Indeed, in 2019, before Covid hit, 86 per cent of all children under the age of one had received a first dose of measles vaccine by their first birthday, according to the WHO. But by 2021, this had fallen to 81 per cent – the lowest number since 2008.

Roughly 67 million children missed out on shots over this two-year period, while vaccination coverage fell across 112 countries.

The consequences of this are now starting to crystallise. Dr Gunter Boussery, a senior health specialist for Unicef who works across South Asia, said outbreaks of measles are occurring in Afghanistan, India, Pakistan, Nepal – and even in the Maldives, where it was classed as eliminated in 2020.

‘First sign of a weakening health system’

The disease is a particular threat in countries where health services are poor and hunger is rife. Set against this backdrop, measles is often one of the first diseases to rear its ugly head.

“Measles is the canary in the coal mine,” says Dr Boussery. “It’s the first sign of a weakening health system.”

While South Asia has particularly struggled with measles in the wake of the pandemic, other regions are also contending with outbreaks: from richer countries such as Austria, to middle-income ones like Russia, Turkey and South Africa.

In the UK, there were nearly as many cases of the disease in the first four months of this year as there were in the whole of 2022. The numbers are small – there were 49 cases between January and April compared to 54 in the whole of 2022 – but the disease is so infectious a single case can quickly turn into an outbreak.

Across the rich west, these numbers are only likely to rise, says Dr O’Connor.

“This summer is anticipated to see the largest number of travellers since Covid. It only takes one person who doesn’t realise they’re sick to get on a plane and they take the disease elsewhere.

Measles has an R number (the number of people each infected person can pass the disease onto) of between 15 and 20; by comparison, the original Covid virus had an R number of about 1.4 to 2.4.

Because of this, 95 per cent of children must receive two doses of the measles vaccine to ensure the viral disease is kept at bay in a population. But while there have been enormous strides in vaccinating children over the last 20 years, this golden target has never been achieved – and is now slipping further out of reach.