Health watchdogs have appealed to parents to ensure their children have had both of the necessary doses of MMR jabs, after rises in confirmed cases of measles.
They believe some children did not have a second injection combining immunisation against measles, mumps and rubella before they went to primary school aged five.
The Health Protection Agency (HPA) said 964 cases of measles had been reported in England and Wales in the first six months of the year, more than many annual totals in the last 15 years.
There were 497 cases in the first six months of 2011 and 1,086 in the year as a whole, up from an annual total of 380 in 2010, although there were more than 1,000 cases in both 2008 and 2009.
Recent outbreaks in Gypsy and Traveller communities and in Sussex and Merseyside have contributed to the high number this year.
Health authorities have had to remind parents about ensuring children are given both doses of MMR since links were first suggested between the MMR vaccine and autism in 1998 by Andrew Wakefield, who was struck off the medical register in 2010.
Take-up in England is now 93% for the first jab and 87% for the second, and in Wales 95% and 88%, whereas in 2002 the coverage for the first dose was 80%.
Cases of rubella have also risen in the first part of this year, to 57, more than the total recorded in each of the nine previous years. Many of these have been linked to travel to Europe.
Mary Ramsay, head of immunisation at the HPA, said: “It’s extremely concerning that measles cases are continuing to rise. Measles can be very serious and parents should understand the risks associated with the infection, which in severe cases can result in death.
“Although uptake of the MMR has improved in recent years, some children do not get vaccinated on time and some older children who missed out when uptake was lower have not had a chance to catch up. Therefore, there are still enough people who are not protected to allow some large outbreaks to occur among unvaccinated individuals.”
She urged parents to ensure their children had received the two doses before they returned to school.