Serological survey to determine measles and rubella immunity gaps across age and geographic locations in The Gambia: a study protocol

Abstract

Vaccine coverage and disease surveillance data are valuable for monitoring protection against vaccine-preventable diseases; however, they do not directly measure population immunity. High-quality, representative serological studies can provide key insights into immunity gaps, outbreak susceptibility, and inform targeted vaccination strategies, even in high-performing immunization programs. This study aims to estimate location-specific and age-specific immunity profiles for measles and rubella while evaluating the predictive value of indirect immunity estimates derived from vaccination and surveillance data against direct serological measurements. Additionally, it seeks to model the risk of measles outbreaks and assess the impact of mitigation strategies. A multi-stage, stratified cluster sampling design will be implemented across six districts in The Gambia's North Bank and Upper River Regions. Survey clusters (i.e. 5 km × 5 km areas) encompassing all settlements within their boundaries will be selected, proportionally to district population sizes. Cluster selection ensures representativeness of both the population and vaccine coverage within each district. Based on detecting a 10% difference in protective immunity across vaccine coverage levels, power analysis assumes an intraclass correlation coefficient (ICC) of 0.01. In each cluster, 70 children aged 9 months to 14 years will be recruited, yielding a total sample size of 1,750 children across 25 selected clusters. Dried blood samples will be collected and tested for anti-measles and anti-rubella IgG using enzyme immunoassays (EIA). District-specific measles seroprevalence will be estimated using a hierarchical spatial model. This study will generate key evidence needed to refine immunization strategies and reduce the risk of measles and rubella outbreaks.

Keywords: Measles; immunity; rubella; serology; serosurvey.

Figure 1.

(A) Annual measles incidence rate per million population in the Gambia, 2011–2023; (B) Vaccination status of confirmed measles cases in the Gambia, 2014–2023; (C) Measles case distribution by district and year across the seven health regions in the Gambia. Note: The red horizontal dashed line is the WHO threshold for measles elimination of ≤ 1 case per million population. Data source: Gambia measles case-based surveillance database. CRR = Central River Region, LRR = Lower River Region, NBER = North Bank East Region, NBWR = North Bank West Region, URR = Upper River Region, WR1 = Western Region 1, WR = Western Region 2.

Figure 2.

Spatial locations of villages/settlements in the Basse and Farafenni Health and demographic surveillance systems (HDSS) located in the Upper River and North Bank Region of the Gambia.

Figure 3.

The distribution of randomly selected serosurvey clusters across the (A) four districts in the Basse HDSS area and (B) two districts in the Farafenni HDSS area. Note: The green, yellow, and red tiles represent MCV1 coverage data at a 5 km by 5 km resolution based on The Gambia DHS (2019–2020) [33]. Blue dots indicate settlements/villages within the Basse and Farafenni HDSS sites, and red squares indicate the 25 selected 5 km by 5 km clusters.