Persistence of oral polio vaccine virus after its removal from the immunisation schedule in New Zealand.

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.