Seroprevalence of IgG Antibodies to Measles, Mumps, Rubella and Varicella in Adolescents with Documented Two-Dose MMR Vaccination: Cross-Sectional Study.

Background: Measles, mumps, and rubella outbreaks continue to occur despite high vaccination coverage, raising concerns about long-term vaccine-induced immunity. This study assessed serological protection against measles, mumps, rubella, and varicella in fully vaccinated Russian adolescents to identify immunity gaps and inform revaccination strategies. Methods: Serum IgG was quantified by commercial ELISA. Analytical cut-offs were: measles ≥ 0.18 IU/mL, rubella ≥ 30 IU/mL, mumps index ≥ 1.0 (index = OD/0.467), and varicella ≥ 1.0 IU/mL; equivocal results were classified as negative in the primary analysis and as positive in a sensitivity analysis. Geometric mean concentrations (GMC) were computed on the natural-logarithm scale. Results: Seropositivity differed markedly between infections (χ2 55.1; p < 0.001): rubella 81.4% (95% CI 74.6-86.7), varicella 57.7% (95% CI 49.8-65.2), mumps 53.2% (95% CI 45.4-60.9) and measles 41.0% (95% CI 33.6-48.9). GMC was lowest for measles (0.132; 95% CI 0.111-0.158) and highest for rubella (61.9; 95% CI 55.3-69.2). The measles estimate was sensitive to the treatment of equivocal results, rising to 53.2% (95% CI 45.4-60.9) at the manufacturer's cut-off of ≥0.12 IU/mL; rubella rose to 94.9% at ≥16 IU/mL. Conclusions: Lower seropositivity rates for measles and mumps were identified among fully vaccinated adolescents, which fall below standard epidemiological benchmarks. In contrast, rubella seropositivity was robust at 81.4%. Because the measles estimate depended strongly on the assay cut-off applied, and because enzyme immunoassays are known to underestimate seroprevalence relative to neutralization testing, these findings describe the distribution of circulating antibody rather than the prevalence of susceptibility and warrant confirmation in larger representative samples with neutralization testing.