Seroprevalence of measles-specific IgG and genotype-specific neutralizing antibody responses in clinically confirmed cases during the 2021–2023 measles outbreaks in Liberia

Background Since 2021, Liberia has experienced disruptive outbreaks of measles across all of its 15 counties, despite the availability of a safe, effective vaccine. There is no data on the potential role of vaccine-induced immunity and no study on seroprevalence of measles-specific IgG in Liberia. Furthermore, there is no data on genotype-specific variation in antibody effectiveness in Liberia. This study evaluated the seroprevalence of measles virus (MeV)-specific IgG in clinically diagnosed patients in Liberia and assessed the breadth of neutralization against four genotypes. Methods Samples collected retrospectively from 2021 to 2023 and prospective samples collected in 2023 from clinically diagnosed measles cases were analyzed in this study. Study participants included both vaccinated and unvaccinated individuals. MeV-specific IgG antibody was assessed using an enzyme-linked immunosorbent assay. Neutralizing antibodies against the Edmonston, B3, D4, and D8 MeV genotypes were measured using vesicular stomatitis virus pseudotype-based neutralization assays. Results Overall, 69.9% of 199 individuals, including 100 vaccinees that were clinically diagnosed with measles, tested positive for MeV-specific IgG antibodies. MeV-specific IgG positivity was higher in vaccinated (52.5%) than in unvaccinated individuals (8.6%). Strong neutralization was observed against the D8 genotype in 98.9% of seropositive individuals, compared to 94.7% against Edmonston and D4 genotypes and 91.6% against B3. Significantly higher neutralization titers were observed in vaccinated compared to unvaccinated individuals for all four genotypes (p < 0.0001). There was a weak positive correlation between IgG levels, neutralizing titers, and participant age. Conclusion The findings demonstrate strong genotype-cross-neutralizing immunity among vaccinated and seropositive patients, highlighting the critical role of measles vaccination in maintaining effective antibody protection. The significant percentage of seronegative patients indicates ongoing vulnerability to measles transmission and reinforces the need to improve vaccination coverage.