Public Health Wales has urged parents in the principality to ensure their children receive the MMR vaccine after the number of children with cases of measles passed 500.
PHW figures up to 3 April show there are 541 cases in and around Swansea. With numbers going up by more than 100 in a week, PHW has also begun offering some babies the MMR jab seven months earlier than recommended.
Experts in Wales fear it could reach the level of the Dublin outbreak of 1999-2000, when over 1,200 children were affected and three died.
PHW said that cases continue to be reported across Wales, with the majority in Abertawe Bro Morgannwg, Powys and Hywel Dda health board areas.
Measles contact risk 'increasing every day'
It warned that the risk of unvaccinated children coming into contact with those already infected is "increasing every day".
PHW had previously said it was disappointed in the number of children being vaccinated, with only 100 of the 3,800 susceptible children aged over two in the Swansea area having the jab last week.
But Dr Marion Lyons, director of health protection for PHW, said it appeared the uptake of the MMR was now improving.
"We are starting to see parents bringing unprotected children to get vaccinated, showing that they're taking the outbreak seriously and have confidence in the jab as the best way of protecting their children," she said.
"Unfortunately some parents are continuing to put their children at unnecessary risk of catching this potentially serious disease. They need to get their children vaccinated without delay."
"The MMR jab is recommended by the World Health Organization, UK Department of Health and Public Health Wales as the most effective and safe way to protect children against measles. The alarming numbers of cases of measles and continuing spread of the disease in Wales illustrates how vital it is for parents to get their children vaccinated."
Previous measles outbreaks
There have been other smaller outbreaks more recently in England, including one last year which was centred around Merseyside.
But in the whole of the north west in 2012 there were just 865 measles cases. Most of the more than 500 people affected in Wales are school-age children who have not had the MMR jab; vaccine uptake in affected areas is around 89%.
PHW suggest that, without greater vaccine uptake, it was "just a matter of time" before a child was left with serious and permanent complications, such as eye disorders, deafness or brain damage, or even dies.
Some GP surgeries have responded to the epidemic by offering extra clinics for the MMR vaccine. A free MMR vaccination session will be held at the Princess of Wales Hospital, Bridgend, on 6 April from 10:00 to 16:00.
Posted 03/04/2013 by firstname.lastname@example.org