Polio in Nigeria, 2018–2019: Wild Virus Gone Since 2016, Vaccine-Derived Type 2 Spreading

Nigeria once reported most of Africa's wild poliovirus (WPV) cases — 1,122 in 2006 — and was the source of most WPV outbreaks elsewhere. The last WPV type 3 seen anywhere in the world was in Nigeria, in November 2012. By 2014 cases were down to six of type 1; none were found from August 2014 to July 2016. Then, in August–September 2016, four WPV1 cases turned up in Borno State, among families freed by military action to reach camps. No WPV has been detected since October 2016.

The insurgency

An insurgency that began in 2009, with fighters seizing territory from 2012 in Borno, Adamawa and Yobe states and neighboring parts of Cameroon, Chad and Niger, displaced 1.8 million people. By 2016, an estimated 468,800 children under 5 lived in insurgent-held Borno with no health services — no vaccination, no surveillance. People have since kept moving out, but many settlements remain unreachable, and about 60,000 children there were still unvaccinated.

Routine immunization

Coverage with the third polio vaccine dose (Pol3) by 12 months has been below 60% nationally since 2002, and lower in the north: a 2016 survey found 33% nationally and under 25% in seven of 13 northern states.

Surveillance

• Community informants. With health facilities destroyed, 1,018 informants in Borno reported 220 verified acute flaccid paralysis (AFP) cases in 2018–2019, especially since February 2018, with no WPV found. Stool samples are taken when patients leave insurgent-held areas for evaluation; adequacy was 61% in 2018 and 79% in 2019 so far.
• Sewage testing. 113 sites in 25 of 37 states (68%); no WPV, but 85 type 2 vaccine-derived isolates, January 2018–May 2019.

Vaccine-derived type 2 outbreaks

Oral polio vaccine (OPV) contains weakened live virus; where immunization is low, it can spread and revert into vaccine-derived poliovirus (VDPV) that paralyzes, called circulating (cVDPV) once it spreads in a community. From 2006 to 2015, more than 94% of cVDPVs worldwide were type 2. So in April 2016 the world switched from trivalent OPV to bivalent OPV (types 1 and 3), added injectable inactivated vaccine (IPV) — which protects individuals but, unlike OPV, doesn't stop fecal shedding — and kept monovalent type 2 OPV (mOPV2) for outbreaks. Type 2 wild virus was declared eradicated in 2015.

Since January 2018 (to June 25, 2019), Nigeria has had two separate cVDPV2 outbreaks:

Outbreak responses so far haven't interrupted transmission.

Reaching the children

Campaigns. In 2018: two national campaigns with bivalent OPV, and several subnational rounds with bivalent OPV, some with fractional IPV; more in 2019, with Gombe holding three. Multiple mOPV2 outbreak rounds were held — but quality varied: nationally, 64% to 90% of sampled local government areas reached the target of vaccinating 90% of children.

Insurgent-held Borno:

Satellite imagery estimates settlement populations, and tracking with geographic information systems records where teams go. Of about 104,330 children under 5 still in insurgent-held areas, 43,840 (42.0%) had received at least one bivalent OPV dose; most unreached children live in settlements scattered over a wide area. Of children in reached settlements, 79.8% (Reaching Every Settlement) and 26.2% (Reaching Inaccessible Children) had been offered three or more doses. Because few mOPV2 doses reached these areas, those 104,330 children have had no type 2 OPV.

Inhabited settlements reached with bivalent oral polio vaccine by standard house-to-house teams, Reaching Every Settlement and Reaching Inaccessible Children. CDC

What it means

• Fewer unvaccinated children in insurgent-held areas reflects new vaccination approaches and people moving out to accessible areas.
• With surveillance assessments under way, the African Regional Certification Commission might certify the interruption of wild poliovirus in Africa in 2020 — though ongoing cVDPV2 transmission in Nigeria or the eight other African countries with active outbreaks could complicate it.
• Stopping cVDPV2 will take better campaign quality and reaching insurgent-held areas.

Sources

Based on Adamu US, Archer WR, Braka F, et al., "Progress Toward Poliomyelitis Eradication — Nigeria, January 2018–May 2019," MMWR volume 68, number 29, Centers for Disease Control and Prevention; a work of the United States government in the public domain. The report dates its figures on unreached children inconsistently (December 2016–May 2017 in its summary, May 2018 in its text), so they are given here without a date; it prints one onset date as "April 3, 2109," which is 2019.