More than 200 at-risk school children are to be given the MMR jab after a measles case was confirmed in a region recovering from a major epidemic.
Pupils at a south Wales school will be given the triple vaccine as concerned health chiefs act to avert a new outbreak of the potential killer illness.
A massive programme to inoculate thousands of children was launched in the greater Swansea area in the spring as it battled a major measles outbreak.
Large numbers of previously unprotected children were given the triple measles, mumps and rubella (MMR) jab as a result.
The new case comes three months after the end of that epidemic - the biggest outbreak of measles ever to hit Wales.
More than 1,200 people contracted measles, 88 serious enough to visit hospital, and one person died in the outbreak which began in November last year.
Public Health Wales (PHW) has received two notifications of measles in pupils at Cwmtawe School, in Port Talbot, one of which has been laboratory confirmed.
The area affected falls under Abertawe Bro Morgannwg University Health Board, based in Swansea, which was at the centre of the previous vaccine programme.
It is arranging for the MMR jab to be offered to all pupils at the secondary school who are either not vaccinated, or have not had a second jab, next week.
Of 1,260 pupils at the school, 62 are recorded as having received no doses of the MMR vaccine and 142 have received only one dose.
Two doses of MMR are needed to provide the maximum 99% protection against measles.
Dr Jorg Hoffmann, PHW consultant in communicable disease control, said: "This case of measles comes only three months after the end of Wales's biggest ever measles outbreak, which was centred on the Abertawe Bro Morgannwg health board area, so we are obviously concerned and want to act quickly.
"Although we vaccinated more than 75,000 people during the outbreak, we are aware of more than 30,000 children in the 10 to 18 age group who remain unvaccinated in Wales and have always warned that this could lead to another outbreak.
"We have written today to all parents of unvaccinated children in Cwmtawe School to urge them to arrange for their children to be vaccinated.
"In the meantime, to prevent the spread of infection, we would urge parents of any children who become unwell with measles-like symptoms over the weekend to keep them at home and seek medical advice, and not to allow them to attend events where other children will be present."
The first dose of MMR is usually given to babies at 12 months old and the second at three years and four months of age, but it is never too late to catch up on missed doses.
The MMR vaccine is recommended by the World Health Organisation, UK Department of Health and Public Health Wales as the most effective and safe way to protect children against measles.
Although there are no other reported cases of measles at present, PHW is reminding all parents in Wales that their children are at risk if not fully vaccinated with two doses of MMR. Vaccination can be arranged at any time via the family GP.
The symptoms of measles include a fever, fatigue, runny nose, conjunctivitis (pink eye) and a distinctive red rash.
Measles is very contagious and can cause serious complications and in rare cases can be fatal.
Anyone who thinks they or their child has the symptoms of measles should contact their GP or NHS Direct Wales on 0845 46 47.