Bridging the Gap: Measles Susceptibility and Neutralization Test-Confirmed Immunity in a Spanish HIV-PrEP Cohort.

BACKGROUND: Measles is re-emerging in highly vaccinated settings, where small gaps in adult immunity may persist. Standard IgG assays are widely used to assess measles immunity, but negative or borderline results may not reflect absence of functional protection. HIV pre-exposure prophylaxis (HIV-PrEP) programs provide regular preventive-care contact and may offer an opportunity to review adult measles immunity. We assessed measles immunity in a Spanish HIV-PrEP cohort using chemiluminescence immunoassay (ChLIA) and neutralization testing (NT). METHODS: We performed a cross-sectional analysis of 155 participants enrolled in a prospective HIV-PrEP cohort in Córdoba, Spain. Measles-specific IgG was measured by ChLIA. All ChLIA-negative or borderline samples, together with 30 randomly selected ChLIA-positive samples, underwent NT. Confirmed susceptibility was defined as ChLIA-negative/borderline and NT negative. RESULTS: Among participants, 149 (96.1%) were male and the median age was 37 years. ChLIA classified 125 participants (80.6%) as IgG-positive, 27 (17.4%) as negative, and 3 (1.9%) as borderline. Among 30 ChLIA-negative/borderline samples, 23 (76.7%) were NT positive and 7 (23.3%) NT negative. All 30 randomly selected ChLIA-positive samples were NT positive. Overall, 7 participants (4.5%) lacked confirmed measles immunity. All were Spanish-born men aged 26-39 years, and 4 had documented 2-dose measles-mumps-rubella vaccination completed at least 20 years before inclusion. CONCLUSIONS: Confirmed measles susceptibility was uncommon in this HIV-PrEP cohort. Most participants with negative or borderline ChLIA results retained neutralizing antibodies, indicating that isolated ChLIA findings may overestimate susceptibility. Routine adult preventive-care encounters may provide opportunities to review measles immunity documentation.