Background: Measles continues to pose a significant public health challenge in Nigeria, despite global efforts to eliminate the disease. Emerging data suggest a shift in disease burden toward older age groups, especially in settings with suboptimal routine immunization and delayed introduction of the second dose of the measles-containing vaccine (MCV2). This study assessed the age distribution and risk of measles among older children and adolescents in Rivers State, Nigeria, from 2020 to 2022. Methods: A retrospective review of measles surveillance data from January 2020 to December 2022 was conducted. Only IgM-confirmed cases were analyzed. Cases were stratified by age group: <9 months, 9–59 months, 5–9 years, 10–14 years, and ≥15 years. Descriptive statistics summarized trends over time. Chi-square analysis was used to assess the association between age group and measles IgM positivity. Logistic regression was used to predict positivity, with age treated as a continuous variable. Results: A total of 958 confirmed measles cases were recorded. Children aged 5 years or older accounted for 55% of cases, while those aged 15 years or older constituted 23%. Vaccination history was undocumented or absent in most older children. The association between age group and measles positivity was statistically significant (Chi-square = 35.81, p < 0.05). Logistic regression failed due to perfect separation, indicating a strong relationship between age and measles positivity. Conclusion: Measles outbreaks in Rivers State disproportionately affect older children and adolescents, suggesting an expanding immunity gap. Immunization strategies should extend beyond under-fives to include school-aged children and adolescents through campaigns, defaulter tracking, and school-based entry checks to accelerate measles elimination.

Measles, Age Shift, Adolescents, Surveillance, Nigeria, Vaccine Coverage, MCV2, Chi-Square Analysis