Measles is making a return in Europe. More than 21,000 cases were reported in the region in 2017 - a 300 per cent increase on 2016, according to the World Health Organization (WHO).

Measles cases dipped to a record low in Europe in 2016. But last year saw large outbreaks in 15 countries that resulted in 35 deaths. Romania, Italy and Ukraine had the most cases, but outbreaks affecting more than 100 people were also seen in the UK, Germany and France, among others.

It is thought that at least 95 per cent of a population needs to be vaccinated against measles to prevent outbreaks. Last year, the proportion of children in England receiving the vaccine dropped to nearly 91 per cent.

In 2014, the WHO set itself the target of eliminating measles in Europe by 2020. "To meet the goal of eliminating measles, high rates of vaccination need to be sustained," says Helen Bedford at University College London.