The number of measles cases in the outbreak centred on Swansea has passed 800 with an extra 43 cases being reported to Public Health Wales in two days.
Vaccinations in secondary schools in the area started on Wednesday, as health officials targeted 10- to 18-year-olds who did not get jabs when they were infants.
Children and young adults have been badly affected in the outbreak. Catch-up clinics have also been organised again at hospitals in Swansea, Bridgend and Neath this Saturday in an attempt to bring the outbreak under control.
A total of 808 cases – of whom 77 have needed hospital treatment – have been reported. Most have been in the area covered by the Abertawe Bro Morgannwg University health board but there have also been 70 in Powys.
Parents who have been put under mounting pressure to sign consent forms for their children to be inoculated with the triple measles, mumps and rubella jab at school have been reminded that anyone with symptoms of measles should not attend vaccination sessions to avoid putting others at risk. They should instead telephone their GP for advice.
Marion Lyons, director of health protection for Public Health Wales, said: "We continue to offer unprecedented opportunities for parents to vaccinate their children against measles, but parents need to make sure they take these opportunities.
"Although we want children of all ages who have missed vaccinations to catch up now, we are particularly concerned about those aged between 10 and 18. These are the children who would have missed vaccination because of concerns about the safety of MMR in the late 1990s."
She said the vaccine was safe, effective and the only protection against a potentially fatal disease.
"We can't bring this outbreak to an end unless the parents of unvaccinated children either arrange vaccination with their GP, call into one of the weekend drop-in sessions or ensure that if their child attends a school where vaccinations are being offered they have signed a consent form for them to be vaccinated," Lyons said.
"Unvaccinated children aged between six and 12 months living in or travelling to the outbreak areas of Swansea and Neath Port Talbot can be offered vaccination by their GP. There's no adverse effect to this extra jab and those children would still need to receive the recommended two doses at 12 months and three years and four months of age."