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.