Health experts are issuing a warning about measles after an unexpectedly high number of cases this summer.
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.
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.
"We've had quite a few cases in children of school age and we're worried those cases are going to take themselves into school when term starts soon."
Officials fear this year's figures could end up beating last year's - and those marked a 10-year high.
In previous decades, measles was a far bigger problem, causing on average 20 deaths a year. That is why officials like Dr Ramsay are nervous that the numbers are creeping up again.
She said: "Although the numbers are still small, compared to the history of measles, we're always worried about measles because very rarely it can kill.
"We hadn't had any deaths from measles since the early 1990s but unfortunately we had one death last year and we don't want any more."
Ollie Mullen is one of this summer's measles statistics. Aged eight months, he was too young for vaccination.
His mother, Anna-Maria, was astonished to find that her youngest son's runny nose, temperature and rash were symptoms of a serious disease.
She said: "I feel quite shocked and disappointed that people still haven't got their children vaccinated against measles - and also that I was walking around with an infectious child.
"It's a really horrible illness. Ollie was very miserable and quite lethargic for a week. I would say definitely do get your children immunised."
The latest figures on measles in Britain show high numbers in the east and south-east of England, Yorkshire and Humberside and London.
Doctors' surgeries at Hackney in the east of the capital are feeling the consequences of the backlash against vaccination.
There have been more than 120 measles cases in the borough in the past three months - most of them in children aged under five who have not been immunised.
Dr Michael Fitzpatrick is a GP in Hackney who is vehement about the need for parents to get behind MMR.
"I am angry that the effect of this campaign against the MMR vaccine has ended up in outbreaks of measles like this," he said.
"This was inevitable and I think the only surprise is this hasn't happened earlier, and hasn't happened on a bigger scale."
MMR coverage began to drop in the late 1990s - though the medical establishment is relieved that uptake levels are now rising again.
The research which led to questions about the triple vaccine is being scrutinised at the moment in a hearing at the medical regulator.
Parents have held demonstrations outside the General Medical Council in support of Andrew Wakefield, the doctor who questioned the safety of MMR. The outcome of that inquiry will once again put the issue under the spotlight.
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.
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 four years of age.