Seroprevalence of Immunoglobulin G against measles and rubella over a 12-year period (2009-2021) in Kilifi, Kenya and the impact of the Measles-Rubella (MR) vaccine campaign of 2016.

BACKGROUND

Measles and rubella have been targeted for elimination by the World Health Organization. Age-specific population immunity to measles and rubella is important to assess progress towards elimination but data are scarce. We conducted seroprevalence surveys to identify disease-specific population immunity profiles in children and adults in Kilifi.

METHODS

Sera from cross-sectional surveys in the Kilifi Health Demographic Surveillance System (2009-2021) were analysed using a fluorescent bead-based multiplex immunoassay. Bayesian multilevel regression with post stratification was used to obtain seroprevalence estimates adjusted for the underlying population and assay performance. Associations between seropositivity and age, sex, location and ethnic group were assessed using a mixed effects logistic regression.

RESULTS

Measles-adjusted seroprevalence showed a significant increase from 88 % in 2009 to 93 % in 2021 (τ = 0.875, P = 0.01). Seropositivity was significantly higher in all age groups compared to those under 9 months. Seroprevalence among children ineligible for the first measles vaccine dose (MCV1) remained low (10-57 %), whereas MCV1-eligible children (9-17 months) had higher seroprevalence (68-91 %). Adult measles seroprevalence exceeded 96 %. Rubella seroprevalence followed a similar pattern, with adults above 88 %. Following the MR campaign, measles seroprevalence increased from 92 % to 96 % in eligible children, while rubella seroprevalence rose from 45 % to 82 %.

CONCLUSION

Population immunity for measles significantly increased over the 12-year period suggesting improvement in immunisation program performance. To reduce reliance on frequent SIAs, efforts should focus on optimizing both the timing and coverage of routine doses, particularly ensuring higher coverage of MCV2. The introduction of rubella vaccination has positively impacted immunity in children. Sustaining this immunity is essential to prevent potential gaps in older age groups, which could increase the risk of Congenital Rubella Syndrome (CRS) in infants.