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Review 1: "SARS-CoV-2 Antibody Prevalence in Sierra Leone, March 2021: A Cross-sectional, Nationally Representative, Age-Stratified Serosurvey"

Published onApr 20, 2022
Review 1: "SARS-CoV-2 Antibody Prevalence in Sierra Leone, March 2021: A Cross-sectional, Nationally Representative, Age-Stratified Serosurvey"
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key-enterThis Pub is a Review of
SARS-CoV-2 antibody prevalence in Sierra Leone, March 2021: a cross-sectional, nationally representative, age-stratified serosurvey

ABSTRACTBackgroundAs of 26 March 2021, the Africa CDC had reported 4,159,055 cases of COVID-19 and 111,357 deaths among the 55 African Union Member States; however, no country has published a nationally representative serosurvey as of May 2021. Such data are vital for understanding the pandemic’s progression on the continent, evaluating containment measures, and policy planning.MethodsWe conducted a cross-sectional, nationally representative, age-stratified serosurvey in Sierra Leone in March 2021 by randomly selecting 120 Enumeration Areas throughout the country and 10 randomly selected households in each of these. One to two persons per selected household were interviewed to collect information on socio-demographics, symptoms suggestive of COVID-19, exposure history to laboratory-confirmed COVID-19 cases, and history of COVID-19 illness. Capillary blood was collected by fingerstick, and blood samples were tested using the Hangzhou Biotest Biotech RightSign COVID-19 IgG/IgM Rapid Test Cassette. Total seroprevalence was was estimated after applying sampling weights.FindingsThe overall weighted seroprevalence was 2.6% (95% CI 1.9-3.4). This is 43 times higher than the reported number of cases. Rural seropositivity was 1.8% (95% CI 1.0-2.5), and urban seropositivity was 4.2% (95% CI 2.6-5.7).InterpretationAlthough overall seroprevalence was low compared to countries in Europe and the Americas (suggesting relatively successful containment in Sierra Leone), our findings indicate enormous underreporting of active cases. This has ramifications for the country’s third wave (which started in June 2021), where the average number of daily reported cases was 87 by the end of the month—this could potentially be on the order of 3,700 actual infections, calling for stronger containment measures in a country with only 0.2% of people fully vaccinated. It may also reflect significant underreporting of incidence and mortality across the continent.FundingThis study was supported by NIAID K08 AI139361, the Sierra Leone Ministry of Health and Sanitation, and the Africa CDC.

RR:C19 Evidence Scale rating by reviewer:

  • Reliable. The main study claims are generally justified by its methods and data. The results and conclusions are likely to be similar to the hypothetical ideal study. There are some minor caveats or limitations, but they would/do not change the major claims of the study. The study provides sufficient strength of evidence on its own that its main claims should be considered actionable, with some room for future revision.



In this manuscript, Barrie et al. describe a cross-sectional, age-stratified nationally representative serosurvey to estimate SARS-CoV-2 antibody prevalence in Sierra Leone in March 2021. This is an important study for understanding the extent of SARS-CoV-2 spread and, at the time of writing, no nationally representative serosurvey had yet been published by an African Union Member State. The authors employed a randomized, multi-stage sampling framework. Since the majority of Sierra Leone’s population is young, they intentionally oversampled adults so that they could compare seroprevalence between similar numbers of individuals within each age strata (5-9, 10-19, 20-39, 40-59 and ≥60 years old). They used a rapid serological test to screen for presence of SARS-CoV-2-specific IgG and IgM antibodies. They found that, after post-stratifying by district population, seroprevalence was 2.6% (95% CI 1.9-3.4), 43 times higher than the reported number of COVID-19 cases. They also found higher seroprevalence in urban compared to rural areas. Seroprevalence was highest among individuals aged 40-59, although differences by age were not significant.

The two key conclusions from this work, which are supported by the results, are:

1) An enormous number of SARS-CoV-2 infections in Sierra Leone have gone (and likely continue to go) unreported.

2) A large proportion of the population remains susceptible to SARS-CoV-2 infection.

The main limitation of this study, which the authors discuss at the end of the manuscript, is that it is unknown how sensitive the serological test is for identifying mild/asymptomatic infections, or infections that occurred earlier on in the pandemic and for which antibody levels have waned. These limitations have likely resulted in under-estimating seroprevalence, and it’s hard to say by how much given currently available information.

I have a few additional questions concerning the methods and results:

1) The authors performed their own validation of the Hangzhou Biotest Biotech RightSign COVID-19 Rapid Test to detect anti-SARS-CoV-2 IgG and IgM antibodies using a small control panel. This is important, and the results are encouraging, but additional details are needed for interpretation. Were the 10 negative samples from RT-PCR negative individuals in Sierra Leone with no known history of COVID-19? Were the 8 positive samples from RT-PCR positive individuals in Sierra Leone, did they have symptoms, and if so, how long were serum samples taken after symptom onset?

2) It was not clear to me from the methods exactly how the age-stratified sampling was done, and how that might have biased the seroprevalence estimates. In a future revision of the manuscript, it would be helpful to have more details there. For example, how often were one vs. two individuals selected per household, and how was this handled in households with multiple eligible participants in each age stratum?

3) How did seroprevalence vary by the other risk factors listed in Table 1 (e.g., number of people per household, high-risk occupations, and current smoker)?

Overall, I do not expect the answers to the questions above to change the main conclusions of this study. The implications of these data, as described by the authors, are clear: in addition to strong containment measures, there is critical need for access to and rapid distribution of SARS-CoV-2 vaccines in Sierra Leone.

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