On March 13, 2020, this report was posted online as an MMWR Early Release.
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.
TABLE
Characteristics of coronavirus disease 2019 (COVID-19) cases, by linkage to other
known cases (N = 100) — Singapore, January–February 2020
Characteristic
No. (%) of COVID-19 cases
P-value
Total
Linked*
Unlinked†
Age group (yrs)
<30
17 (17.0)
17 (20.5)
0 (—)
0.12
30–39
28 (28.0)
23 (27.7)
5 (29.4)
40–49
20 (20.0)
16 (19.3)
4 (23.5)
50–59
24 (24.0)
20 (24.1)
4 (23.5)
≥60
11 (11.0)
7 (8.4)
4 (23.5)
Sex
Male
60 (60.0)
46 (55.4)
14 (82.4)
0.06
Female
40 (40.0)
37 (44.6)
3 (17.6)
Ethnic group
Chinese
87 (87.0)
74 (89.2)
13 (76.5)
0.21
Indian
6 (6.0)
4 (4.8)
2 (11.8)
Malay
2 (2.0)
1 (1.2)
1 (5.9)
Other
5 (5.0)
4 (4.8)
1 (5.9)
Primary detection method
Contact tracing
53 (53.0)
52 (62.7)
1 (5.9)
<0.001
Case definition at medical consult
20 (20.0)
16 (19.3)
4 (23.5)
Enhanced surveillance
16 (16.0)
6 (7.2)
10 (58.8)
Provider clinical discretion
11 (11.0)
9 (10.8)
2 (11.8)
* Patients who were epidemiologically linked to other COVID-19 patients or had recent
travel to China.
† Patients whose source of infection could not be determined.
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.
The figure consists of two histograms, epidemiologic curves showing the date of symptom
onset and the date of report for 100 cases of coronavirus disease 2019 (COVID-19)
in Singapore during January 14–February 28, 2020, based on status of importation and
linkage to other cases or travel to China.
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
The figure is a line graph showing the interval from symptom onset to isolation or
hospitalization, using a 7-day moving average, of 100 coronavirus disease 2019 COVID-19)
cases in Singapore during January 14–February 28, 2020, by importation status.
Discussion
In this assessment of the measures that Singapore, a small city-state, put in place
to identify COVID-19 patients and contain disease spread in the early outbreak phase,
approximately one quarter of cases were detected through enhanced surveillance among
hospitalized patients with pneumonia and ICU patients (16 cases [16%]) and through
providers’ clinical discretion (11 [11%]). A recent study considered Singapore to
have the highest surveillance capacity for COVID-19 among all countries (
6
). The study estimated that if other countries had similar detection capacities as
Singapore, the global number of imported cases detected would be 2.8 times higher
than the observed current number. The surveillance methods in Singapore complemented
one another to identify infected persons, with the overlapping components constituting
safety nets; none of the methods alone would have detected all patients. The case
definition was important for clinicians to use as a foundation, and active case finding
around COVID-19 patients through contact tracing was useful in detecting new patients
early for isolation.
The enhanced surveillance measures of SARS-CoV-2 testing of all patients with pneumonia,
surveillance of ICU patients with severe illness and deaths potentially attributable
to COVID-19, and clinical discretion in requesting testing were all important in detecting
initially unlinked patients for further investigation. Adoption of multiple surveillance
mechanisms can ensure broad coverage because each missed case can lead to chains of
transmission that might be difficult to contain subsequently.
Singapore has implemented aggressive measures to contain local transmission of COVID-19.
After an initial increase in locally transmitted cases, the number of newly identified
cases decreased after approximately 1 month, determined by symptom onset dates. This
decrease is likely a result of the early implementation of surveillance and detection
measures while the numbers of patients were still small and individual-level containment
was possible; a larger number of cases would have driven community transmission. The
decline in the 7-day moving average of interval from onset to isolation in hospital
and quarantine was also indicative of efforts to contain disease spread across time.
Singapore has also implemented other measures to reduce the spread of COVID-19. To
prevent imported cases from seeding local transmission, border control measures included
temperature screening initially for travelers on flights from Wuhan before expanding
to include all travelers entering Singapore at air, sea, and land checkpoints (
7
). Short-term visitors with travel in the past 14 days to selected countries or regions
(initially mainland China and later expanded to South Korea, northern Italy, and Iran)
were denied entry; Singapore residents returning from these areas were placed under
a mandatory 14-day self-quarantine. To reduce community spread, public education messages
were focused on personal hygiene and seeking early medical care and self-isolation
when having respiratory symptoms. As of March 5, 2020, schools have not closed because
there was no widespread community transmission in Singapore and few cases among children;
precautionary measures such as reducing mixing across classes or schools have been
implemented to limit possible disease transmission.
The findings in this report are subject to at least three limitations. First, the
7-day moving average interval from symptom onset to isolation could fluctuate for
recent dates as additional patients are detected and might be insufficient as a single
measure to evaluate the effectiveness of containment. Further indicators to assess
effectiveness of containment measures should be investigated. Nevertheless, the downward
trend was significant from the outbreak’s start until early February. Second, the
case detection methods were primarily focused on symptomatic patients. Further studies
are needed to assess the number of asymptomatic patients in the community and their
potential to transmit disease and whether additional measures targeting asymptomatic
patients would have resulted in further case reductions. Finally, generalizability
of results is limited because of the small sample size and lack of cases in settings
such as long-term nursing facilities and health care settings.
Singapore implemented strong surveillance and containment measures, which appear to
have slowed the growth of the outbreak. These measures might be useful for detection
and containment of COVID-19 in other countries that are experiencing the start of
local COVID-19 outbreaks. Singapore is a small island city-state, and nations with
other characteristics might need to adapt and augment Singapore’s approaches to achieve
the same level of effectiveness.
Summary
What is already known about this topic?
First detected in China in late 2019, coronavirus disease 2019 (COVID-19) transmission
has spread globally.
What is added by this report?
Singapore implemented a multipronged surveillance and containment strategy that contributed
to enhanced case ascertainment and slowing of the outbreak. Based on review of the
first 100 cases, the mean interval from symptom onset to isolation was 5.6 days and
declined after approximately 1 month.
What are the implications for public health practice?
A multipronged surveillance strategy could lead to enhanced case detection and reduced
transmission of highly infectious diseases such as COVID-19.