Why OPV cessation?
Oral polio vaccine (OPV) is extremely safe and effective at protecting children against lifelong polio paralysis. Over the past ten years, more than 10 billion doses of OPV have been given to nearly three billion children worldwide. More than 16 million cases of polio have been prevented, and the disease has been reduced by more than 99%. It is the appropriate vaccine through which to achieve global polio eradication.
OPV contains attenuated (weakened) polioviruses. On extremely rare occasions, use of OPV can result in cases of polio due to vaccine-associated paralytic polio (VAPP) and circulating vaccine-derived polioviruses (cVDPVs). For this reason, the global eradication of polio requires the cessation of all OPV in routine immunization, as soon as possible after the eradication of wild poliovirus (WPV) transmission.
Phased approach to OPV cessation – trivalent OPV to bivalent OPV switch
Historically, OPV has been available in different formulations:
- Trivalent OPV – containing type 1, 2 and 3 serotypes
- Bivalent OPV – containing type 1 and 3 serotypes
- Monovalent OPV – containing one serotype (ie type 1, 2 or 3)
A mix of all three formulations was being used to eradicate polio during supplementary immunization activities (SIAs). However, only trivalent OPV was used in routine immunization programmes. Bivalent OPV was exclusively used during SIAs to more rapidly interrupt the transmission of WPV1 and 3 – the only remaining WPV strains in circulation. WPV2 has been eradicated since 1999.
With the transmission of WPV2 already successfully interrupted, the only cases of type 2 paralytic polio were being caused by the type 2 serotype component in trivalent OPV. Over 90% of cVDPV cases were due to the type 2 component, which was also responsible for up to 38% of VAPP cases (approx. 200 cases per year worldwide).
In April 2016 a switch was implemented from trivalent OPV to bivalent OPV in routine immunization programmes.
Following WPV1 and WPV3 eradication, use of all OPV in routine immunizations will be stopped.
The switch from trivalent OPV to bivalent OPV is associated with significant public health benefits. More 90% of all cVDPV cases, currently caused by the type 2 component of trivalent OPV, and up to 38% of all VAPP cases, will no longer occur.
In addition to these significant humanitarian benefits, OPV type 2 cessation will provide the GPEI with a ‘push’ for global OPV cessation of all OPVs. Feasibility of OPV cessation was underscored in practice, and ensured a ‘trial run’ for all OPV cessation. Key lessons were learnt to ensure that this process can be implemented in the safest and most efficient manner.
Programmatic implications of trivalent OPV to bivalent OPV switch
Since 2014, countries have been preparing for the switch from trivalent OPV to bivalent OPV to minimise the risks associated with the phased removal of OPV.
The primary risk associated with the switch is the increase in susceptible populations to poliovirus type 2, which in turn increases the risk of new cVDPV type 2 emergence in the immediate period following OPV type 2 cessation. Several strategies are being used to safely manage these risks.
Inactivated polio vaccine (IPV):
To maintain immunity levels to type 2 polio, high-risk countries introduced IPV into routine immunization programmes prior to the switch.
Access to bivalent OPV in routine immunization:
Bivalent OPV was previously licensed for use during SIAs only. Access to sufficient supplies of bivalent OPV licensed for routine immunization use was made available for the switch.
Maintaining outbreak response capacity:
At the same time, outbreak response capacity is being maintained, including ensuring the supply and management of stockpiles of vaccines containing OPV type 2 to facilitate an appropriate outbreak response, should it be necessary.
All WPV2 and Sabin type 2 viruses must be placed under appropriate biocontainment levels on a timely basis to minimize the risk of re-introduction into a type 2 polio-free world.