The warning was issued by the World Health Organisation and the US Centers for Disease Control and Prevention.
Reported measles cases fell by more than 80 per cent last year compared with 2019 but a higher number of children missing their vaccine doses leaves them vulnerable, a joint report by the WHO and the US CDC showed on Wednesday.
About three million more children missed the shots in 2020 than the previous year- the largest increase in two decades, threatening global efforts to eventually eradicate the highly infectious viral disease.
"Large numbers of unvaccinated children, outbreaks of measles and disease detection and diagnostics diverted to support COVID-19 responses are factors that increase the likelihood of measles-related deaths and serious complications in children," the US CDC's immunisation head Kevin Cain said.
Measles is one of the most contagious diseases known, more so than COVID-19, Ebola, tuberculosis or flu.
It can be especially dangerous for babies and young children, with pneumonia among the possible complications.
In 2019, reported cases of measles were at their highest in almost a quarter of a century.
The latest report said 24 measles vaccination campaigns originally planned for 2020 in 23 countries were postponed, leaving more than 93 million people at risk.
"It's critical that countries vaccinate as quickly as possible against COVID-19 but this requires new resources so that it does not come at the cost of essential immunisation programs," said Dr Kate O'Brien, director of the WHO's department of immunisation, vaccines and biologicals.
"Routine immunisation must be protected and strengthened; otherwise, we risk trading one deadly disease for another," she said.
AAP with The Project