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714 www. thelancet. com Vol 400 September 3, 2022 risk of vaccine-associated paralytic poliomyelitis caused by Sabin 2. 6 Although most countries eliminated Sabinlike poliovirus type 2 (SL2) transmission following the switch, some cVDPV2 transmission continued, leading to sporadic cVDPV2 outbreaks as poliovirus type 2 mucosal immunity declined. 7, 8 Sabin monovalent OPV type 2 (mOPV2) was used to address these outbreaks, but many such campaigns were delayed and provided insufficient coverage, leading to further emergence and transmission of cVDPV2. 8, 9 The COVID-19 pandemic compounded the issue; routine polio vaccination, surveillance, and cVDPV2 outbreak response activities were suspended for 4 months in 2020 to protect communities and health workers, enabling further spread of cVDPV2. 8 Furthermore, many of the large cVDPV2 outbreaks have emerged in countries such as Afghanistan, which are beset by conflict, with highly mobile populations and inadequate health infrastructures. The discovery of VDPV2 in London and New York (NY, USA) should come as a wake-up call. As the full extent of VDPV2 transmission in these cities is evaluated, the factors underlying such transmission in settings with exclusive inactivated polio vaccine (IPV) use, high national vaccine coverage, and good sanitation must also be fully assessed. A possible explanation would be that VDPV2 has emerged following shedding and subsequent circulation of SL2 by individuals who were vaccinated overseas with Sabin mOPV2, and ongoing VDPV2 transmission is occurring in undervaccinated communities. Rockland (NY, USA), the county where a case of poliovirus has been identified, has routine childhood immunisation coverage of 42%, the lowest in New York, 10 and inactivated polio vaccine vaccination coverage in London has fallen in recent years, with coverage as low as 73· 6% in some boroughs. 11 However, the finding of virus in sewage over several months from a large population and with high genetic diversity of the isolated viruses implies that transmission could be widespread. 12 The USA and the UK have exclusively used IPV since 2000 and 2004, respectively. 13, 14 Although two or more doses of IPV provide excellent humoral immunity, protecting fully-vaccinated individuals from developing poliomyelitis caused by all strains of poliovirus, including VDPV2, IPV isless effective than OPV at inducing intestinal mucosal immunity, meaning IPV-vaccinated individuals might theoretically be able to shed polioviruses in their stool, facilitating transmission. 15, 16 However, there are currently few real-world data to evidence this, and the previous success of the IPV in eliminating polio in the Netherlands and Scandinavian countries suggests that a more complex picture of poliovirus immunity and VDPV2 transmission in London and New York mightemerge. 17, 18 There is evolving evidence of genetic linkages between the polioviruses detected in London and New York, and also with SL2 viruses identified in sewage from Jerusalem in June, 2022. 3 Israel boasts high vaccination coverage with both the IPV and bivalent OPV (97· 6% and 88· 3% for the IPV and bivalent OPV, respectively), but despite this, has also recently identified circulating vaccine-derived poliovirus type 3, with one case of poliomyelitis occurring on March 7, 2022. 19

The rise in VDPV2 transmission presents a conundrum for global polio eradication. Eradication of both wild polioviruses and VDPVs is dependent on complete interruption of transmission; this is assumed to be conditional on induction of robust mucosal immunity through use of the OPV. However …