Measles cases are spreading with "alarming" speed despite urgent calls to ensure children have vital MMR protection.
Health chiefs warned again today that an epidemic in Wales could leave unprotected children brain-damaged or dead.
Experts fear it could rival an outbreak which ravaged Dublin more than a decade ago, when three people died.
The latest warning is fuelled by concerns that increasingly urgent public pleas for parents to act do not appear to be getting through.
The outbreak, centred on Swansea and the surrounding area, has seen the numbers infected rise from just over 200 on March 4 to 432 now.
By the end of last week the disease had spread to children in 111 secondary and primary schools, nurseries and play groups.
Experts with Public Health Wales (PHW) expressed rising concern last week when the number of new cases rose by 42 to 306.
Since then a further 116 cases have been reported and the rate at which the disease is spreading shows no sign of slowing.
Dr Marion Lyons, PHW director of health protection, warned: "Measles is now spreading at an alarming rate across areas of Wales.
"Worryingly there are still tens of thousands of susceptible children across Wales, yet our weekly monitoring of vaccination rates shows only a slight increase in numbers receiving MMR jabs."
She added: "If the numbers of parents bringing their children for MMR jabs does not dramatically increase, measles will continue to spread and quickly reach levels last seen in the outbreak in Dublin in 1999/2000. In that outbreak over 1,200 children were infected and three died.
"The risk to unvaccinated children in Wales is increasing as the disease spreads. You only need one or two people who haven't had the vaccination in a community to put at risk babies, toddlers and anyone else who is vulnerable, such as children with leukaemia who cannot have the vaccination and pregnant women who haven't been vaccinated.
"A simple and safe jab from your GP will protect your child's health, could save their life, and will help protect other children too. It is the only precaution you can take at this time, and I would urge parents to contact their GP today to get their children's MMR vaccinations up to date.
"If you haven't thought about it, think about it today, and make an appointment with your GP for your child to get the MMR jab.
"If you're worried about the jab, I would reassure you that the jab is safe and offers the only protection against measles. Get your child vaccinated.
"If you are planning to have your child vaccinated, contact your GP today and get the vaccination as soon as possible."
The majority of cases so far are in the Swansea area but an increasing number are being reported across the Mid and West Wales regions.
The areas in question are largely covered by Abertawe Bro Morgannwg, Powys and Hywel Dda local health board areas.
Public Health Wales is urging parents of all children aged between one and 18 in Wales who have not been fully vaccinated to contact their GP for advice and to arrange vaccination as soon as possible.
PHW has issued the following advice to avoid the spread of the disease:
* If your child is unwell and you suspect it is measles, contact your GP. Your child should not attend school or nursery for four days after the rash starts;
* Telephone your GP surgery to inform them you or your child has a rash illness before attending, so that arrangements can be made in advance for minimising contact with other vulnerable patients;
* Avoid going to A&E unless you are seriously ill, and if you do attend, telephone ahead to let them know you or your child may have measles;
* Avoid contact with pregnant women, people with weak immune systems and babies who are too young to be vaccinated, as they are more vulnerable to infection and there are very few treatments available to help them if they do catch measles;
* If any family members are pregnant, receiving chemotherapy, or aged under one, it is vital that all other family members are up to date with their MMR vaccinations;
* Maternity wards, midwives and health visitors are being asked to share information with parents to encourage them to check the vaccination status of all children in the family to avoid further household spread among vulnerable groups.