Inaccurate and misleading anti-vaccination messages in social media are leading to more and more children not being vaccinated against measles.
The analysis of UNICEF charity data shows that over half a million children in the UK have not been vaccinated against measles for a period of eight years. As a result, they are susceptible to a disease that can lead to disability or death.
Unicef says 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 for maintaining good health.
"With the number of measles cases quadrupling in England in just one year, it is irresponsible that everyone is afraid of vaccines, and social media companies should have a zero-tolerance approach to this danger
Children need two doses of measles vaccine for protection, with the World Health Organization (WHO) recommending 95% coverage to achieve herd immunity, which provides protection against the spread of the disease in the community.
In the UK, there were 259 measles cases in 2017, compared to 966 in 2018.
In 2016 and 2017, the first dose of MMR-Jab, which includes measles vaccine, exceeded 95% of five-year-olds in the UK for the first time.
However, two doses of MMR vaccine are required to ensure complete protection against measles. The second dose of MMR in five-year-old children is 88% – well below the WHO 95% target.
Mary Ramsay, Head of Public Health UK's Vaccinations, said: "The UK has achieved the status of WHO's measles eradication in 2017, so the overall risk of measles to the UK population is low.
"However, there will still be cases due to the ongoing measles outbreak in Europe, especially in non-immunized individuals.
"This could lead to a certain spread in communities with low MMR coverage and in age groups with very little mixing.
"Measles can be very serious, so make sure you and your family are protected."
The measles virus is extremely contagious and spreads through coughing and sneezing, but can live up to two hours in the air. 90% of those who are near an infected person become infected themselves if they are not immune.
Symptoms include runny nose, sneezing and coughing, sore eyes, high temperatures, and small grayish-white patches on the inside of the cheeks.
A few days later, a reddish-brown spotted rash appears. This usually begins on the head or upper neck before it expands to the rest of the body. The symptoms usually go away after seven to ten days, but in some cases the consequences can be devastating.
Complications can be hepatitis, meningitis, or brain encephalitis infection.
In rare cases it can cause vision problems, heart and nervous system problems and even death.