Porthmadog measles outbreak linked to Ysgol Eifionnydd affects 30
Health chiefs say another 11 children in Gwynedd have been diagnosed with measles, bringing the total number of cases to 30.
All the cases are associated with Ysgol Eifionydd School in Porthmadog.
Doctors in the area are offering vaccinations to children who have not had both doses of the MMR vaccine.
Public Health Wales and Betsi Cadwaladr University Health Board are urging parents in Gwynedd to get their children vaccinated.
They say the majority of the cases have either had no MMR vaccination or only one dose.
Two doses of the vaccine is the only protection against the virus.
Dr Chris Whiteside, consultant in communicable disease control for Public Health Wales, said: "Measles is highly contagious and spreads very easily.
"We predicted that the number of cases would rise but it is sad nonetheless that it has risen so sharply.
"As long as there are children who have not had their two MMR vaccinations, there is the potential for more people to become ill with measles."
Two sessions were arranged last week which saw a total of 30 children vaccinated.
If parents suspect their children are unwell due to measles they are asked to contact their GP or NHS Direct Wales on 0845 4647.