A measles epidemic is spreading at "alarming" speeds - with the number of cases doubling this month.
The Welsh outbreak has prompted health chiefs to stress again the vital need for proven MMR protection.
Health officials warned that the epidemic in the Swansea area could leave unprotected children brain-damaged or dead.
Experts are worried that it could rival an outbreak which killed three people and infected more than 1,200 children in Dublin more than a decade ago.
The new plea is fuelled by concerns that increasingly urgent public warnings for parents to immunise their children do not appear to be getting through.
Dr Marion Lyons, Public Health Wales director of health protection, said: "If the numbers of parents bringing their children for MMR jabs does not dramatically increase, measles will continue to spread."
The Swansea infection rate has rocketed from just over 200 on 4 March to 432 now. Experts' fears were heightened when the number of new cases soared by 42 to 306 last week. The disease has spread to pupils in 111 secondary and primary schools, nurseries and playgroups and its spread shows no sign of slowing.
Dr Lyons said there are still tens of thousands of susceptible children across Wales, yet her weekly vaccination rate count shows only a small rise in numbers receiving MMR jabs.
Copyright Press Association 2013