Background Many studies assume that the serologic correlate of protection from measles disease is 120 mIU/mL. We systematically reviewed the literature to examine the evidence supporting this correlate of protection. Methods We searched peer-reviewed and gray literature for articles reporting a measles correlate of protection. We excluded studies focusing on special populations, infants aged <9 months, and those using animal models or nonstandard vaccines or administration routes. We extracted and synthesized data from full-text articles that met inclusion criteria. Results We screened 14 778 articles and included 5 studies in our review. The studies reported either preexposure antibody concentrations of individuals along with a description of symptoms postexposure, or the proportion of measles cases that had preexposure antibody concentrations above a threshold of immunity specified by the authors. Some studies also described secondary antibody responses upon exposure. The variation in laboratory methods between studies made comparisons difficult. Some of the studies that assumed 120 mIU/mL as a correlate of protection identified symptomatic individuals with preexposure titers exceeding this threshold. Conclusions Our findings underscore the scant data upon which the commonly used 120 mIU/mL measles threshold of protection is based, suggesting that further work is required to characterize the measles immunity threshold.

Background We conducted a systematic review to assess whether measles humoral immunity wanes in previously infected or vaccinated populations in measles elimination settings. Methods After screening 16,822 citations, we identified nine articles from populations exposed to wild-type measles and 16 articles from vaccinated populations that met our inclusion criteria. Results Using linear regression, we found that geometric mean titers (GMTs) decreased significantly in individuals who received two doses of measles-containing vaccine (MCV) by 121.8 mIU/mL (95% CI -212.4, -31.1) per year since vaccination over one to five years, 53.7 mIU/mL (95% CI -95.3, -12.2) five to ten years, 33.2 mIU/mL (95% CI -62.6, -3.9) ten to 15 years, and 24.1 mIU/mL (95% CI -51.5,3.3) 15 to 20 years since vaccination. Decreases in GMT over time were not significant after one dose of MCV or after infection. Decreases in the proportion of seropositive individuals over time were not significant after one or two doses of MCV, or after infection. Conclusions Measles antibody waning in vaccinated populations should be considered in planning for measles elimination.