Inaccurate and misleading anti-vaccination messages on social media are leading to more and more children not being immunised against measles.
Analysis of data by the charity Unicef shows that more than half a million children in the UK were not vaccinated against measles over an eight-year period, leaving them vulnerable to a disease that can cause disability or death.
And Unicef says that nearly 170 million children in the world under the age of 10, including 2.5 million in the US, are not vaccinated against measles.
Simon Stevens, chief executive of NHS England, said: "Getting yourself and your children vaccinated against killer diseases is essential to staying healthy, and vaccine rejection is a serious and growing public health timebomb.
"With measles cases almost quadrupling in England in just one year, it is grossly irresponsible for anybody to spread scare stories about vaccines, and social media firms should have a zero tolerance approach towards this dangerous content."
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Children need two doses of the measles vaccine for protection, with the World Health Organisation (WHO) recommending 95% coverage to achieve herd immunity, which offers protection against the disease spreading in the community.
In the UK in 2017, there were 259 measles cases in England, rising to 966 in 2018.
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In 2016 and 2017, uptake of the first dose of the MMR jab, which includes the measles vaccine, in five-year-olds in the UK exceeded 95% for the first time.
However, two doses of MMR vaccine are required to ensure full protection from measles. Uptake of the second dose of MMR in five-year-old children is 88% – well below the 95% WHO target.
Mary Ramsay, Public Health England's head of immunisations, said: "The UK achieved WHO measles elimination status in 2017, so the overall risk of measles to the UK population is low.
"However due to ongoing measles outbreaks in Europe, we will continue to see cases, particularly in unimmunised individuals.
"This could lead to some spread in communities with low MMR coverage and in age groups with very close mixing.
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