All but two of Wales' 22 counties have reported measles cases
More than 10,000 children in three south Wales counties are still at risk from a potentially fatal measles outbreak, say public health chiefs.
The worst measles outbreak in Wales in years has seen at least 374 cases.
A drive to boost MMR take-up in Neath Port Talbot, Swansea and Bridgend has seen 948 children given both doses.
But the National Public Health Service (NPHS) for Wales says the outbreak "could continue for some time" without more children having the MMR vaccine.
The NPHS said the outbreak, most serious since the introduction of the MMR vaccine, is affecting 20 of Wales' 22 counties, with a new outbreak just declared in Merthyr Tydfil.
Public health chiefs said since the start of the campaign to increase local MMR uptake, 948 children in Swansea, Neath Port Talbot and Bridgend have received both doses of the vaccine, which offered the maximum 99% protection against measles.
A further 3,494 children have received one dose of the vaccine since May this year, which provided 90% protection, said the NPHS.
But 10,820 youngsters have not had a single dose.
Dr Mac Walapu chairs the team tackling the outbreak in Swansea, Bridgend and Neath Port Talbot area.
He said: "It's good news that 4,619 parents have been encouraged to protect their children with the MMR vaccination since we started our campaign to increase uptake and that almost 1,000 children who were previously vulnerable are now fully vaccinated.
"However, unfortunately we still have thousands of unprotected children who have not benefited from the routine childhood immunisation programme.
"We continue to see new measles cases not only in this area but across Wales, with a new outbreak just declared in Merthyr Tydfil.
"With so many children still unprotected, the outbreak could continue for some time and can only be halted by an increase in MMR uptake."
The NPHS said children should receive their first dose of the MMR vaccine at 13 months of age and the second at around three years and four months of age.
Letters had been sent to every household in the area which has children who have not had the vaccine, said the NPHS.
Dr Annie Delahunty, Neath Port Talbot's public health director, said: "Remember, if you say no to the vaccine, you are saying yes to measles, as well as mumps and rubella.
"In our area alone, there have been 133 cases of measles to date and 41 children have been hospitalised across Wales, so no one can afford to be complacent about the risk to our most vulnerable children."
The NPHS said many people who catch measles will have a fever, cough, red eyes, and blocked nose and feel generally unwell. The blotchy rash appears a few days later beginning on the face and spreading downwards to the rest of the body over several days.
Typically people will be infectious from the day before their first symptoms until four to five days after the appearance of the rash.