A vaccination clinic is being held at Cwmtawe School in Pontardawe today after one confirmed and two suspected cases of measles at the school.
A further two cases have are suspected in children at Ysgol Y Cribarth in Powys.
More than 200 children at Cwmtawe are not fully vaccinated.
It is just three months after the end of Wales's biggest ever measles outbreak, which was also centred on the Swansea area.
More than 1,200 people contracted measles, 88 serious enough to visit hospital, and one person died.
In spite of vaccinating more than 75,000 people during the last outbreak, health officials are concerned that at least 30,000 children remain unvaccinated in Wales.
The symptoms of measles include a fever, fatigue, runny nose, pink eyes and a distinctive red rash.
Doctors are urging parents to keep children at home if they suspect measles and to seek medical advice.
They say that any unvaccinated children should receive the MMR jab as soon as possible.
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