Children who missed their MMR jab are being targeted by a campaign aimed at cutting the threat of measles.
The move follows an epidemic in Swansea, which affected 900 people.
Health officials say a similar outbreak could occur anywhere, and the vaccination campaign run through schools, GP surgeries and community groups, will target more than one million children aged between 10 and 16.
It is feared many of them were not vaccinated for measles, mumps and rubella because of a scare about the jabs a decade ago.
Research that has since been discredited, linked the injections to autism.