Three clusters of coronavirus disease 2019 (COVID-19) linked to a tour group from China, a company conference, and a church were identified in Singapore in February, 2020.
We gathered epidemiological and clinical data from individuals with confirmed COVID-19, via interviews and inpatient medical records, and we did field investigations to assess interactions and possible modes of transmission of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). Open source reports were obtained for overseas cases. We reported the median (IQR) incubation period of SARS-CoV-2.
As of Feb 15, 2020, 36 cases of COVID-19 were linked epidemiologically to the first three clusters of circumscribed local transmission in Singapore. 425 close contacts were quarantined. Direct or prolonged close contact was reported among affected individuals, although indirect transmission (eg, via fomites and shared food) could not be excluded. The median incubation period of SARS-CoV-2 was 4 days (IQR 3–6). The serial interval between transmission pairs ranged between 3 days and 8 days.
SARS-CoV-2 is transmissible in community settings, and local clusters of COVID-19 are expected in countries with high travel volume from China before the lockdown of Wuhan and institution of travel restrictions. Enhanced surveillance and contact tracing is essential to minimise the risk of widespread transmission in the community.
We report three clusters showing local transmission of COVID-19 in Singapore. The three clusters involved a Chinese tour group, company conference, and a church and show the transmissibility of COVID-19 in community settings beyond household clusters. A household cluster was previously reported in Shenzhen, Guangdong, China.These findings have implications for public-health measures and outbreak investigations that other countries might consider to detect and contain transmission.The first indication of human-to-human transmission in Singapore was detected 11 days after confirmation of the first imported case. China imposed a lockdown of Wuhan on Jan 23, 2020, and suspended all outgoing tour groups beginning Jan 27, 2020. Despite these measures, local clusters were subsequently identified in Singapore, with probable links to travellers from China before the measures to curb travel were implemented. Although the primary case or cases in all three clusters cannot be conclusively identified, they are postulated to be tourists or business visitors from China. The tour group implicated in cluster A originated from Guangxi at a time when relatively few cases had been reported in that region of China.
Temperature screening for inbound travellers on flights from Wuhan was implemented in Singapore on Jan 3, 2020, and expanded to all flights from China on Jan 22, 2020. Nonetheless, it is unlikely that these primary cases could be identified by temperature screening, because there was no indication that they were febrile on arrival.
Therefore, it is important that countries, particularly those with a high travel volume from China, enhance their surveillance system to identify local cases, not merely among people with a travel history to China because cases could be present in the community. The local cases in cluster C were identified through enhanced pneumonia surveillance of people who had no history of travel to China, whereas the initial cases in cluster A were identified because of heightened suspicion by health-care workers, leading to testing. Among the first 84 confirmed cases in Singapore, ten were detected by enhanced pneumonia surveillance and testing of patients in intensive care units, and another eight affected individuals were detected based on doctors' discretion to test patients whom they viewed with suspicion for clinical or epidemiological reasons. Such surveillance systems are, therefore, important to identify cases in the community who would be missed if the focus were only on travellers, and they have enabled prompt investigation and containment measures. Furthermore, with swift response in contact tracing and quarantine of close contacts, two close contacts developed symptoms and were confirmed to be infected with SARS-CoV-2 during their period of quarantine, ensuring no further onward transmission of the virus. Only two of 425 close contacts identified by contact tracing developed COVID-19 (ie, most cases identified in the clusters did not transmit the SARS-CoV-2 virus to their close contacts), raising the hypothesis that a few spreading events can result in clusters of transmission, whereas for most cases, transmission ended.
The ease of international travel makes containment difficult. Cluster B was identified because of sharing of case data in Malaysia through the IHR focal point, and Singapore shared information on attendees at the company conference to respective IHR focal points where participants at the conference had originated. This work led to early identification of cases globally and highlights the importance of international cooperation and sharing of information under IHR to aid in active case-finding and containment.
The cases of COVID-19 in these three clusters are probably attributable to close or prolonged interactions. Direct physical contact was reported between shop assistants and tourists at the health products shop (cluster A). Handshaking and physical contact during team-building activities and sharing of meals were reported among participants of the business meeting (cluster B).Therefore, direct transmission could be possible by contact or indirect transmission (eg, via fomites and shared food). We have shown environmental contamination of common-touch surfaces in the isolation room of a confirmed case. This finding highlights the importance of emphasising personal and hand hygiene as a key public-health message that countries should adopt.
Recall bias could affect the accuracy of symptom-onset dates reported by cases. In cluster A, the date of onset of symptoms for AC2 was uncertain because the individual only sought primary-care treatment on Jan 30, 2020, 7 days after reported onset of symptoms. Other exposures are plausible. AC2 reported having symptoms on the day of the tour group's arrival, and we could postulate that AC2 acquired infection from other tourists before Jan 23, 2020, and seeded infections among colleagues, the tour guide, and the two people from Guangxi on Jan 23, 2020. However, case AJ1 had no interactions with AC2, unless AC2 acquired infection serendipitously at a similar time, independently. Similarly, in cluster C, C1 did not report symptoms until 5 days and C2 until 3 days, after attending church, and this difference could be attributable to recall bias of symptom-onset dates, particularly if symptoms were mild. Other study limitations include the small sample size used to ascertain the incubation period, because primary cases could not be identified with certainty. Moreover, symptom-onset dates and the movement of and exposure history of cases detected overseas were either based on media reports or were unknown. Based on symptom-onset dates of 17 local cases, the median incubation period (4 days) corroborates other published findings .Although there is interest on asymptomatic transmission, we are unable to address this point in our study, and further studies should be done to better understand disease transmissibility of asymptomatic cases.
In conclusion, as importation and local transmission could have occurred in other countries before the lockdown of Wuhan and institution of travel restrictions by China, local clusters outside China can be expected and efforts should be focused on surveillance for locally acquired cases and containment efforts to prevent widespread community transmission. It is important for countries to do active case-finding among close contacts of cases to prevent clusters from spreading.
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