Two good reasons to drop type 2 virus from oral polio vaccine

Abstract

Since October, 1999, wild poliovirus type 2 has not been detected in community-acquired infection anywhere, despite eradication-quality surveillance.1Anon Apparent global interruption of wild poliovirus type 2 transmission.MMWR Morb Mortal Wkly Rep. 2001; 50: 222-224PubMed Google Scholar We can, therefore, be confident that the virus has been globally eradicated, giving us the first reason to drop the type 2 component from the oral polio vaccine.The second reason is more important. Vaccine viruses are genetic mutants of wild virus strains, with greatly reduced neurovirulence and transmissibility. Neurovirulence can increase as a result of back-mutations, occurring during replication and transmission. As such, oral polio vaccine can give rise to rare sporadic vaccine-associated paralytic polio (VAPP) in vaccinated children or in unvaccinated contacts. If transmissibility is regained, the resultant circulating vaccine-derived poliovirus (cVDPV) could cause outbreaks of VAPP.2Kew OM Wright PF Agol VI et al.Circulating vaccine-derived polioviruses: current state of knowledge.Bull World Health Organ. 2004; 82: 16-23PubMed Google Scholar Reduced immunisation coverage in a community results in children who are shedding vaccine virus coming into contact with those not immunised against polio, providing an optimum environment for the emergence of cVDPVs.2Kew OM Wright PF Agol VI et al.Circulating vaccine-derived polioviruses: current state of knowledge.Bull World Health Organ. 2004; 82: 16-23PubMed Google Scholar In the past, all children not immunised would have been protected through contact with wild viruses. We cannot predict the probability of emergence of cVDPV if oral polio vaccine was stopped in the posteradication era, when birth cohorts will have no immunity from vaccine or wild virus infection. The eradication of type 2 wild virus offers an opportunity to assess this risk by discontinuing type-specific vaccine while surveillance quality remains excellent.Those involved in the Global Polio Eradication Initiative and other science opinion leaders and representatives of agencies that have funded the global mission, should meet urgently to discuss this possibility. If all agree, oral polio vaccine without type 2 should be manufactured and licensed with minimum lead-time. Since October, 1999, wild poliovirus type 2 has not been detected in community-acquired infection anywhere, despite eradication-quality surveillance.1Anon Apparent global interruption of wild poliovirus type 2 transmission.MMWR Morb Mortal Wkly Rep. 2001; 50: 222-224PubMed Google Scholar We can, therefore, be confident that the virus has been globally eradicated, giving us the first reason to drop the type 2 component from the oral polio vaccine. The second reason is more important. Vaccine viruses are genetic mutants of wild virus strains, with greatly reduced neurovirulence and transmissibility. Neurovirulence can increase as a result of back-mutations, occurring during replication and transmission. As such, oral polio vaccine can give rise to rare sporadic vaccine-associated paralytic polio (VAPP) in vaccinated children or in unvaccinated contacts. If transmissibility is regained, the resultant circulating vaccine-derived poliovirus (cVDPV) could cause outbreaks of VAPP.2Kew OM Wright PF Agol VI et al.Circulating vaccine-derived polioviruses: current state of knowledge.Bull World Health Organ. 2004; 82: 16-23PubMed Google Scholar Reduced immunisation coverage in a community results in children who are shedding vaccine virus coming into contact with those not immunised against polio, providing an optimum environment for the emergence of cVDPVs.2Kew OM Wright PF Agol VI et al.Circulating vaccine-derived polioviruses: current state of knowledge.Bull World Health Organ. 2004; 82: 16-23PubMed Google Scholar In the past, all children not immunised would have been protected through contact with wild viruses. We cannot predict the probability of emergence of cVDPV if oral polio vaccine was stopped in the posteradication era, when birth cohorts will have no immunity from vaccine or wild virus infection. The eradication of type 2 wild virus offers an opportunity to assess this risk by discontinuing type-specific vaccine while surveillance quality remains excellent. Those involved in the Global Polio Eradication Initiative and other science opinion leaders and representatives of agencies that have funded the global mission, should meet urgently to discuss this possibility. If all agree, oral polio vaccine without type 2 should be manufactured and licensed with minimum lead-time.