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.
