Paired serological testing of rubella IgG: monitoring antibody stability over time.

Abstract

Background: Rubella immunity plays a key role in preventing congenital rubella syndrome, but longitudinal data on antibody stability remain limited.

Objective: To evaluate rubella IgG seroprevalence, seroconversion, and changes in rubella IgG concentrations over time.

Methods: We conducted a retrospective cohort study of 2,022 consecutive individuals from Region Zealand, Denmark, each with at least two serum samples submitted for routine rubella IgG testing between 11 September 2018 and 30 May 2025. Pairedsamples from each individual were analysed.

Results: At baseline, 88.6% of participants were seropositive, increasing to 90.6% at follow-up. The cohort was predominantly female (99.4%) and mean age at first sample was 29.8 years. Among initially seronegative individuals, 75 seroconverted, while 34 individuals initially seropositive seroreverted. Median IgG levels among seroconverts were 23.5 IU/mL (range 10-350 IU/mL). Individuals who remained seropositive showed a modest yet statistically significant median decrease of 8.4% between the first and second samples (Hodges-Lehmann ratio = 0.92, 95% CI: 0.91-0.93, p < 0.0001), with minimal association between antibody change and time interval (Spearman's ρ = -0.058, p = 0.015).

Conclusions: In this cohort of routinely tested individuals, rubella IgG levels remained broadly stable, indicating sustained rubella seropositivity over time. Seroconversion and seroreversion occurred in a small subset of individuals, supporting the continued monitoring of rubella immunity, particularly among women of reproductive age. Maintaining high vaccination coverage remains important to sustain rubella elimination and prevent congenital rubella syndrome.