The optimal age for measles-containing vaccine (MCV) administration depends on various factors, mainly the duration of the protection induced by transplacentally acquired maternal immunity, the maturity of the infant’s immune system, and the average age of measles infection in different geographical areas. 1 Demographics and vaccine coverage also contribute. 2 Initially, experts thought that if an area had a high risk of measles outbreaks, a first dose of MCV at 9 months of age could be enough to protect most infants, as those younger than 9 months could avoid infection through transplacentally acquired immunity. Subsequently, it was shown that this supposition was partly wrong, as many measles cases, sometimes very severe, were diagnosed in infants younger than 6 months. 3 This finding was attributed to a shorter-than-expected duration (9–12 months) of passive maternal immunity, evidenced in both infants born from immunised mothers and those born from mothers with naturally acquired immunity. It was shown that vaccination stimulated the immune system less than natural infection, and infants born from vaccinated mothers had lower specific antibody titres with more rapidly waning protection than infants born from naturally infected mothers. 4 Nevertheless, with the introduction of MCV administration, the circulation of measles virus progressively declined and, without this natural booster, even previously naturally infected mothers transferred reduced antibody concentrations to their fetuses. 5 To address the problem of measles infection in young infants, an early MCV dose before 9 months of age was suggested. Infants living in or travelling to countries with frequent measles outbreaks, or considered at risk because of HIV exposure or infection, and refugees or people living in conflict zones were the target populations. However, because of the risk that an early MCV dose could lead to poor short-term and long-term protection, this dose was considered only supplemental.