The proportion of five-year-olds having both MMR jabs in North Lincolnshire has fallen slightly, and immunisation rates for the vaccine are significantly below target.
The latest Public Health England figures for 2017-18 show that 89.2% of five-year-olds had received both MMR jabs before their fifth birthday, down from 90.7% the previous year.
This means that 67 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 2,111 five-year-olds living in North Lincolnshire, 2,044 had the first MMR jab, and 1,884 had both jabs.
That means that in addition to those with no protection, 160 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."