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SARS-CoV-2 seroprevalence and transmission risk factors among high-risk close contacts: a retrospective cohort study

Our take —

This study sought to estimate the transmission rate and risk factors for SARS-CoV-2 infection among close contacts of all COVID-19 cases in Singapore. Using detailed questionnaires and contact-tracing methods, they identified 1,114 index cases and 7,770 contacts from January 23 to April 3, 2020. They found that household contacts had the highest clinical attack rate (5.9%) compared to work contacts (1.3%) and social contacts (1.3%), though after accounting for asymptomatic cases, the estimated secondary infection rate increased to 11% for household contacts, 4% for social contacts, and 5% for work contacts. Being spoken to by (regardless of masks or being indoors) an index case for any amount of time and sharing a bedroom with an index case were associated with transmission. This study included all people in Singapore, representing an important contribution to population-based estimates of transmission.

Study design

Retrospective Cohort

Study population and setting

The study objective was to estimate the transmission risks of COVID-19 given close household contacts. The study was conducted from January 23 to April 3, 2020, and included contact-tracing of all confirmed COVID-19 cases in Singapore. Household contacts were defined as living with the index case, while non-household contacts were defined as having contact for 30 minutes or more within 2 meters of the index case (indoors or outdoors, regardless of mask use). Contacts were all required to quarantine for 14 days and complete symptom monitoring 3 times per day via telephone and received PCR testing for SARS-CoV-2 if they reported symptoms. If they did not report symptoms or received a negative PCR test after reporting symptoms, they were eligible to also receive a serological test and complete further risk assessments. The study used Bayesian models to estimate asymptomatic cases and missed diagnoses.

Summary of Main Findings

From January 23 to April 3, 2020, there were 1,114 index cases identified, and 7,770 close contacts traced. These included 1,863 household contacts, of which 2,319 were contacts from work and 3,588 were contacts from social gatherings. Using a symptoms-based testing strategy, 188 (2.42%) secondary cases were identified among contacts, which, when limited to those with complete data available, reflected a clinical attack rate of 5.9% (95% CI: 4.9 – 7.1%) for household contacts, 1.3% for work contacts (0.9 – 1.9%), and 1.3% (1.0 – 1.7%) for social contacts. Bayesian modeling coupled with serological testing of all individuals who completed the quarantine without symptoms or a positive PCR test estimated that 62% (95% CrI: 55 – 69%) of COVID diagnoses were missed under this testing strategy, and 36% (95% CrI: 27 – 45%) of infected individuals were asymptomatic. Among participants who completed a detailed risk assessment, household contacts who shared a bedroom with a case (OR: 5.38, 95% CI: 1.82 – 15.84) was associated with transmission; household, work, and social contacts who were spoken to by an infected person for any amount of time (ORs: 2.5 – 7.86) were also significantly associated with transmission. Meal-sharing, lavatory co-usage, and indirect contact were not significantly associated with infection.

Study Strengths

The study was able to model and estimate the proportion of missed diagnoses and asymptomatic infections, despite not actively testing all asymptomatic cases. These modeled estimates allowed them to estimate the true infection rate that may be occurring. Additionally, because cases were legally required to quarantine and report symptoms, there was a high rate of participation and large proportion of respondents with complete data. Using PCR testing on all symptomatic contacts increased the validity of these results by minimizing false negatives and positives and providing a high degree of accuracy, especially when coupled with the subset of serological testing. Their risk assessment also included 70-items to pinpoint specific types of exposure that may have occurred, adding granularity to these data.


As the study noted and estimated, a number of cases were likely missed under this testing schema, which highlights the importance of testing regardless of symptom-status, in order to not miss asymptomatic cases. Singapore had a number of strict infection protocols that individuals were legally required to adhere to, and therefore these results may not be generalizable to other contexts where prevention measures are less common. For instance, it is not standard medical protocol that everyone who tests positive for SARS-CoV-2 infection receives inpatient hospital care, which will likely reduce the rate of transmission.

Value added

This study examined household and non-household contacts among all Singapore COVID-19 cases.

This review was posted on: 14 January 2021