Rationale and timelines for OPV withdrawal
Why replace trivalent OPV with bivalent OPV?
Until April 2016, tOPV was an important component of routine immunization programmes in 155 countries and territories around the world. tOPV contains all three poliovirus serotypes (1, 2 and 3), and the use of this vaccine has led to the eradication of wild poliovirus type 2 (WPV2), with the last case occurring in 1999. The last detected case of WPV3 was in 2012. Furthermore, four of the six WHO regions have been certified as polio-free.
Even as the remaining strains of wild poliovirus are being eradicated, the switch from tOPV to bOPV was a major step to combat cVDPV and VAPP. Over 90% of cVDPV cases, and approximately 40% of VAPP cases, are due to the type 2 component of tOPV. The type 2 component of tOPV also interferes with the immune response to poliovirus types 1 and 3.
Given the risk the type 2 component of tOPV poses to a world free of WPV2, tOPV was replaced with bOPV in routine programmes and supplementary immunization activities (SIAs). bOPV contains type 1 and 3 serotypes only, and can help stop transmission of WPV1 and 3 and reduce the risk of VAPP and cVDPVs.
The introduction of IPV will help to reduce risks associated with the withdrawal of OPV type 2, facilitate interruption of transmission with the use of monovalent OPV type 2 in the case of outbreaks, and hasten eradication by boosting immunity to poliovirus types 1 and 3.
Key dates around the switch
The World Health Assembly endorsed the process and tentative timelines.
Target date for national operational plans to be finalized, based on the guidelines available.
As part of a readiness review, SAGE assessed the epidemiology of persistent type 2 cVDPVs.
Two week window for the switch from tOPV to bOPV, followed by a two-week validation phase.
tOPV is no longer used globally in routine immunization, nor in SIAs.