Uptake of the MMR vaccine in Wales has still to hit the 95% target
Health officials in north Wales are urging parents to ensure their children are vaccinated against measles, after identifying 20 suspected cases.
The disease had been confirmed in four individuals, all in families associated with Ysgol Ffordd Dyffryn and Ysgol John Bright in Llandudno, Conwy.
Parents are being asked to make sure their children have had two doses of the measles, mumps and rubella vaccine.
GPs are also being alerted to possible increases in demand for the vaccine.
Dr Chris Whiteside, consultant in communicable disease control for the National Public Health Service for Wales (NPHS), said: "Measles is a highly contagious infectious disease and children who have not been immunised are at risk.
"Parents can easily protect their children by having them immunised with the MMR vaccine. After completing a two-dose course of MMR, 99.9% of children will be protected against measles."
The NPHS said it was also working with North Wales NHS Trust to provide Ysgol Ffordd Dyffryn and Ysgol John Bright with vaccination sessions on Friday morning for pupils who have not had two doses of the MMR vaccine.
Dr Whitside said: "If your child is unwell and you suspect it is measles you should contact your GP. Your child should not attend school or nursery for five days after the rash starts.
"Most people make a full recovery but there is a rare possibility of severe complications including serious eye disorders, deafness, mental difficulties and even death."
Latest figures for measles cases in Wales and England show that there has been an almost 40% increase in the infection compared to 2007.
In 2008, the Health Protection Agency recorded 39 confirmed cases of the disease in Wales.
The widespread rise in measles across Britain followed now discredited research suggesting the triple MMR vaccine could have been linked to autism.
The target in Wales is to see 95% of children receive a double dose of the vaccine.
However, the NPHS said that this take-up level has yet to be reached, and in north Wales stands at just under 85% of children who are up-to-date with their measles jabs.