Evaluation of the Effectiveness of Surveillance and Containment Measures for the First 100 Patients with COVID-19 in Singapore — January 2–February 29, 2020

Yixiang Ng, MSc; Zongbin Li, MBBS; Yi Xian Chua, MSc; Wei Liang Chaw, MSc; Zheng Zhao, MSc; Benjamin Er, MSc; Rachael Pung, MSc; Calvin J. Chiew, MPH; David C. Lye, MBBS; Derrick Heng, MPH; Vernon J. Lee, PhD

Disclosures

Morbidity and Mortality Weekly Report. 2020;69(11):307-311. 

In This Article

Abstract and Introduction

Introduction

Coronavirus disease 2019 (COVID-19) was first reported in Wuhan, China, in December 2019, and has since spread globally, resulting in >95,000 confirmed COVID-19 cases worldwide by March 5, 2020.[1] Singapore adopted a multipronged surveillance strategy that included applying the case definition at medical consults, tracing contacts of patients with laboratory-confirmed COVID-19, enhancing surveillance among different patient groups (all patients with pneumonia, hospitalized patients in intensive care units [ICUs] with possible infectious diseases, primary care patients with influenza-like illness, and deaths from possible infectious etiologies), and allowing clinician discretion (i.e., option to order a test based on clinical suspicion, even if the case definition was not met) to identify COVID-19 patients. Containment measures, including patient isolation and quarantine, active monitoring of contacts, border controls, and community education and precautions, were performed to minimize disease spread. As of March 5, 2020, a total of 117 COVID-19 cases had been identified in Singapore. This report analyzes the first 100 COVID-19 patients in Singapore to determine the effectiveness of the surveillance and containment measures. COVID-19 patients were classified by the primary means by which they were detected. Application of the case definition and contact tracing identified 73 patients, 16 were detected by enhanced surveillance, and 11 were identified by laboratory testing based on providers' clinical discretion. Effectiveness of these measures was assessed by calculating the 7-day moving average of the interval from symptom onset to isolation in hospital or quarantine, which indicated significant decreasing trends for both local and imported COVID-19 cases. Rapid identification and isolation of cases, quarantine of close contacts, and active monitoring of other contacts have been effective in suppressing expansion of the outbreak and have implications for other countries experiencing outbreaks.

On January 2, 2020, days after the first report of the disease from China, the ministry of health (MOH) in Singapore, a small island city-state in Southeast Asia with a population of approximately 5.7 million, developed a local case definition (Supplementary Table, https://stacks.cdc.gov/view/cdc/85735) and advised all medical practitioners to be vigilant for suspected COVID-19 patients.[2] A confirmed case was defined as a positive test for SARS-CoV-2, the virus that causes COVID-19, by reverse transcription–polymerase chain reaction (RT-PCR),[3] or a positive viral microneutralization antibody test using a SARS-CoV-2 virus isolate (BetaCoV/Singapore/2/2020; GISAID accession 76 number EPI_ISL_407987) and conducted using previously published protocols.[4] At hospitals, patients with suspected COVID-19 received chest radiographs and RT-PCR testing on at least two nasopharyngeal swabs collected 24 hours apart.[5] Physicians are mandated to report all suspected and confirmed COVID-19 patients through a centralized disease notification system.

The case definition was updated five times following the outbreak's start to adapt to the evolving global situation (Supplementary Table, https://stacks.cdc.gov/view/cdc/85735). The MOH carried out contact tracing around confirmed cases to identify persons who might have been infected. Contacts with fever (temperature ≥100.4°F [≥38°C]) or respiratory symptoms were transferred directly to a hospital for further evaluation and testing. Close contacts were defined as having close (within 6.6 ft [2 m]) and prolonged (generally ≥30 minutes) contact with the COVID-19 patient. Contacts at lower risk were persons who had some interactions with the COVID-19 patient for shorter periods of time. Asymptomatic close contacts were placed under compulsory quarantine for 14 days, and contacts at lower risk were placed under active monitoring. All contacts were assessed by telephone for fever or respiratory symptoms by public health officials during the quarantine or monitoring period, thrice daily for close contacts and once daily for contacts at lower risk. Contacts who became symptomatic were transferred to a hospital. Surveillance was enhanced in late January 2020 by testing the following groups for COVID-19: 1) all hospitalized patients with pneumonia (later expanded to include patients with pneumonia evaluated in primary care settings); 2) ICU patients with possible infectious causes as determined by the physician; 3) patients with influenza-like illness at sentinel government and private primary care clinics included in the routine influenza surveillance network; and 4) deaths from possible infectious causes. In addition, medical practitioners could choose to test patients if there was clinical (e.g., prolonged respiratory illness with unknown cause) or epidemiologic (e.g., association with known clusters) suspicion.

The effectiveness of Singapore's surveillance and containment efforts was assessed from the outbreak's start until February 29 by calculating the 7-day moving average of the interval from symptom onset to isolation in hospital or quarantine. This measure provides an indication of the time spent within the community when a person with COVID-19 is potentially infectious. Differences in the percentages of cases detected through the different surveillance components were tested using the chi-squared or Fisher's exact test. All analyses were conducted using R statistical software (version 3.5.1; The R Foundation).

Among the first 100 confirmed COVID-19 cases in Singapore, the average patient age was 42.5 years (median = 41 years; interquartile range [IQR] = 34–54 years) (Table). The majority (72%) of patients were aged 30–59 years, and 60% of patients were male. RT-PCR confirmed 99% of cases, and one case was confirmed by viral microneutralization testing. Twenty-four cases were imported, and the rest resulted from local transmission. Fifteen patients were ever in the ICU; no deaths have been reported to date. Contact tracing contributed to the primary detection of approximately half (53%) of COVID-19 patients. Another 20 (20%) patients were identified at general practitioner clinics or hospitals because they met the case definition; 16 were identified through enhanced surveillance (15 from pneumonia surveillance and one from the ICU), and another 11 through medical providers' clinical discretion. No patients were identified through surveillance for influenza-like illness. A significant difference was found in the percentage of cases detected by the various surveillance methods, depending on whether the cases were linked to another COVID-19 patient or by travel to China, compared with cases that could not be linked to another case (p<0.001). Among linked cases, the largest proportion (62.7%) was detected through contact tracing, whereas among unlinked cases, the largest proportion of cases (58.8%) was detected through enhanced surveillance (Table). The earliest symptom onset date reported by a COVID-19 patient was January 14 (Figure 1). The epidemic curve peaked on January 30, when nine patients were identified, before declining to two to five patients per day on February 11 and continuing forward. International importations accounted for a majority of cases at the outbreak's start before more local cases were detected. The mean interval from symptom onset to hospital isolation or quarantine was 5.6 days (median = 5 days; IQR = 2–8 days). The 7-day moving average of the interval from symptom onset to isolation declined significantly across the study period for both imported and local cases, from 9.0 and 18.0 days to 0.9 and 3.1 days, respectively (p<0.001) (Figure 2). Among the 53 patients first identified through contact tracing, 13 (24.5%) were contacted on or before the date of symptom onset.

Figure 1.

Date of symptom onset and date of report for cases of coronavirus disease 2019 (COVID-19) (N = 100), based on importation and linkage*,† status — Singapore, January 14–February 28, 2020
*Linked patients defined as those who were found to be epidemiologically linked to other COVID-19 patients or who had recent travel to China.
Unlinked patients defined as those whose source of infection could not be determined.

Figure 2.

Interval from symptom onset to isolation or hospitalization (7-day moving average), of coronavirus disease 2019 (COVID-19) cases (N = 100), by importation status — Singapore, January 14–February 28, 2020

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