By Carol A. Kemper, MD, FACP

Clinical Associate Professor of Medicine, Stanford University, Division of Infectious Diseases, Santa Clara Valley Medical Center

SOURCE: Ng OT, Marimuthu K, Koh V, et al. SARS-CoV-2 seroprevalence and transmission risk factors among high-risk close contacts: A retrospective cohort study. Lancet 2021;21:333-343.

Early on during the COVID-19 pandemic, Singapore adopted a comprehensive approach to prevention, diagnosis, and management of COVID-19, with clear guidance for the community and recommendations for mask wearing and social distancing. A network of 800 public health and community clinics was activated to quickly perform contact tracing and quarantine of contacts in the home and in the workplace and to test contacts who developed symptoms, with a medical leave plan for those who became ill. From Jan. 23 to April 3, 2020, 13,026 close contacts were identified, including 1,863 household contacts (with 578 distinct contact groups), 2,319 work contacts (with 225 distinct contact groups), 3,588 social contacts (with 346 distinct contact groups), 2,626 transportation contacts, and 2,630 other contacts.

Of these, 468 household contacts, 332 work contacts, and 458 social contacts were polymerase chain reaction (PCR) tested based on the presence of symptoms. A total of 188 cases were identified as secondary cases based on symptom-driven PCR screening, and another 7,582 completed quarantine without a COVID-19 diagnosis. Based on symptom-based screening, the secondary attack rate was 5.9% for household contacts, 1.3% for work contacts, and 1.3% for social contacts. Cases clearly clustered together within certain households and a few work groups; 86.3% of household contact groups and 91.6% of work contact groups had no apparent secondary cases based on symptom-based PCR screening. Convalescent serologic testing was performed in 30% of household contacts, 9% of work contacts, and 11.8% of social contacts who completed quarantine without a COVID-19 diagnosis. An additional 5.5% of household contacts, 2.5% of work contacts, and 2.1% of social contacts were identified as secondary cases based on positive serology. Among these, two-thirds were asymptomatic and one-third had developed symptoms but had tested SARS-CoV-2 PCR negative.

Activities that increased the risk of transmission included sharing a bedroom, sharing a vehicle, or being spoken to by a COVID-19 PCR-positive person for > 30 minutes. Indirect contact, sharing objects or equipment, sharing a bathroom, and sharing a meal were not associated with SARS-CoV-2 transmission.

Secondary transmission was much more likely for individuals within households (11.4%) than in the workplace (3.8%) or social situations (3.4%), and infections clearly clustered in some households and a few workplaces.

Efforts to control secondary transmission should be given to households and those contact groups where any case of secondary transmission has been identified. Symptom-based PCR screening of contacts missed nearly half of those who developed secondary infection. At least one-third of secondary transmission cases remained asymptomatic.