Geneva: The World Health Organization gave countries two weeks to replace a problem polio vaccine blamed for some outbreaks of the crippling disease, which the UN health agency is hoping to wipe out once and for all.
WHO has warned that the live polio virus used in some vaccines is one of the biggest obstacles to eradicating the disease.
For the past year and a half, 155 countries and territories have been gearing up to make the shift away from the problem vaccine, with the big switch set to begin on Sunday and to be completed by May 1, WHO said.
"The switch is a significant milestone in the effort to achieve a polio-free world," WHO said in a statement.
A massive global effort has in recent decades come close to wiping out polio, a crippling and potentially fatal viral disease that mainly affects children under the age of five.
Cases have decreased by 99 percent since 1988, when polio was endemic in 125 countries and 350,000 cases were recorded worldwide.
The wild version of the virus now exists only in Afghanistan and Pakistan, but a type of vaccine that contains small amounts of weakened but live polio still causes occasional outbreaks elsewhere.
Oral polio vaccine (OPV) replicates in the gut and can be passed to others through faecal-contaminated water -- meaning it won't hurt the child who has been vaccinated, but could infect their neighbours.
Long-term, WHO recommends that the OPV should be phased out worldwide and replaced by the inactivated polio vaccine (IPV).
But due to a range of constraints, including shortages of supply, the shift is expected to take a while.
In the meantime, the WHO Strategic Advisory Group of Experts, which advises the agency on immunisation, has called for the withdrawal of one form of OPV considered to be at the root of the problem.
The trivalent vaccine protects against all three types of polio, including type 2, which no longer exists in the wild but which causes most vaccine-derived outbreaks.
By the end of this month, countries will simultaneously swap out that vaccine with the bivalent version of OPV, which only protects against polio types 1 and 3.
Since the problem vaccine contains a small dose of type 2 polio, the switch needs to be coordinated to prevent outbreaks in places where the trivalent vaccine is no longer being used.
When the switch is done, "approximately 300 million doses of bivalent OPV will be used in routine immunisation programmes around the world" annually, WHO spokesman Oliver Rosenbauer said.
Thousands of independent monitors will deploy worldwide to confirm that the problem vaccine is no longer being used, WHO said....