Global Withdrawal of Trivalent Oral Polio Vax Achieving Goal

— Detection of Sabin-2 poliovirus down but not out

MedpageToday

Prevalence of type 2 poliovirus declined after the global withdrawal of the serotype 2 oral poliovirus vaccine blamed in part for a resurgence in polio cases, researchers found.

After the serotype 2 oral poliovirus vaccine was withdrawn in April 2016, Sabin-2 poliovirus declined in stool samples from children with non-polio acute flaccid paralysis over the next 2 months (prevalence 3.9%, 95% CI 3.5%-4.3%, in March 2016 to 0.2%, 95% CI 0.1%-2.7%, in June 2016; P<0.001 by a chi-square test), reported Isobel M. Blake, PhD, of Imperial College London in England, and colleagues.

Prevalence of Sabin-2 poliovirus in examined sewage samples also dropped from 71% (95% CI 61.0%-80.0%) at the time of withdrawal to 13% (95% CI 8.0%-20.0%) 2 months later.

But there was continued detection of Sabin-2 poliovirus in stool and sewage samples 12 months after withdrawal, which was likely attributed to using another oral poliovirus vaccine in response to outbreaks of vaccine-derived poliovirus, the authors wrote in the New England Journal of Medicine.

They noted that the live attenuated oral poliovirus vaccine, which is "inexpensive and easy to administer," has also been linked with vaccine-associated paralytic poliomyelitis or outbreaks of circulating vaccine-derived polioviruses that cause poliomyelitis. In April 2016, the World Health Organization (WHO) therefore recommended a "globally synchronized withdrawal" of serotype 2 oral poliovirus vaccine, replacing the trivalent vaccine with a bivalent vaccine "to prevent further emergence of circulating [serotype 2 vaccine-derived poliovirus]."

But they added that this withdrawal could be associated with further outbreaks of serotype 2 vaccine-derived poliovirus resulting from continued circulation of this virus caused by use of trivalent oral poliovirus vaccine. Moreover, any response to an outbreak of circulating serotype 2 vaccine-derived poliovirus will include the use of monovalent serotype 2 oral poliovirus vaccine, they said, which could cause more cases of serotype 2 vaccine-derived poliovirus, "thus risking an escalation in the use of [serotype 2 oral poliovirus vaccine] and 'cessation failure.'"

Blake and colleagues examined data from about 495,000 children ages 0 to 14 in the Polio Information System, analyzing two stool samples from each child with acute flaccid paralysis. They also looked at epidemiologic and laboratory data from cases of acute flaccid paralysis in 118 countries, and environmental surveillance, including testing of about 8,500 sewage samples, in four high-risk countries (Afghanistan, Pakistan, Nigeria, and Kenya).

Nine outbreaks of serotype 2 vaccine-derived poliovirus occurred from May 2016 to July 2018, and the authors cited "low routine immunization coverage, low serotype 2 population immunity, and low population density" as risk factors for these outbreaks.

At 12 months after serotype 2 oral poliovirus vaccine withdrawal, prevalence of Sabin-2 poliovirus in stool samples was <0.1% (95% CI <0.1%-0.1%), though the authors noted that the virus was "detected for a longer time after monovalent [serotype 2 oral poliovirus vaccine] campaigns in sewage samples than stool samples from children with non-polio acute flaccid paralysis." Prevalence of Sabin-2 poliovirus in sewage samples was 8.0% (95% CI 5.0%-13.0%) at 12 months after withdrawal.

Limitations to the data include that isolations of Sabin-2 poliovirus were not available from the Americas or the western Pacific region, and that the rate for reporting acute flaccid paralysis varies across populations. Also the authors acknowledged that they "did not consider the association of seasonality with the detection of Sabin-2 poliovirus," which could have affected their projections' accuracy.

An accompanying Perspective piece by Mark A. Pallansch, PhD, of the CDC, warned that surveillance will have to continue following the withdrawal of serotype 2 oral poliovirus vaccine, as new countries use monovalent serotype 2 oral poliovirus vaccines to combat outbreaks of circulating serotype 2 vaccine-derived poliovirus.

"It will be important to monitor whether there are any observable changes over time in the disappearance of OPV2-related virus in these regions where new and past outbreaks have occurred," he wrote. "The unfolding experience following the OPV2 switch will provide lessons that improve our understanding of problems confronting the endgame strategy of OPV cessation."

Disclosures

This study was supported by grants from the Bill and Melinda Gates Foundation and the WHO.

Blake disclosed support from the Bill and Melinda Gates Foundation and the WHO.

Other co-authors disclosed support from the Bill and Melinda Gates Foundation, the WHO, the CDC, and being a member of the WHO SAGE Polio working group.

Pallansch disclosed no conflicts of interest.

Primary Source

New England Journal of Medicine

Source Reference: Blake IM, et al "Type 2 poliovirus detection after global withdrawal of trivalent oral vaccine" N Engl J Med 2018; DOI: 10.1056/NEJMoa1716677.

Secondary Source

New England Journal of Medicine

Source Reference: Pallansch M "Ending use of oral poliovirus vaccine -- A difficult move in the polio endgame" N Engl J Med 2018; DOI: 10.1056/NEJMp1808903.