Background
Measles is a highly contagious viral disease. Vaccinated mothers transfer fewer antibodies during pregnancy, resulting in shortened infant immunity. Earlier primary vaccination might avert the gap in protection.

Methods
Healthy 5–7-month-old Danish infants were assigned in a 1:1 ratio to M-M-RVaxPro or placebo (solvent) in a double-blind, randomized trial between April 15, 2019 and November 1, 2021 (ClinicalTrials.gov NCT03780179, EudraCT 2016-001901-18). Eligibility criteria were birth weight >1000 g and gestational age ≥32 weeks.

Immunogenicity was measured by plaque reduction neutralization test (PRNT) and IgG ELISA before intervention, four weeks after intervention and routine MMR. Reactogenicity data were collected for six weeks and measured by hazard ratios (HR).

Findings
647 and 6540 infants participated in the immunogenicity and reactogenicity study, respectively; 87% and 99% completed follow-up. After early MMR, seroprotection rates (SPRs) were 47% (13%) in measles PRNT; 28% (2%), 57% (8%) in mumps and rubella IgG (placebo). For measles PRNT, geometric mean ratio was 4.3 (95% CI: 3.4–5.3) between randomization groups after intervention and 1.5 (95% CI: 1.3–1.9) after routine MMR.

Reactogenicity was independent of randomization (HR, 1.0; 95% CI: 0.9–1.1). Severe adverse events occurred in 25 infants (HR, 1.8; 95% CI: 0.8–4.0); none deemed vaccine related.

Interpretation
Early MMR elicited low SPRs but did not negatively impact short-term responses to a subsequent MMR. MMR at 5–7 months was safe and not associated with higher rates of reactogenicity than placebo.