Around 117 million children worldwide risk contracting measles because dozens of countries are curtailing their vaccination programmes as they battle Covid-19, the United Nations warned on Tuesday.
Currently 24 countries, including several already dealing with large measles outbreaks, have suspended widespread vaccinations, the World Health Organization and the UN's children's fund Unicef said.
An additional 13 countries have had their vaccination programmes interrupted due to Covid-19.
In a joint statement, the Measles and Rubella Initiative (M&RI) said it was vital that immunisation capacity was retained during and after the current pandemic.
"Together, more than 117 million children... could be impacted by the suspension of scheduled immunisation activities," it said.
"The M&RI supports the need to protect communities and health workers from Covid-19 through a pause of mass campaigns, where risks of the disease are high."
"However, this should not mean that children permanently miss out."
Measles, a highly contagious disease, effects around 20 million people every year, the majority of whom are aged under five.
Despite a cheap and readily available vaccine, measles cases have surged in recent years, largely in part to what the WHO terms "vaccine hesitancy".
In 2018, 140 000 measles deaths, mostly among children and babies, were recorded - most were preventable, meaning that the countries they occurred in had a vaccination programme.
Of the two dozen countries to have officially suspended measles vaccine programmes - ostensibly to protect health workers and prioritise Covid-19 response - several have seen worrying rises in measles cases in recent years.
In particular, Bangladesh, Brazil, the Democratic Republic of Congo, South Sudan, Nigeria, Ukraine and Kazakhstan are all battling large outbreaks.
DR Congo alone has had 6000 measles deaths in its current epidemic.
The country last week also recorded a new case of Ebola - just days before the UN was due to announce an end to that outbreak.
Robin Nandy, Unicef's chief of immunisation, told AFP that Covid-19 was likely to place additional strain on already overburdened healthcare systems.
"We have to be mindful of the impact of Covid-19, threatening outbreaks of measles, an extremely contagious and potentially lethal disease for which there already exists a safe and effective vaccine," he said.
"We are therefore urging countries to prepare and plan now for intensive catch-up vaccinations once physical distancing restrictions are lifted."
Billions of people around the world face weeks of lockdown as governments figure out their response to the Covid-19 pandemic.
Experts have warned since the start of the outbreak that response programmes to other infectious diseases - from polio to tuberculosis - are likely to suffer as health services triage workers to Covid-19 cases.
And while Covid-19 is overwhelmingly more serious in older patients, many communicable diseases, including measles, inordinately target children.
"Children younger than 12 months of age are more likely to die from measles complications, and if the circulation of measles virus is not stopped, their risk of exposure to measles will increase daily," said the M&RI.