Discussion

The number of cVDPV outbreaks detected worldwide increased from nine in six countries during the January 2017–June 2018 reporting period (3) to 29 in 15 countries during January 2018–June 2019; 25 (86%) outbreaks were cVDPV2 emergences, 18 (72%) of which were detected during the first half of 2019 in Central and Western Africa. cVDPV2 cases primarily occurred in type 2-naïve children who were born after the switch from tOPV to bOPV and who were therefore at high risk because they were born in areas with chronically low routine and supplementary polio immunization coverage. Seven new cVDPV2 outbreaks were detected in Angola and CAR, countries with no mOPV2 use after the withdrawal of type 2 OPV, but which border DRC, where mOPV2 was used in outbreak responses. Similarly, new cVDPV emergences have occurred in areas of countries that were not part of the mOPV2 response areas (Angola, DRC, and Nigeria). This reflects the increasing susceptibility to type 2 infection and cVDPV2 outbreaks because >3 years have passed since OPV2 cessation. International cVDPV2 spread of JIS-1 from Nigeria to Benin, Cameroon, Ghana, and Niger, and of BAN-1 from Somalia to Ethiopia suggests that multiple mOPV2 responses after detection in each of the countries were of insufficient quality, delayed, or too limited in scope to prevent further spread that, in some cases, led to international transmission.

cVDPV1 and cVDPV3 outbreaks can emerge in countries with suboptimal routine and supplementary immunization coverage; at the subnational level, areas with very wide gaps in immunity carry a higher risk for VDPV emergence and circulation. bOPV campaigns in response to cVDPV1 and cVDPV3 emergences effectively controlled outbreaks in Papua New Guinea (cVDPV1) and Somalia (cVDPV3). cVDPV2 outbreak control requires the use of mOPV2, the release of which depends on the decision of the Director-General of the World Health Organization with the advice from the mOPV2 Advisory Group. Early cVDPV2 detection and timeliness of response are key in addressing circulating VDPV2s; a geographically limited scale mOPV2 campaign should be conducted within 14 days after laboratory cVDPV2 confirmation before larger scale rounds are implemented.

Since April 2016, approximately 300 million mOPV2 doses have been administered in response to cVDPV2 outbreaks (7). Although the effective means to stop cVDPV2 outbreaks is mOPV2, the risks associated with its use include seeding of new VDPV2 emergences with the potential for further circulation. The increase in the frequency of new emergences of cVDPV2 outbreaks outside of mOPV2 response areas has led to enhanced surveillance activities and scaling the geographic distribution of mOPV2 campaigns to 1–4 million persons aged <5 years. GPEI partners are providing a surge in technical assistance staffing to outbreak countries to improve the timeliness and quality of mOPV2 responses to aid in more rapid control of outbreaks and limit new emergences. A novel OPV type 2 vaccine, stabilized to decrease the likelihood of reversion to neurovirulence during replication, is in clinical trials (8) and, if found to be safe and effective, could be available in limited supply for emergency use as early as mid-2020, and in larger supply at a later date. Expansion of environmental surveillance provides critical indicators for early VDPV detection (9); for example, environmental surveillance detection in Cameroon and Ghana in 2019 confirmed circulation of the cVDPV2 emergence of JIS-1 outside Nigeria in the absence of detection of AFP (Cameroon) or before detection (Ghana) of AFP cases.

Since 2000, 1,085 cases of paralysis caused by cVDPV have been reported, 932 (86%) of which were type 2. During the same period, approximately 12 million cases of paralytic polio have been averted through polio eradication efforts. Vaccine-associated paralytic polio can occur in children who receive the vaccine, usually after the first dose, or in their susceptible close contacts, totaling about 2–4 cases per birth cohort of 1,000,000 children before the switch from tOPV to bOPV. Since the switch, an estimated 160–240 cases per year of type 2 vaccine-associated paralytic polio have been averted. In addition, there have been no new cases of VDPV2 excretion identified in persons with primary immunodeficiency (iVDPV) since the switch from tOPV-to-bOPV. Cessation of all OPV use after certification of polio eradication will eliminate the risk for VDPV emergence and spread.