Serologic Testing Helps Scientists Connect Dots on COVID-19 Clusters

Active case-finding and contact tracing were undertaken for all COVID-19 cases in the region. Diagnosis for acute disease was confirmed with RT-PCR testing. When epidemiological information suggested that people might have been nodes of disease transmission but had recovered from illness, SARS-CoV-2 IgG serology testing was used to establish past infection.

The epidemiological study shows how SARS-CoV-2 serological analysis (ELISA detecting IgG and VNT detecting neutralizing antibodies), in addition to the use of traditional epidemiological methods, was important in establishing links among locally transmitted COVID-19 cases and tracing the transmission chain to an imported source. Three clusters of COVID-19, comprising 28 locally transmitted cases, were identified in Singapore; these clusters were from two churches (Church A and Church B) and a family gathering. The clusters in Church A and Church B were linked by an individual from Church A (A2), who transmitted SARS-CoV-2 infection to the primary case from Church B (F1) at a family gathering they both attended on January 25, 2020. All cases were confirmed by RT-PCR testing because they had active disease, except for A2, who at the time of testing had recovered from their illness and tested negative. This individual was eventually diagnosed with past infection by serological testing. ELISA assays showed an optical density of more than 1·4 for SARS-CoV-2 nucleoprotein and receptor binding domain antigens in titres up to 1/400, and viral neutralization was noted in titres up to 1/320.

As shown in the study, serological testing enabled the detection of a convalescent case, which could be the key to initial containment efforts to discover transmission links to support containment efforts. Serological testing also detects people with mild or asymptomatic disease who have recovered, allowing for more accurate determination of the number of people probably infected in a cluster or the population. Identifying people who were probably infected in household or school clusters could help ascertain attack rates by age, particularly among children who mostly manifest less severe disease.