
Group D. A 37-year-old man (patient D1) traveled to the Philippines from February 23 to March 2, where he was in contact with a patient with pneumonia who later died. Patient D1 likely transmitted the infection to his 35-year-old wife (patient D2) during her presymptomatic period. Both patients developed symptoms on March 8.
Group E. A 32-year-old man (patient E1) traveled to Japan from February 29 to March 8, where he was probably infected, and subsequently transmitted the infection to a 27-year-old woman with whom he shared a home (patient E2), before of developing symptoms. Both presented symptoms on March 11.
Group F. A 58-year-old woman (patient F1) attended a singing class on February 27, where she was exposed to a confirmed COVID-19 patient. She attended a religious service on March 1, where she likely infected a 26-year-old woman (patient F2) and a 29-year-old man (patient F3), both sitting one row behind her. Patient F1 developed symptoms on March 3, and patients F2 and F3 developed symptoms on March 3 and March 5, respectively.
Group G. A 63-year-old man (patient G1) traveled to Indonesia from March 3 to 7. He met a 36-year-old woman (patient G2) on March 8 and likely transmitted SARS-CoV-2 to her; she developed symptoms on March 9, and patient G2 developed symptoms on March 12.
The investigation of these groups did not identify other patients who could have transmitted COVID-19 to infected people. In four groups (A, B, F, and G), exposure to presymptomatic transmission occurred 1-3 days before patient 1 developed symptoms. For the remaining three groups (C, D, and E), the exact timing of exposure to transmission could not be determined because people lived together and exposure was continuous.
Discussion
This research identified seven COVID-19 clusters in Singapore in which pre-symptomatic transmission likely occurred. Among the 243 COVID-19 cases reported in Singapore as of March 16, 157 were locally acquired; 10 of the 157 (6.4%) locally acquired cases fell into these groups and were attributed to presymptomatic transmission.
These findings are supported by other studies suggesting that presymptomatic transmission of COVID-19 may occur. An examination of transmission events among cases in Chinese patients outside Hubei Province, China, suggested that 12.6% of transmissions may have occurred before the onset of symptoms in the source patient.
Presymptomatic transmission can occur through the generation of respiratory droplets or possibly through indirect transmission.
| Speech and other vocal activities , such as singing, have been shown to generate air particles, with the rate of emission corresponding to the volume of the voice. |
Media reported that during a choir practice in Washington on March 10, pre-symptomatic transmission likely played a role in transmitting SARS-CoV-2 to approximately 40 of the 60 choir members.
Environmental contamination with SARS -CoV-2 has been documented, and the possibility of indirect transmission through fomites by presymptomatic people is also a concern. Objects can be contaminated directly by droplets or by contact with the contaminated hands of an infected person and transmitted through non-rigorous hygiene practices.
The possibility of pre-symptomatic transmission of SARS-CoV-2 increases the challenges of COVID-19 containment measures, which rely on early detection and isolation of symptomatic individuals. The magnitude of this impact depends on the extent and duration of transmissibility while a patient is presymptomatic, which, to date, has not been clearly established.
In four groups (A, B, F and G), it was possible to determine that exposure to presymptomatic transmission occurred 1-3 days before the source patient developed symptoms. Such transmission has also been observed in other respiratory viruses such as influenza. However, transmissibility by presymptomatic people requires further study.
The results of this report are subject to at least three limitations .
- First, although these cases were carefully investigated, there is a possibility that an unknown source may have started the clusters described.
- Second, recall bias could affect the accuracy of symptom onset dates reported by cases, especially if symptoms were mild, leading to uncertainty about the length of the presymptomatic period.
- Finally, due to the nature of screening and surveillance activities that focus on testing symptomatic individuals, underdetection of asymptomatic disease is expected. Remember that interviewer bias (i.e., the expectation that some symptoms will be present, no matter how mild), could have contributed to this.
Conclusions
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