Across England, take-up of the vaccine has fallen, with NHS chief executive Simon Stevens blaming anti-vaxxers increasing prominence as “part of the fake news movement”.
The latest figures show that in North Yorkshire, between April and September 2018, 89.5% of children turning five had received the recommended two measles, mumps and rubella (MMR) jabs.
This means around 308 children in the area are not fully vaccinated.
MMR take-up in North Yorkshire has dropped since 2014, when 90.3% of five year olds had the full course of vaccinations, according to Public Health England.
The target, set by the World Health Organisation, is for 95% coverage.
Across England, the proportion of five year olds fully immunised against MMR has dropped from 88.5% in 2014 to 86.3% in 2018.
There were more than three times as many measles cases in 2018, as in the previous year.
Speaking at a health summit organised by the Nuffield Trust, Mr Stevens said: “Across the world, two to three million lives are saved each year by vaccination.
“But as part of the fake news movement, actually the vaccination deniers are getting some traction.
“We have seen a five-year steady decline in the vaccination uptake.”
Mr Stevens explained a parent at his own daughter’s primary school had used WhatsApp to express concern about children’s immune systems being “loaded up” with vaccines.
“We are not being helped on this front by the fact that although nine in 10 parents support vaccination, half of them say they have seen fake messages about vaccination on social media,” he said.
“Frankly it’s as irresponsible to tell parents that their children shouldn’t be vaccinated as it is to say don’t bother to look both ways when they cross the road.”
The MMR vaccination is made up of two jabs, the first when babies are one year old, and then before they start school aged three or four.
North Yorkshire had a higher take-up of the first jab in 2018, with 96.6% of five year olds having had it.
The NHS says up to one in 10 children are not fully immune after the first jab, whereas fewer than one in 100 are fully immunised after the second dose.
The Royal College of Nursing’s Helen Donovan said: “Challenging misinformation is vital to reverse the decline in vaccination uptake and ensure people recognise the protection it offers.”
She said the rise in measles was “exacerbated by myths propagated largely online”.