Measles: 43 new cases in south west Wales outbreak
Health officials say 43 more measles cases have been reported in the Swansea area in the last week, taking the outbreak to 252 with 38 hospitalised.
In total, 64 secondary and primary schools and nurseries across have been affected and Public Health Wales says the spread is a "serious concern".
Parents are being urged to make sure children of all ages have MMR jabs.
The majority of the new cases are in the Swansea area but cases are being reported across south east Wales.
Dr Marion Lyons, director of health protection for Public Health Wales (PHW), said: "The numbers of cases of measles and continuing spread of the disease is of serious concern to us.
"We cannot emphasise enough that measles is an illness that can kill, or leave people with permanent complications including severe brain damage, and the only protection is the measles, mumps and rubella (MMR) vaccination.
"The continuing spread of the disease in the Swansea area means that it is only a matter of time before we have a child whose health is seriously damaged by measles.
"We are urging parents of unvaccinated children to make immediate arrangements with their GP for their children to receive the safe and highly effective MMR vaccine."
Dr Roland Salmon, director of PHW's communicable disease surveillance centre, said: "We have pockets in Wales, of which Swansea and the surrounds are one, that don't have high enough enough level of MMR vaccine uptake to effectively damp down that spread."
He said the vaccination rate for the city was rising again but there was "pocket of people" in the area who were not vaccinated as young children.
He said public health officials wanted parents of older children to make sure their youngsters have had both MMR jabs.
Typical symptoms of measles include fever, cough, conjunctivitis and a rash.
Complications are quite common even in healthy people, and around 20% of reported measles cases experience one or more complication.
These can include ear infections, vomiting and diarrhoea, pneumonia, meningitis and serious eye disorders.