Public health officials are warning that the number of measles cases has risen to 14, after an outbreak earlier this year.
There are now 13 cases linked to Cwmtawe School in Pontardawe, and a case of suspected measles with no apparent links to the school, at a nursery in the Swansea area.
Public Health Wales says there is "potential for the outbreak to spread quickly", unless children who have not received two doses of the MMR are vaccinated urgently.
Two weeks ago, vaccination clinics were held at the secondary school in Pontardawe, after the first case was confirmed.
The last outbreak, centred on Swansea and Neath Port Talbot, was declared over in July, having started last November.
It resulted in more than 1,200 cases.
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