Almost 43,000 at-risk children and teenagers are being targeted in an urgent vaccination effort in the face of a spreading measles epidemic.
Health chiefs in Wales warned on Thursday that the epidemic concentrated in the Swansea area could spread across the principality without action.
Measles cases in the greater Swansea area have continued to rise, now totalling 1,039.
An extra 28 cases in the region since Tuesday show the epidemic, which began in November 2012, continues to spread.
Public Health Wales (PHW) revealed that more than 33,000 non-routine vaccinations have been given across Wales during the outbreak.
The good news is that those who already had the three-in-one measles, mumps and rubella jab (MMR) before the outbreak have had almost total protection.
PHW figures released on Thursday reveal the vaccination has been more than 99% effective in the current outbreak.
There have been fewer than 10 confirmed cases of measles in people who have previously been vaccinated.
But with the 10 to 18 age group most at risk, and also the hardest hit, thousands remain unprotected across the principality.
Most cases so far have been in the greater Swansea area with Abertawe Bro Morgannwg, Hywel Dda and Powys health board areas.
But of the 33,000 vaccinations given in response to the epidemic, less than 25%, or 8,000, have been to 10 to 18-year-olds.
That leaves 43,000 unvaccinated within that crucial age group in Wales and their parents are being urged to act immediately.
The headline total for measles throughout Wales now stands at 1,170 cases.
The number of laboratory confirmed cases in the outbreak stands at 370 out of a total of 850 samples tested.
Dr Marion Lyons, PHW director of health protection, said: "The efforts to vaccinate susceptible young people children across Wales have been excellent, with non-routine vaccinations being given in their thousands by GPs, in schools and in emergency drop-in clinics.
"This undoubtedly will have reduced the length and severity of the outbreak, but the number of unvaccinated people in the hardest hit age group remains a cause for concern.
"Our message is clear - the MMR vaccine is safe and it works, and there is no reason why children not vaccinated in the late 1990s because of fears about the safety of the vaccine should not be immunised now.
"While not enough 10-18 year olds in particular are vaccinated, this outbreak can easily spread anywhere in Wales.
"Therefore we urge young people themselves and the parents of children, to take up opportunities to receive the MMR vaccine as a matter of urgency.
"Vaccination sessions are continuing in schools throughout Wales and I urge pupils and their parents to take advantage of these.
"Those not vaccinated are highly likely to catch measles, which is highly contagious. It is just a matter of time before a child is left with serious and permanent complications such as eye disorders, deafness or brain damage, or dies."
The MMR vaccine is recommended by the World Health Organisation, UK Department of Health and Public Health Wales as the most effective and safe way to protect children against measles.
Unvaccinated children aged between six and 12 months living in or travelling to the outbreak areas of Swansea and Neath Port Talbot or North Powys can be offered vaccination by their GP.
There is 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.