A Dutch study found the immunity that children get from their mother wanes several months before routine vaccination begins.
This protection disappeared sooner after birth in children whose mothers were vaccinated with MMR than infants whose mothers had previously contracted the disease.
Infants are unprotected against the disease for several months until their first routine vaccination at 12-13 months, researchers said.
The findings support the existing UK policy of offering MMR vaccination to infants aged six months to 12 years in the event of a measles outbreak. This is now being advised to GPs in south Wales, the centre of the ongoing measles outbreak in the country.
Authors said that in the long term, vaccination schedules will need to change to offer the MMR jab earlier as vaccination rates among women of childbearing age rise over the next decade.
The NHS in Wales has vaccinated more than 50,000 people in the last two months, but cases continue to rise and now stand at 1,105.
In the study, researchers tested blood samples from Dutch infants and women and compared antibody levels in the general population with those of the orthodox protestant community, which has low vaccination rates and has seen outbreaks of measles, mumps and rubella.
Children of vaccinated mothers were protected against measles for 3.3 months after birth on average, whereas this protection lasted two months longer in unvaccinated mothers.
Researchers said the MMR vaccine may induce lower antibody levels than natural infection.
Study authors concluded: 'The average age at which a child loses the protection of its maternal antibodies and becomes susceptible to measles, mumps, and rubella lies well before the age of first MMR vaccination. It is extremely important to protect this large number of susceptible children, who have a high probability of a severe outcome when infected.'
The vaccine may be less effective in younger infants because their immune systems are not fully developed, they warned.
In the Netherlands, MMR vaccination is recommended for children aged 6-14 months who are set to travel abroad to areas with measles outbreaks, they said. Widespread vaccination of this agegroup would be activated if an outbreak occurred within the country.