Intense and Mild Wave of COVID-19 in The Gambia: a Cohort Analysis

BackgroundThe SARS-CoV-2 pandemic is evolving differently in Africa compared to other regions, with lower transmission and milder clinical presentation. Reasons for this are not fully understood. Recent data from Eastern and Southern Africa suggest that transmission may be higher than measured. Detailed epidemiological data in different African settings is urgently needed. MethodsWe calculated cumulative rates of SAR-CoV-2 infections per 1,000 people at risk in The Gambia (2.42 million individuals) using publicly available data. We evaluated these rates in a cohort of 1,366 employees working at the MRC Unit The Gambia @LSHTM (MRCG) where systematic surveillance of symptomatic cases and contact tracing was implemented. Cumulative rates among the Gambian population were stratified by age groups and, among MRCG staff, by occupational exposure risk. SARS-CoV-2 testing was conducted on oropharyngeal/nasopharyngeal samples with consistent sampling and laboratory procedures across cohorts. FindingsBy September 2020, 3,579 cases of SARS-CoV-2 and 115 deaths had been identified; with 67% of cases detected in August. Among them, 191 cases were MRCG staff; all of them were asymptomatic/mild, with no deaths. The cumulative incidence rate for SARS-CoV-2 infection among MRCG staff (excluding those with occupational exposure risk) was 129 per 1,000, at least 20-fold higher than the estimations based on diagnosed cases in the adult Gambian population. InterpretationOur findings are consistent with recent African sero-prevalence studies reporting high community transmission of SAR-CoV-2. Enhanced community surveillance is essential to further understand and predict the future trajectory of the pandemic in Africa.