Modeling Poliovirus Transmission in Borno and Yobe, Northeast Nigeria

Abstract

Beginning in 2013, multiple local government areas (LGAs) in Borno and Yobe in northeast Nigeria and other parts of the Lake Chad basin experienced a violent insurgency that resulted in substantial numbers of isolated and displaced people. Northeast Nigeria represents the last known reservoir country of wild poliovirus (WPV) transmission in Africa, with detection of paralytic cases caused by serotype 1 WPV in 2016 in Borno and serotype 3 WPV in late 2012. Parts of Borno and Yobe are also problematic areas for transmission of serotype 2 circulating vaccine‐derived polioviruses, and they continue to face challenges associated with conflict and inadequate health services in security‐compromised areas that limit both immunization and surveillance activities. We model poliovirus transmission of all three serotypes for Borno and Yobe using a deterministic differential equation‐based model that includes four subpopulations to account for limitations in access to immunization services and dynamic restrictions in population mixing. We find that accessibility issues and insufficient immunization allow for prolonged poliovirus transmission and potential undetected paralytic cases, although as of the end of 2019, including responsive program activities in the modeling suggest die out of indigenous serotypes 1 and 3 WPVs prior to 2020. Specifically, recent and current efforts to access isolated populations and provide oral poliovirus vaccine continue to reduce the risks of sustained and undetected transmission, although some uncertainty remains. Continued improvement in immunization and surveillance in the isolated subpopulations should minimize these risks. Stochastic modeling can build on this analysis to characterize the implications for undetected transmission and confidence about no circulation.

Review of poliovirus modeling performed from 2000 to 2019 to support global polio eradication

“…Although not captured in the review, modeling of one of the last known reservoirs of WPV3 transmission (i.e. Borno and Yobe, Nigeria) published in 2020 [ 233 , 234 ] also supported the 2019 decision by the Global Certification Commission to certify the global eradication of indigenous WPV3 [ 235 ].…”

Modeling Undetected Live Poliovirus Circulation After Apparent Interruption of Transmission: Borno and Yobe in Northeast Nigeria

“…We model LPV transmission within Borno and Yobe by characterizing individuals in the population using eight immunity states further subdivided by a five‐stage process of waning of immunity to infection, and infections using a 20‐stage poliovirus reversion process and a six‐stage infection process for both fecal‐oral and oropharyngeal routes of transmission (Kalkowska, Wassilak, Cochi, Pallansch, & Thompson, 2020b). We divide the population into 11 age groups and assign these to three preferentially mixing age groups, which reflect the tendencies of individuals to mix more with other individuals of similar age (Kalkowska et al., 2020a). We model Borno and Yobe as one epidemiological block, but divide it into a general population, an undervaccinated subpopulation, and two distinct isolated subpopulations.…”

“…We model Borno and Yobe as one epidemiological block, but divide it into a general population, an undervaccinated subpopulation, and two distinct isolated subpopulations. We assume the two distinct isolated populations are part of the undervaccinated subpopulation prior to 2013, then they gradually become increasingly isolated in 2013 and 2014, become completely isolated from 2015 to early 2016, and then gradually become slightly more accessible after early 2016 with the implementation of special interventions (Kalkowska et al., 2020a). The model specifies demographic information, poliovirus transmissibility and seasonality, mixing among the four subpopulations, and the history of poliovirus vaccination, including both OPV and inactivated poliovirus vaccine (IPV) use for routine immunization (RI) and supplemental immunization activities (SIAs) as previously described (Kalkowska et al., 2020a).…”

“…Northeast Nigeria represents an important epidemiological region (Kalkowska et al., 2020a), with the last globally reported cases of WPV serotype 3 (WPV3) in Borno and Yobe in November 2012 (Kew et al., 2014), the last African region reported cases of WPV serotype 1 (WPV1) in Borno in 2016 (Nnadi et al., 2017; World Health Organization [WHO], 2016), and ongoing challenges with reported cases of circulating vaccine derived polioviruses (cVDPVs) of serotype 2 (cVDPV2s) in the same area (Etsano et al., 2016). Since Nigeria went for a nearly two‐year period with no reported cases from July 24, 2014, in Kano (with a reported case in April 19, 2014, in Yobe) until the detection of WPV1 transmission in Borno on July 4, 2016, the African Region previously began considering regional certification of all WPVs.…”

“…The detection of cases also restarted the time since the last reported case that occurred in Borno and Nigeria to August 21, 2016. Now that three full years have passed since the detection of the last reported WPV1 case, we apply a previously developed differential equation based model (Kalkowska et al., 2020a) and stochastic approach (Kalkowska et al., 2015) to explore the confidence about no circulation of poliovirus transmission as a function of time without detected events for conditions relevant to Borno and Yobe. We use the model to consider the effect of different assumptions about the quality of AFP surveillance on the confidence about no circulation.…”