The UN children's agency, Unicef, says a measles campaign about to start in Bangladesh is the biggest such vaccination programme ever undertaken.
Measles kills nearly 20,000 children annually in Bangladesh
The campaign is aimed at inoculating 33.5m children in Bangladesh where Unicef estimates nearly 20,000 children a year die from the disease.
The campaign is a joint effort by the Unicef and the Bangladesh government.
Children aged between nine months and 10 years will be covered in the 20-day campaign that starts on 25 February.
The inoculation programme is aimed at 62 districts and five cities. Last September nearly 1.4m children in two districts were given the measles vaccine.
"If ever Bangladesh needed a truly all-out effort to fight measles... it is now," Louis-Georges Arsenault, Unicef's Bangladesh representative says.
Unicef says only three in four children in Bangladesh receive their routine measles vaccine.
It says the 40 million doses of measles vaccines have been prepared for the campaign.