It marked the beginning of the largest and fastest globally coordinated rollout of a vaccine into routine immunization programs in history, said Farhan Haq, the deputy UN spokesman, at a daily news briefing held here Friday.

Between April 17 and May 1, 155 countries and territories in the world will stop using the trivalent oral polio vaccine (tOPV), which protects against all three strains of wild poliovirus, and replace it with bivalent OPV (bOPV), which protects against the remaining two wild polio strains, types 1 and 3, Haq said.

"This transition, referred to as the global vaccine 'switch,' is possible because type 2 wild polio has been eradicated," he said.

The switch is a significant milestone in the effort to achieve a polio-free world. In 2015, there were fewer cases reported in fewer countries than before.

This year, the focus is on reaching every child with the polio vaccine and stopping the virus in its final strongholds.

The World Health Organization (WHO) and the UN Children's Fund (UNICEF) consider that the world is closer than ever to ending polio worldwide.

The oral polio vaccine (OPV) has been used to stop polio in most of the world. On very rare occasions in under-immunized populations, the live weakened virus contained in OPV can mutate and cause circulating vaccine-derived polioviruses (cVDPV).

More than 90 percent of cVDPV cases in the last 10 years have been caused by the type 2 vaccine strain. Withdrawing tOPV and replacing it in routine immunization programs with bOPV will eliminate the risks associated with the type 2 vaccine strain and, just as importantly, boost protection against the two remaining wild strains of the virus.

The switch must be globally synchronized because if some countries continue to use tOPV, it could increase the risk of the spread of type 2 poliovirus to those no longer using tOPV. The switch is the first major step toward the eventual removal of all OPV after wild polio transmission has been stopped.

To ensure that the switch takes place as planned, thousands of independent monitors will confirm the absence of tOPV at public and private service facilities and cold chain stores.

In countries at higher-risk of a polio outbreak, a dose of inactivated polio vaccine (IPV) has been added to routine immunization schedules, in addition to bOPV, to further boost immunity. To protect against the very small risk of an outbreak of cVDPV type 2 after the switch, a global stockpile of monovalent OPV (mOPV) type 2 is ready to be dispatched if an outbreak occurs.