Research Letters 394 www.thelancet.com Vol 366 July 30, 2005 The global eradication of wildtype poliomyelitis by mass immunisation campaigns with live-attenuated oral poliovirus vaccine (OPV) is imminent, despite outbreaks of this disease in Nigeria in 2004–05 and persisting small reservoirs in Africa and Asia (http://www. polioeradication.org). A priority is to develop strategies of when and how to stop OPV immunisation once poliomyelitis is eradicated. However, whether vaccine virus transmission is sustained after withdrawal of OPV from immunisation schedules remains unknown. This question is important, since persistent circulation of OPV viruses increases the risk of reversion to fully neurovirulent vaccine-derived poliovirus strains in unvaccinated populations. 1 After OPV vaccination, poliovirus is excreted by healthy children for 2–3 months and its persistence in populations is limited. 1 Reports 2 from several developing countries though indicate that circulating neurovirulent vaccine-derived poliovirus strains can be sustained for extended periods and cause poliomyelitis when population immunity is low. Since 1961, New Zealand has maintained OPV coverage of about 85%. However, after two instances of vaccine-associated paralytic poliomyelitis, inactivated poliovirus vaccine (IPV) replaced OPV in the infant immunisation schedule on Feb 1, 2002. This change provided an opportunity to monitor the persistence of OPV strains excreted by the last cohorts of children immunised with OPV. We did systematic population-based surveillance for OPV virus circulation and evolution before, during, and after the OPV/IPV switch with combined paediatric-inpatient, acute flaccid paralysis, enterovirus laboratory, and environmental surveillance systems. Based on Cuba’s experience of annual mass immunisation campaigns, we postulated that polioviruses would be isolated during the preswitch period and then decline over a 2-month transitional period, after which no further polioviruses would be isolated. 3 The Wellington Ethics Committee approved this study on behalf of Auckland and Waikato Ethics Committees. All patients or their parents or guardians provided written consent. We surveyed paediatric inpatients for 8 months (3 months before, 2 months during, and 3 months after the OPV/IPV switch) at three hospitals in Auckland, Hamilton, and Wellington. To detect a decline in OPV strain prevalence from 4·5% (previous national enterovirus surveillance data) to 0·5% (80% power, 95% significance, two-tailed test of difference in proportions), we approached every month 35 children (younger than age 15 years) consecutively admitted to each of the three hospitals with expectation of 80% participation. Of 861 patients recruited, 633 (74%) provided stool samples for testing. The results of paediatric-inpatient surveillance indicate that vaccine viruses disappeared quickly after the switch (figure 1). During the preswitch and transition periods, we isolated polioviruses from 18 Lancet 2005; 366: 394–96 Published online May 3, 2005 DOI: 10.1016/S0140-6736(05) 66386-6 See Comment page 351 Institute of Environmental Science and Research, PO Box 50348, Porirua, New Zealand (Q S Huang PhD, G Greening PhD, M G Baker FAFPHM, J Hewitt MSc, D Hulston NZCS, L van Duin BMLS, A Fitzsimons BSc); Department of Paediatrics, Wellington School of Medicine and Health Sciences, University of Otago, Wellington, New Zealand (Prof K Grimwood MD); Faculty of Health, Auckland University of Technology, Auckland, New Zealand (N Garrett MSc); Waikato Hospital, Hamilton, New Zealand (D Graham FRACP); South Auckland Clinical School, Middlemore Hospital, Auckland, New Zealand (Prof D Lennon FRACP); Department of Virology II, National Institute of Infectious Diseases, Tokyo, Japan (H Shimizu PhD, T Miyamura MD); Persistence of oral polio vaccine virus after its removal from the immunisation schedule in New Zealand Q Sue Huang, Gail Greening, Michael G Baker, Keith Grimwood, Joanne Hewitt, Debbie Hulston, Lisa van Duin, Amanda Fitzsimons, Nick Garrett, David Graham, Diana Lennon, Hiroyuki Shimizu, Tatsuo Miyamura, Mark A Pallansch On Feb 1, 2002, inactivated poliomyelitis vaccines replaced live-attenuated oral poliovirus vaccine (OPV) in New Zealand’s immunisation schedule, allowing systematic monitoring of OPV virus circulation. Findings of paediatric-inpatient surveillance indicate that 7% of children excreted polioviruses before this switch, but none did so 1 month afterwards. Acute flaccid paralysis surveillance detected no poliovirus during and after the switch, whereas enterovirus surveillance detected poliovirus only once during the switch. Environmental surveillance identified polioviruses in sewage samples until May, 2002, after which they were detected infrequently. Intratypic differentiation and sequencing showed that all polioviruses were Sabin-like. Multiple surveillance methods hence showed that OPV strains did not persist for extended periods after a vaccine switch in a developed country with a temperate climate. Sequence homology with Sabin vaccine parent strains indicated that polioviruses detected more than 4 months after the switch were of recent origin, consistent with importation from OPV-using countries. Preswitch period Transitional period Month sample collected p0·0001 Post-transitional period 0 10 8 6 4 2 18 16 14 12 20 Positive for polioviruses (%) Jan, 2002 Dec, 2001 Nov, 2001 Positive samples Total samples Proportion positive May, 2002 Apr, 2002 Mar, 2002 Feb, 2002 Jun, 2002 4 60 7% 7 91 8% 7 110 6% 6 70 9% 0 79 0% 0 70 0% 0 80 0% 0 73 0% Figure 1: Poliovirus prevalence (95% CI) between November, 2001, and June, 2002 by paediatric inpatient surveillance