WHO’s SAGE Gives Final Nod to April Polio Vaccine Switch
Although circumstances are less than ideal, the World Health Organization’s immunization advisory panel recommends that countries move forward with a worldwide switch in oral polio vaccines (OPV) in April 2016. While a vaccine considered important to the switch scenario is in short supply, WHO’s Strategic Advisory Group of Experts on Immunization (SAGE) concluded October 20 that the benefits still outweigh the risks and that the move should go ahead as originally planned.
The vaccine switch calls on all national immunization programs to move from a trivalent OPV that provides immunity for all three types of wild, or naturally occurring, poliovirus to a bivalent version that addresses only types 1 and 3. The shift will greatly reduce the incidence of both circulating vaccine-derived polioviruses and vaccine-associated paralytic polio since the type 2 vaccine virus is responsible for 90% and 40% of such cases, respectively. The Global Commission for the Certification of the Eradication of Poliomyelitis determined earlier this year that type 2 wild poliovirus has been eradicated, paving the way for the type 2 vaccine to be eliminated.
Moving from trivalent to bivalent OPV is the first stage in eventual worldwide withdrawal of all OPV and replacement with inactivated poliovirus vaccine (IPV). As a killed virus vaccine, IPV carries no risk of vaccine-related disease. An attenuated live virus vaccine, OPV can in rare cases cause paralysis and even mutate into a form that circulates among populations with low polio immunity. Most higher income countries have long used the injectable IPV in their immunization systems, but 126 countries were still using only OPV as of January 2013. Further, since it is substantially less expensive and easier to administer, OPV is the vaccine of choice for mass polio vaccination campaigns. Many countries will continue using both vaccines for the near future.
In making its recommendation, SAGE considered the potential complications resulting from a shortage of IPV as well as other issues. IPV manufacturers working with the GPEI said they had underestimated the complexity of the task and are behind on their projected vaccine output. As a result, not all countries will have included at least one dose of IPV into their immunization systems before the OPV switch is made, a measure SAGE has recommended previously. The introduction of IPV, which contains all three poliovirus types, would boost global immunity during the OPV switch. The highest risk countries will still be able to introduce IPV before the April switch date, but some lower risk countries will need to wait for several months after the change for supply to meet demand. The delay will affect only a small fraction of the global birth cohort in countries at low risk for a polio outbreak, according to GPEI officials, but will require clear engagement and coordination with national immunization officials.
In addition to the IPV shortage, other potential complicating factors related to the OPV switch include recent cases of vaccine-derived poliovirus in Guinea and South Sudan caused by the type 2 vaccine virus and concerns about country readiness to contain or destroy materials containing type 2 poliovirus, including vials of trivalent OPV.
While delaying the switch would give IPV manufacturers more time to create an adequate vaccine supply and ensure better containment preparations, SAGE listed several “pros” in recommending the switch as planned. These include reducing the incidence of type 2 vaccine-related polio cases and the current global momentum toward the shift. Countries have been gearing up for the switch by increasing stocks of bivalent OPV, including IPV in their immunization systems, and working on type 2 poliovirus containment strategies. If the change were delayed, it would likely be April 2017 before it could be undertaken again. SAGE said in addition that as wild poliovirus is further reduced, polio surveillance and outbreak capacity could decline. Both systems need to be in top form during the switch to monitor for and then quickly respond to any type 2 poliovirus outbreaks.
In preparing for the switch, SAGE advised the GPEI to respond vigorously to any vaccine-derived poliovirus outbreaks and asked Pakistan to review its polio vaccination schedule to ensure sufficient population immunity before the switch. The GPEI has several polio vaccine campaigns planned for each of nearly 30 countries between September 2015 and March 2016.
As the SAGE notes, the switch is not risk free. All materials containing the type 2 virus must be contained or destroyed worldwide to avoid any type 2-related outbreaks. Nonetheless, it is a first step to removing OPV globally and halting vaccine-related paralysis. The GPEI and SAGE will continue to monitor both the IPV supply and for outbreaks of vaccine-derived disease as the switch date approaches. GPEI communications and support for countries will be critical as they tackle this ambitious and complicated task.