Characterising the evolving SARS-CoV-2 seroprevalence in urban and rural Malawi between February 2021 and April 2022: a population-based cohort study

ObjectivesTo investigate the changing SARS-CoV-2 seroprevalence and associated health and sociodemographic factors in Malawi between February 2021 and April 2022.MethodsFour three-monthly serosurveys were conducted within a longitudinal population-based cohort in rural Karonga District and urban Lilongwe, testing for SARS-CoV-2 S1 IgG antibodies using an enzyme-linked immunosorbent assay. Population seroprevalence was estimated in all and unvaccinated participants. Bayesian mixed effects logistic models estimated the odds of seropositivity in the first survey, and of seroconversion between surveys, adjusting for age, sex, occupation, location, and assay sensitivity/specificity.ResultsOf 2005 participants (Karonga,n=1005;Lilongwe,n=1000), 55.8% were female and median age was 22.7 years. Between Surveys 1-4, population-weighted SARS-CoV-2 seroprevalence increased from 26.3% to 89.2% and 46.4% to 93.9% in Karonga and Lilongwe, respectively. At Survey 4, seroprevalence did not differ by COVID-19 vaccination status in adults, except for those aged 30+ in Karonga (unvaccinated:87.4%,95% credible interval 79.3-93.0%; 2 doses:98.1%,94.8-99.5%). Location and age were associated with seroconversion risk. Individuals with hybrid immunity had higher SARS-CoV-2 seropositivity and antibody titres, than those infected.ConclusionsHigh SARS-CoV-2 seroprevalence combined with low morbidity and mortality indicate that universal vaccination is unnecessary at this stage of the pandemic, supporting change in national policy to target at-risk groups.