Measles cases across Europe soared by 400% last year, according to the World Health Organisation.
A total of 21,315 cases were reported on the continent in 2017 and the infection was the cause of 35 deaths.
It came after a record low of 5,273 measles cases the previous year.
WHO said a decline in immunisations, interruptions in vaccine supply, low rates of vaccination among marginalised groups and poor monitoring had contributed to the crisis.
Dr Zsuzsanna Jakab, WHO regional director for Europe, described it as “a tragedy we cannot accept”.
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Symptoms of measles include cold-like symptoms, sore red eyes, a high temperature and a blotchy rash. Although it usually clears in 7-10 days, complications of the illness can be fatal.
Last September, it was announced that measles had been officially eradicated in the UK – meaning it had not freely circulated for three years – thanks to widespread immunisation with the MMR vaccine.
But there was a slight surge in cases this winter, and the WHO recorded a total of 282 last year.
According to Public Health England, the outbreak is linked to the rise in Europe and 36 cases were reported in 2018.
Dr Mary Ramsey, head of immunisation at PHE, said those who had not been vaccinated and had recently travelled to Romania, Italy or Germany were at risk.
“This serves as an important reminder for parents to take up the offer of MMR vaccination for their children,” she said.
Some 15 European countries experienced serious outbreaks of measles last year, but the worst affected were Romania, with 5,562 cases, Italy with 5,006 and Ukraine with 4,767.
Other states to be seriously affected were Greece with 967 cases, Germany with 927, and Belgium with 369.
“Every new person affected by measles in Europe reminds us that unvaccinated children and adults, regardless of where they live, remain at risk of catching the disease and spreading it to others who may not be able to get vaccinated,” Dr Jakab said.
Around 95% of cases of measles happen in low-income countries, and globally the fatality rate of measles is one in five.
That’s much more than Europe’s current outbreak, which is less than 0.2%.
Most deaths from measles result from severe complications such as diarrhoea, dehydration, brain inflammation or respiratory infections, according to Medicins Sans Frontiers.
“Elimination of both measles and rubella is a priority goal that all European countries have firmly committed to, and a cornerstone for achieving the health-related Sustainable Development Goals,” the WHO’s Dr Jakab said.
“This short-term setback cannot deter us from our commitment to be the generation that frees our children from these diseases once and for all.”