Purpose <p>Measles outbreaks have occurred across England since mid-2023. We estimated measles and rubella antibody seroprevalence among individuals in 2018 in English regions outside London, and estimated the effective reproduction number (Re) for measles to predict the potential for outbreaks.</p> Methods <p>Using validated enzyme-linked immunosorbent assays, anti-Measles and anti-Rubella IgG antibodies were measured in residual sera from 3758 1-30-year-olds born after introduction of measles-mumps-rubella vaccination who submitted samples to clinical laboratories outside London. The measles Re was calculated using seronegatives defined by the manufacturer’s cutoff, mixture modelling, and vaccination coverage data.</p> Results <p>Using the manufacturer’s cutoffs, the overall proportion seronegative to measles was 9.2% (95% confidence interval 8.3–10.1), and 10.3% (9.4–11.3) had equivocal results. The respective estimates for rubella were lower at 5.2% (4.6-6.0) and 5.4% (4.7–6.1). For both viruses, equivocal proportions increased with age, consistent with antibody waning. Mixture modelling for measles identified a common seronegative distribution across age groups, with lower proportions seronegative than using the manufacturer’s cutoff. Re for measles using the manufacturer’s seronegative cutoff (~ 150 mille international units/mL) was 1.00, versus 0.38 and 0.51 using the mixture model and vaccination coverage, respectively.</p> Conclusions <p>Re for measles estimated from seroepidemiology using an antibody cut-off similar to that considered a correlate of protection for measles was a more accurate predictor of recent measles resurgences outside London than those estimated using mixture modelling of seronegatives or coverage data. Seroepidemiological studies are a useful adjunct to coverage data in monitoring population immunity and in predicting the potential for measles outbreaks.</p>