Vaccine-derived poliovirus serotype 2 outbreaks and response in the Democratic Republic of the Congo, 2017-2021

Abstract

Vaccine-derived polioviruses (VDPVs) can emerge from Sabin strain poliovirus serotypes 1, 2, and 3 contained in oral poliovirus vaccine (OPV) after prolonged person-to-person transmission where population vaccination immunity against polioviruses is suboptimal. VDPVs can cause paralysis indistinguishable from wild polioviruses and outbreaks when community circulation ensues. VDPV serotype 2 outbreaks (cVDPV2) have been documented in The Democratic Republic of the Congo (DRC) since 2005. The nine cVDPV2 outbreaks detected during 2005-2012 were geographically-limited and resulted in 73 paralysis cases. No outbreaks were detected during 2013-2016. During January 1, 2017-December 31, 2021, 19 cVDPV2 outbreaks were detected in DRC. Seventeen of the 19 (including two first detected in Angola) resulted in 235 paralysis cases notified in 84 health zones in 18 of DRC's 26 provinces; no notified paralysis cases were associated with the remaining two outbreaks. The DRC-KAS-3 cVDPV2 outbreak that circulated during 2019-2021, and resulted in 101 paralysis cases in 10 provinces, was the largest recorded in DRC during the reporting period in terms of numbers of paralysis cases and geographic expanse. The 15 outbreaks occurring during 2017-early 2021 were successfully controlled with numerous supplemental immunization activities (SIAs) using monovalent OPV Sabin-strain serotype 2 (mOPV2); however, suboptimal mOPV2 vaccination coverage appears to have seeded the cVDPV2 emergences detected during semester 2, 2018 through 2021. Use of the novel OPV serotype 2 (nOPV2), designed to have greater genetic stability than mOPV2, should help DRC's efforts in controlling the more recent cVDPV2 outbreaks with a much lower risk of further seeding VDPV2 emergence. Improving nOPV2 SIA coverage should decrease the number of SIAs needed to interrupt transmission. DRC needs the support of polio eradication and Essential Immunization (EI) partners to accelerate the country's ongoing initiatives for EI strengthening, introduction of a second dose of inactivated poliovirus vaccine (IPV) to increase protection against paralysis, and improving nOPV2 SIA coverage.

Keywords: Acute flaccid paralysis; Africa; Circulating vaccine-derived poliovirus type 2; Democratic Republic of the Congo; Monovalent oral polio vaccine type 2; Novel oral polio vaccine type 2; Polio eradication; Poliovirus environmental surveillance; Poliovirus outbreak response; Supplemental immunization activities; Surveillance.

Fig. 1

Democratic Republic of the Congo (DRC): Acute flaccid paralysis (AFP) cases with ambiguous (aVDPV2) or circulating (cVDPV2) vaccine-derived poliovirus serotype 2 isolated from specimens, with onset of paralysis January 1, 2004-December 31, 2016, cumulative, by province. In 2006 and 2013, no AFP cases in DRC had any VDPV in specimens. For details, by province, see references 7 and 8.

Fig. 2A

Democratic Republic of the Congo (DRC): Acute flaccid paralysis (AFP) cases (◯) confirmed positive for circulating vaccine-derived poliovirus serotype 2 (cVDPV2), by emergence group and semester (S) of year of paralysis onset, January 1, 2017–December 31, 2019. The map for S1 2017 indicates all DRC province names; in subsequent maps, only provinces with AFP cases confirmed positive for cVDPV2 are labelled.

Fig. 2B

Democratic Republic of the Congo (DRC): Acute flaccid paralysis (AFP) cases (◯) confirmed positive for and environmental samples (□) with circulating vaccine-derived poliovirus serotype 2 (cVDPV2), by emergence group and semester (S) of year of paralysis onset or sample collection, January 1, 2020–December 31, 2021.The map for S1 2017 indicates all DRC province names (Fig. 2A); in subsequent maps, only provinces with AFP cases confirmed positive for or environmental samples with cVDPV2 are labelled. DRC-TPA-2 and CAR-BNG-1 do not appear of these maps because genetically linked virus was not detected in AFP cases in DRC.

Fig. 3

Democratic Republic of the Congo (DRC): Numbers of monovalent oral polio vaccine Sabin-strain serotype 2 (mOPV2) Supplementary Immunization Activities (SIAs) conducted in response to circulating vaccine-derived poliovirus serotype 2 (cVDPV2) outbreaks, by health zone, June 2017–April 2021.