Nic Lochlainn, Laura Mde Gier, Brechjevan der Maas, Nicolinevan Binnendijk, RobStrebel, Peter MGoodman, Traceyde Melker, Hester EMoss, William JHahné, Susan J M2019-10-032019-10-032019-09-201474-44573154808110.1016/S1473-3099(19)30396-2http://hdl.handle.net/10029/623256Our search retrieved 1156 records and 85 were excluded due to duplication. 1071 records were screened for eligibility, of which 351 were eligible for full-text screening and 21 were eligible for inclusion in the review. From 13 studies, the pooled proportion of infants seropositive after two MCV doses, with MCV1 administered before 9 months of age, was 98% (95% CI 96-99; I2=79·8%, p<0·0001), which was not significantly different from seropositivity after a two-dose MCV schedule starting later (p=0·087). Only one of four studies found geometric mean titres after MCV2 administration to be significantly lower when MCV1 was administered before 9 months of age than at 9 months of age or later. There was insufficient evidence to determine an effect of age at MCV1 administration on antibody avidity. The pooled vaccine effectiveness estimate derived from two studies of a two-dose MCV schedule with MCV1 vaccination before 9 months of age was 95% (95% CI 89-100; I2=12·6%, p=0·29). Seven studies reporting on measles virus-specific cellular immune responses found that T-cell responses and T-cell memory were sustained, irrespective of the age of MCV1 administration. Overall, the quality of evidence was moderate to very low.enLancet Infect Dis 2019; 19(11):1246-54