The proportion of five-year-olds having both MMR jabs in Lancashire has fallen slightly, and immunisation rates for the vaccine are significantly below target.
The latest Public Health England figures for 2017-18 show that 84.6% of five-year-olds had received both MMR jabs before their fifth birthday, down from 85.1% the previous year.
This means that 814 five-year-olds in the area are unprotected against highly-contagious measles.
The target, set by the World Health Organisation, is 95% coverage.
Experts have said that the uptake of the jab across England, where 87.2% of five-year-olds have had both jabs, is "worryingly low".
The MMR jab is a 3-in-1 vaccine for measles, mumps and rubella. It is given to young children in two doses - the first at 12 months, and the second around three years.
The Royal College of Paediatrics and Child Health and the British Society for Immunology are concerned about children getting the first jab, but not the second.
Of the 12,643 five-year-olds living in Lancashire, 11,829 had the first MMR jab, and 10,691 had both jabs.
That means that in addition to those with no protection, 1,138 children missed out on the second dose of MMR and will only be partially protected.
One in 10 children will not be protected against measles after the first dose alone. After the second, that falls to one in a hundred.
BSI chief executive Dr Doug Brown said: "One in 10 people unprotected simply just isn't good enough."
This year there have already been 876 confirmed cases of measles in England, more than three times the number recorded in the whole of 2017.
Although measles is now more common in teenagers and young adults, Dr Brown said that it's more important than ever to make sure young children are fully protected.
He said that reduced uptake rates were "disappointing".
"If we are in a position of increased outbreaks, low immunisation is even more worrying," he said.
Dr Brown added: "Vaccination is one of the few miracles of modern medicine. It is one of the safest and most cost-effective methods we have to prevent the spread of disease."
The Royal College's immunisation expert Dr David Elliman said that a lack of awareness around the importance of getting both jabs, rather than a decision by the parents, could be causing children to miss the second dose.
Dr Elliman said: "The best solution is GPs sending regular reminders to parents, and all GPs should make sure they have an efficient reminder system in place."
He added that anyone who has missed the MMR vaccine can still get it for free at their GP.
"But even healthy children can get nasty measles and die from it," he added. "A third of measles cases will end up in the hospital."
Public Health England said that they are working closely with the NHS and general practice staff to improve uptake.
PHE's Dr Michael Edelstein said: "We are seeing small reductions in uptake for most of the childhood vaccines, which is why we continue to encourage all parents to get the best protection for their children by ensuring they are fully immunised."
The British Society for Immunology is calling on the Government and the NHS to conduct a review of immunisation rates, to learn from the areas that are doing well and apply that to the rest of the country.