Title : Seroprevalence of measles-specific IgG and neutralizing antibodies in clinically-confirmed cases from the ongoing measles outbreak in Liberia
Abstract:
Background: Measles Virus (MeV) causes a highly contagious disease (MeV) associated with significant morbidity and mortality. Despite the availability of an effective two-dose vaccine regimen, measles continues to be a substantial public health problem globally. This study assessed the seroprevalence of MeV antibodies in clinically confirmed patients and evaluated the neutralization breadth of MeV-specific IgG in an ongoing measles outbreak in Liberia.
Methods: The study used samples collected both retrospectively and prospectively from 199 clinically confirmed MeV cases, based on WHO criteria. Samples were collected from 2021 to 2023. The samples were tested for measles-specific IgM and IgG antibodies using an enzyme-linked immunosorbent. We generated vesicular stomatitis virus pseudotypes of circulating MeV genotypes in Africa, namely Edmonston, B3, D4, and D8 MeV genotypes, incorporating the hemagglutinin and fusion glycoproteins. Strong neutralization was defined by a dilution of neutralizing titre that was set at a concentration resulting in a 90% reduction in infectivity. The clinically confirmed MeV cases were characterized as vaccinated and non-vaccinated individuals, and associations between IgG status, age, vaccination record, neutralizing titers, and IgG levels were determined, with statistical significance set at p < 0.05.
Results: We tested 199 samples, revealing that 69.9% (95% CI 63.5-76.3) of individuals had MeV IgG antibodies. Seropositivity was higher in vaccinated individuals (52.5%) than in non-vaccinated individuals (8.6%). We found no association between IgG titre and age. We found that 82.4% of all participants had strong neutralizing titers against B3, while 80.2% of those tested had strong neutralization titers against Edmonston, 70.6% against D4, and 80.1% against D8. We observed significant differences in neutralizing titers between protected and non-protected individuals for all four genotypes (p < 0.0001 in all cases).
Conclusion: We found that during an active measles outbreak in Liberia, only 69.9% of clinically confirmed cases were IgG seropositive. Genotypes of the current outbreak have not been determined; however, our participants showed strong neutralization (>80%) of all MeV genotypes tested, except for D4 (70%). Previous vaccination status was low in our population. Furthermore, participants over the age of 10 years had higher average seropositivity than those less than 10 years of age. Overall, the results indicate effective immunity as well as a potential immunity gap, as seropositivity was below the threshold for herd immunity, potentially increasing outbreak risk. It further supports the continued use and expansion of vaccination programs, as vaccinated individuals had higher immunity compared to non-vaccinated individuals.
