Discussion
This study evaluated the performance of one rapid test and two ELISAs for detecting antibodies to SARS-CoV-2 in asymptomatic individuals in a slum community in Nairobi, Kenya, using capillary blood samples. Most evaluations of serological tests have been carried out by comparison with RT-PCR in both positive and negative COVID-19 cases. In a systematic review by Lisboa Bastos of serological test performance in 40 studies, pooled sensitivity of rapid tests was 66.0% (95% CI 49.3%−79.3%), and of ELISAs measuring IgG or IgM was 84.3% (95% CI 75.6%−90.9%) [6]. In all analyses, pooled sensitivity was lower for rapid tests, the potential point-of-care method; pooled specificity range was 96.6%−99.7%. Sensitivity of commercial rapid tests (49.0%−78.2%) was lower than of non-commercial tests (83.6%−91.3%). Sensitivity was higher at least three weeks after symptom onset (69.9%–98.9%) than in the first week (13.4%­–50.3%). Ghaffari in another systematic review including both rapid tests and ELISAs reported greater variability in sensitivity than in specificity, and suggested serological tests were more effective in the later stages of disease [7].
Typically in response to viral infection, IgM is produced first, with a later switch to IgG production for long-term immune memory [8]. However, one study showed no statistical difference for IgM or IgG seropositivity between testing samples taken from PCR-confirmed COVID-19 cases between 9–17 days and 18–29 days [9]; studies of SARS-associated coronaviruses suggest IgM and IgG often develop around the same time [10,11]. It may therefore not be possible to judge recency of infection in this population group. Studies have also shown IgM and IgG levels are significantly higher in severe COVID-19 cases than in patients with mild or moderate disease [12], suggesting serological tests require high sensitivity to detect antibodies in mild or asymptomatic cases. A Cochrane meta-analysis of antibody studies concluded there was no certainty about how well the tests would work in asymptomatic or milder disease cases [13].
The rapid test showed fair (0.32) and Platelia ELISA moderate (0.6) agreement with the Wantai ELISA. As with other studies, specificity of the rapid test was high (93.42%) but sensitivity lower (61.33%); the Platelia ELISA similarly showed good specificity (93.85%) but poorer sensitivity (83.39%). The number of vaccinated individuals was too small to influence the results. More accurate point-of-care tests for field-based screening for SARS-CoV-2 exposure in population surveys are needed.