BACKGROUND: Invasive meningococcal disease can have devastating outcomes, especially in high-risk groups such as infants. As infants are recommended to receive multiple vaccines during a single office visit, this phase 3 study assessed the safety and immune response to MenACWY-CRM at alternative visits in older infants and concomitant use with measles, mumps, rubella, varicella vaccine (MMRV) at 12 months of age. METHODS: Two age groups were concurrently enrolled: 7- to 9-month-old infants who received 2 doses of MenACWY-CRM at 7-9 and 12 months and were randomized 1:1 to receive MenACWY-CRM with or without MMRV at 12 months, and 12-month-old infants who received MMRV only at 12 months. Using predefined non-inferiority criteria, immune responses to the antigens in MMRV were compared between those who did and did not receive MenACWY-CRM; immune responses to MenACWY-CRM as measured by the percentage of subjects with human serum bactericidal activity (hSBA) titers >/= 8, were compared between those who did and did not receive concomitant MMRV. Adequacy of the immune response to 2 doses of MenACWY-CRM administered at 7-9 and 12 months was also assessed. Local and systemic reactions, adverse events resulting in withdrawal or requiring medical attention and serious adverse events were monitored. RESULTS: Concomitant administration of MMRV with MenACWY-CRM did not affect the immune response to either vaccine. The 2-dose series of MenACWY-CRM induced adequate immune response to all 4 serogroups. No increased reactogenicity was observed with MenACWY-CRM+MMRV compared with MMRV alone, and there were no study-related serious adverse events. CONCLUSIONS: Concomitant administration of MenACWY-CRM with MMRV vaccinations at 12 months was well-tolerated, without safety concerns. Robust immune responses to all components of both vaccines were produced and all criteria for non-inferiority were met, supporting the use of a 2-dose regimen of MenACWY-CRM in this age group.