Parents are being urged to make sure their children have had both doses of the jab against measles, mumps and rubella before the return to school.
The Health Protection Agency has recorded 480 cases of measles this year - and more samples are arriving each day, with about half testing positive. Officials fear this year's figures could end up beating last year's - and those marked a 10-year high.
Doctors and HPA experts said they were concerned about the number of cases. HPA immunisation expert Dr Mary Ramsay said: "We've been very worried because the cases have stayed up over the summer holidays.”
Measles is a highly infectious virus. It starts with a fever and conjunctivitis before a rash develops. The rash often lasts about a week and other complications can include pneumonia and diarrhoea.
The MMR vaccine protects against mumps and rubella, as well as measles. It is given in the national immunisation programme at 12-15 months and at 4 years of age.
The research, which led to questions about the triple vaccine, is being scrutinised at the moment in a hearing at the medical regulator.
The latest figures show that 88% of UK children begin school having had one dose of MMR. But only three-quarters of them have the full protection afforded by both doses.
Until those figures climb far higher, health officials say people should not assume their children are safe from measles.