Measles is now an "imminent threat" across every region in the world, according to the World Health Organization ( WHO ) and the US public health agency.
A joint report by the two health organisations said millions of children were now susceptible to measles, one of the world’s most contagious diseases, due to a fall in vaccine uptake during the pandemic.
The disease is almost entirely preventable through vaccination, but at least 95 percent vaccine coverage is necessary to prevent outbreaks.
Almost 40 million children missed a dose last year because of pandemic-related hindrances.
"We are at a crossroads and it is going to be a very challenging 12-24 months trying to mitigate this." said Patrick O'Connor, the WHO's measles lead.
Around 81 percent of children had received their first dose of the measles vaccine, while only 71 percent got their second dose, which is the lowest global coverage rate of the first measles dose since 2008.
“The record number of children under-immunised and susceptible to measles shows the profound damage immunisation systems have sustained during the COVID-19 pandemic,” Centres for Disease Control and Prevention director Dr Rochelle Walensky said in a statement.
According to the NHS, measles usually starts with cold-like symptoms, followed by a rash a few days later. Some people may also get small spots in their mouth.
A distinctive rash usually appears a few days after the cold-like symptoms.
The spots of the measles rash are sometimes raised and join together to form blotchy patches, but not usually itchy.
It is rare, but sometimes measles can lead to serious problems if it spreads to other parts of the body, such as the lungs or brain.
It could lead to pneumonia, meningitis, blindness or seizures.
In February, health officials in England sounded the alarm that vaccine rates had dropped to their lowest level in a decade.
More than 95 percent of deaths from measles occur in developing countries, mostly in Africa and Asia.
While there is no specific treatment for measles, by having the two-dose vaccine you will be 97 percent protected against severe illness and death.
