Health teams in 155 countries and territories have begun switching to a different polio vaccine.
The changeover, which is due to be completed by May 1, is "a significant milestone in the effort to achieve a polio-free world," the World Health Organization says.
There have been at least 10 cases of the paralyzing disease so far this year -- all of them in Afghanistan and Pakistan.
The new vaccine will protect against the two remaining strains of the virus -- types 1 and 3 -- and will no longer include the type 2 polio virus, which was eradicated in 1999.
The vaccine will still be given as drops in the mouth, so health-care workers will not need new training.
The switch is taking effect mainly in developing countries, but also in richer ones such as Russia.
Based on reporting by Reuters, AFP, and the BBC