Cohort 2: Serological assessment in asymptomatic pregnant women
Samples were available for serological analysis in 598 women in the asymptomatic cohort. Anti-SARS-CoV-2 IgG antibodies were detected in 12 women using the Roche platform and ten women using the Alinity and Architect platform (Table S2). The seroprevalence rate using two independent assays was 1.67% (10/598 95% CI 0·8-3·1%). Anti-SARS-CoV-2 IgM antibodies were observed in twenty-one asymptomatic women (21/598 3.51% CI 2.3-5.5%) (Supplementary table S3a). Of these 21 women, 8 were also IgG positive (8/21, 38%) using both the Roche, Architect and Alinity platforms. Th majority of women who were antibody positive were > 24 weeks gestation. Demographics of all women are presented in supplementary table S4
Nine women from the asymptomatic cohort returned for follow up antibody testing between 101-122 days after the baseline test (Table 1). Four (4/9, 44%) remained IgM positive (Figure 2a). One woman was IgM positive only at baseline testing. Her follow up IgM antibodies were negative. In view of this, and the initial negative PCR and IgG anti SARS-CoV-2, it is likely that this also represents a false positive IgM result. The majority (6/7, 85.7%) of the seven women who were IgG positive at baseline no longer exhibited anti SARS-CoV-2 IgG antibodies (Figure 2b). Retrospective histories taken at follow-up suggests many of these women may have been mildly symptomatic at the time of the initial study