One million children who missed out on the MMR vaccine around a decade ago are to be targeted in a national campaign in Britain to raise the level of protection against measles.
Alarmed by an outbreak in Swansea, where almost 900 children have been diagnosed with the disease, Public Health England is appealing to parents of children who missed one or both of the MMR jabs before the age of 5 to send them for vaccination.
Measles was virtually eliminated in the 1990s following the introduction of MMR vaccine after the disease infected over 500,000 children a year in the 1950s and 1960s.
But the disease re-established itself in the mid-2000s after an unfounded scare linking MMR to autism which caused parents to abandon the vaccine.
Immunisation rates fell nationally to less than 80 per cent in 2005.
Dr Mary Ramsay, head of immunisation at Public Health England, said children born in 2001-2 who were now just starting secondary school had the highest rate of non-vaccination, and this was also the age group in which measles cases were highest.
"We have a legacy of older children who were not vaccinated as toddlers. They are now at secondary school where measles can spread very effectively. We estimate a third of a million 10- to 16-year-olds are completely unvaccinated," she said.
"Measles is a potentially fatal but entirely preventable disease so we are very disappointed that cases have recently increased.
"The only way to prevent outbreaks, such as the one we are seeing in south Wales is to ensure good uptake of the MMR vaccine across all age groups. Measles is not a mild illness it is very unpleasant and can lead to serious complications."