Measles outbreaks may occur as a result of the coronavirus pandemic, officials say, because some vaccination programmes are having to be delayed.
Unicef says 117 million children in 37 countries may not get immunised on time.
There have been several large outbreaks in countries across Europe where MMR vaccine uptake has been low.
The UK has already lost its measles-free status, because of rising cases of the potentially deadly infection.
The disease, which causes coughing, rashes and fever, can be prevented by two doses of the mumps, measles and rubella (MMR) vaccine, available free to all young children in the UK.