Type 2 Polio virus detected after global withdrawal of OPV v
The present study has been published in the recent issue of NEJM. Mass campaigns with oral poliovirus vaccine (OPV) have brought the world close to the eradication of wild poliovirus. However, to complete eradication, OPV must itself be withdrawn to prevent outbreaks of vaccine-derived poliovirus (VDPV). Synchronized global withdrawal of OPV began with serotype 2 OPV (OPV2) in April 2016, which presented the first test of the feasibility of eradicating all polioviruses.

The researchers analyzed global surveillance data on the detection of serotype 2 Sabin vaccine (Sabin-2) poliovirus and serotype 2 vaccine–derived poliovirus (VDPV2, defined as vaccine strains that are at least 0.6% divergent from Sabin-2 poliovirus in the viral protein 1 genomic region) in stool samples from 495,035 children with acute flaccid paralysis in 118 countries and in 8528 sewage samples from four countries at high risk (Pakistan, Afghanistan, Kenya and Nigeria) for transmission; the samples were collected from January 1, 2013, through July 11, 2018.

The results showed that the prevalence of Sabin-2 poliovirus in stool samples declined from 3.9% at the time of OPV2 withdrawal to 0.2% at 2 months after withdrawal, and the detection rate in sewage samples declined from 71% to 13.0% during the same period.

HOWEVER, 12 months after OPV2 withdrawal, Sabin-2 poliovirus continued to be detected in stool samples and sewage samples because of the use of OPV2 in response to VDPV2 outbreaks. Nine outbreaks were reported after OPV2 withdrawal and were associated with low coverage of routine immunization and low levels of population immunity within affected countries.

The authors concluded that high population immunity has facilitated the decline in the prevalence of Sabin-2 poliovirus after OPV2 withdrawal and restricted the circulation of VDPV2 to areas known to be at high risk for transmission. The prevention of VDPV2 outbreaks in these known areas before the accumulation of substantial cohorts of children susceptible to type 2 poliovirus remains a high priority.

Read about the research in detail here: https://www.nejm.org/doi/full/10.1056/NEJMoa1716677