A high level of population-wide immunity at the time the oral polio vaccine 2 was withdrawn has helped to enhance the disappearance of Sabin-2 vaccine poliovirus as well as restrict circulating serotype 2 vaccine-derived poliovirus to regions known to be at high risk for transmission, according to the results of a new study published in the New England Journal of Medicine.

Large vaccination campaigns for the oral polio vaccine have led to the near eradication of wild poliovirus. For eradication of polio to be complete, however, use of the oral polio vaccine must be withdrawn to prevent outbreaks of serotype 2 vaccine-derived poliovirus. Synchronized global withdrawal of the oral polio vaccine began with the polio serotype 2 oral vaccine in April 2016. The current study analyzed surveillance data on the detection of serotype 2 Sabin vaccine poliovirus and serotype 2 vaccine-derived poliovirus. The investigators used samples of stool collected from 495,035 children with acute flaccid paralysis in 118 countries and from 8528 sewage samples from 4 countries at high risk for transmission from 2013 to 2018.

There was a decline in serotype 2 Sabin poliovirus prevalence in stool samples of 3.9% (95% CI, 3.5-4.3), and the detection rate in sewage samples dropped from 71.0% (95% CI, 61.0-80.0) to 13.0% (95% CI, 8.0-20.0) during the same period. Because the serotype 2 oral polio virus had been used during vaccine-derived poliovirus outbreaks, at 12 months serotype 2 Sabin poliovirus continued to be detected in both stool (<.1%; 95% CI, <.1-.1) and sewage samples (8.0%; 95% CI, 5.0-13.0).

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“Our findings offer support for the planned withdrawal of the oral polio vaccine after eradication of wild-type polioviruses is confirmed, provided high immunity and effective surveillance is maintained in high-risk areas,” the study authors concluded, although they added that in 2017, “the number of poliomyelitis cases associated with serotype 2 vaccine-derived poliovirus (96) exceeded those caused by wild poliovirus (22) for the first time and outbreak response campaigns with [serotype 2 oral polio vaccines] are continuing in several countries.”

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Blake IM, Pons-Salort M, Molodecky NA, et al. Type 2 poliovirus detection after global withdrawal of trivalent oral vaccine. N Engl J Med. 2018;379:834-845