Dr Marion Lyons from Public Health Wales has told ITV News that parents must get their children vaccinated before school trips abroad, or other activities, during the easter holidays.
She said new cases are coming through every day, and extra clinics are being put on.
Letters are being sent out to parents across South Wales.
Fresh measles cases have been reported in Powys and Cardiff. A public health expert warned tens of thousands of children could be at risk.
Public Health Wales has information and advice on measles - a highly-infectious viral illness which can have life-threatening complications.