High SARS-CoV-2 seroincidence but low excess COVID mortality in Sierra Leone in 2020-2022

Abstract

While SARS-CoV-2 infection appears to have spread widely throughout Africa, documentation of associated mortality is limited. We implemented a representative serosurvey in one city of Sierra Leone in Western Africa, paired with nationally representative mortality and selected death registration data. Cumulative seroincidence using high quality SARS-CoV-2 serological assays was 69% by July 2021, rising to 84% by April 2022, mostly preceding SARS-CoV-2 vaccination. About half of infections showed evidence of neutralizing antibodies. However, excess death rates were low, and were concentrated at older ages. During the peak weeks of viral activity, excess mortality rates were 22% for individuals aged 30-69 years and 70% for those over 70. Based on electronic verbal autopsy with dual independent physician assignment of causes, excess deaths during viral peaks from respiratory infections were notable. Excess deaths differed little across specific causes that, a priori, are associated with COVID, and the pattern was consistent among adults with or without chronic disease risk factors. The overall 6% excess of deaths at ages ≥30 from 2020-2022 in Sierra Leone is markedly lower than reported from South Africa, India, and Latin America. Thus, while SARS-CoV-2 infection was widespread, our study highlights as yet unidentified mechanisms of heterogeneity in susceptibility to severe disease in parts of Africa.

Fig 1. Timeline of reported COVID infections and cumulative number of people who received two SARS-CoV-2 vaccine doses in Sierra Leone.

Data sources and indicator descriptions: Our World In Data: COVID infections, 7-day rolling averages, and total number of people who received the second dose of an authorized 2-dose vaccine; *duration of HEAL-SL fieldwork: May 2019 to January 2021 (Round 1), April 2021 to October 2022 (Round 2); vertical arrows represent serosurvey periods.

Fig 2. Correlation of titers of receptor binding domain (RBD) assay with neutralizing antibody (top panel) and Relative ratio (RR) by age and distributions of neutralizing and non-neutralizing antibody (bottom panel), in Bo district, Sierra Leone 2021 and 2022.

The top panel illustrates the relationship between inhibitory capacity and the log 10 median of the ratio of raw values for the RBD IgG antibody response, based on 2.5 ul/well sample dilution. A cut off of 20% inhibitory capacity separates neutralizing from non-neutralizing antibodies, and a log10(0.324) cut-off separates positive from negative IgG RBD antibody responses. The relationship is fit using locally estimated scatterplot smoothing and assessed using a Pearson correlation coefficient. The bottom panel illustrates the inhibitory capacity in relation to neutralizing ability.

Fig 3. Age-specific mortality rate per 100,000 study population in Sierra Leone 2020–2022 from HEAL-SL (top panel) and registered deaths from NCRA (bottom panel).

Data sources: Healthy Sierra Leone (HEAL-SL) rounds 1 and 2, National Civil Registration Authority (NCRA); mortality rates from HEAL-SL (2-month rolling averages) were calculated by summing the age-specific study deaths by week and dividing by the total study population enumerated in the same age group during the week of the reported deaths and afterwards. Total registered deaths from NCRA (both sexes): 19,277 in 2020 (7264 at 30–69 years, 3796 at ≥70 years), 19,114 in 2021 (7487 at 30–69 years, 3850 at ≥70 years) and 15,507 in 2022 (6378 at 30–69 years, 2834 at ≥70 years).