Factors associated with laboratory-confirmed measles cases during the 2023 and 2024 outbreaks in Tanzania—a mixed-methods study

Abstract

Background Despite the introduction of the two-dose measles-containing vaccine (MCV2) in 2014, measles remains a persistent public health problem in Tanzania, characterized by recurrent outbreaks and shifting epidemiological patterns. Understanding the epidemiology of measles in the country is therefore essential to inform context-specific elimination strategies. Methods During the 2023 and 2024 measles outbreaks, data and samples from the National Measles Epidemiological Surveillance System were used. Sera were processed to detect specific Measles Immunoglobulin M (IgM) antibodies using an indirect enzyme-linked immunosorbent assay. In addition, in-depth interviews and focus group discussions were conducted with key stakeholders and caregivers, respectively, to explore factors associated with measles outbreaks. Results A total of 17,902 suspected measles cases were enrolled; of these, 11,576 (64.7%) were confirmed measles cases. The overall incidence rate declined markedly from 163.9 cases per 1,000,000 population in 2023 to 15.4 cases per 1,000,000 population in 2024, with the highest incidence occurring among children aged <1 year. Of the total confirmed measles cases, 10,239 (57.2%), 1,311 (7.3%), and 26 (0.15%) were epi-linked, laboratory-confirmed, and clinically compatible, respectively. Higher proportions of confirmed measles cases were observed among children aged >5 years and infants <9 months, compared with children aged 12–59 months ( p < 0.001). A significantly high proportion of cases were enrolled from rural areas and were zero dose. Of 7,637 suspected measles cases tested, measles IgM antibodies were detected in 1,311 (17.2%) cases, with significantly more cases in 2023 than in 2024. Factors that were significantly associated with laboratory-confirmed measles cases in multivariate analysis were age groups 5 + years, under-vaccinated/unvaccinated, and unknown vaccination status. The estimated effectiveness of two doses in preventing laboratory-confirmed measles cases during the 2023 and 2024 outbreaks was 80.7%. Under-vaccinated and unvaccinated due to unavailability of vaccination services, stockouts, and missed opportunities were highlighted by stakeholders as contributing factors to measles outbreaks. Conclusion The study revealed that the vast majority of confirmed cases occurred among inadequately immunized children, with limited access to vaccination services, driven by multiple underlying factors, emerging as a key contributor to measles outbreaks. These findings highlight the importance of implementing context-specific strategies to close immunity gaps.