Outbreak response strategies with type 2-containing oral poliovirus vaccines

Abstract

Despite exhaustive and fully-financed plans to manage the risks of globally coordinated cessation of oral poliovirus vaccine (OPV) containing type 2 (OPV2) prior to 2016, as of 2022, extensive, continued transmission of circulating vaccine-derived polioviruses (cVDPVs) type 2 (cVDPV2) remains. Notably, cumulative cases caused by cVDPV2 since 2016 now exceed 2,500. Earlier analyses explored the implications of using different vaccine formulations to respond to cVDPV2 outbreaks and demonstrated how different properties of novel OPV2 (nOPV2) might affect its performance compared to Sabin monovalent OPV2 (mOPV2). These prior analyses used fixed assumptions for how outbreak response would occur, but outbreak response implementation can change. We update an existing global poliovirus transmission model to explore different options for responding with different vaccines and assumptions about scope, delays, immunization intensity, target age groups, and number of rounds. Our findings suggest that in order to successfully stop all cVDPV2 transmission globally, countries and the Global Polio Eradication Initiative need to address the deficiencies in emergency outbreak response policy and implementation. The polio program must urgently act to substantially reduce response time, target larger populations - particularly in high transmission areas - and achieve high coverage with improved access to under-vaccinated subpopulations. Given the limited supplies of nOPV2 at the present, using mOPV2 intensively immediately, followed by nOPV2 intensively if needed and when sufficient quantities become available, substantially increases the probability of ending cVDPV2 transmission globally.

Keywords: Dynamic modeling; Eradication; OPV; Outbreak response; Polio.

Fig. 1.

Expected global number of polio cases by year for 100 stochastic iterations of the different vaccine characteristics scenarios for 2022–2026 compared to the reference case (see main text for descriptions). Notes: All scenarios use the characteristics of the reference case (Table 1 bottom, row 0). Abbreviations: cVDPV(1,2), circulating vaccine-derived poliovirus (type 1 or 2); RC (A0) indicates the reference case, which uses monovalent Sabin OPV2 (A); B0 indicates the use of novel OPV2 (nOPV2) assuming the same effectiveness, no reversion, and no vaccine-associated paralytic polio (VAPP) [16]; C0 indicates the use of nOPV2 assuming the same effectiveness and same VAPP as mOPV2, but reduced reversion relative to mOPV2; D0 assumes nOPV2 with reduced effectiveness, no reversion, and no VAPP; E0 assumes nOPV2 with reduced effectiveness and VAPP and less reversion relative to mOPV2; and F0 uses mOPV2 until April 30, 2024 and then inactivated poliovirus vaccine (IPV) for the rest of the time horizon.

Fig. 2.

Expected global number of polio cases by year for 100 stochastic iterations of univariate outbreak response SIA change scenarios using mOPV2 for direct comparison to the reference case for 2022–2026 (see main text for descriptions). Notes: All scenarios use monovalent Sabin OPV2 (Table 1, top, row A) and all or all but one of the reference case characteristics (Table 1, bottom, row 0). Abbreviations: cVDPV(1,2), circulating vaccine-derived poliovirus (type 1 or 2); RC (A0) indicates the reference case, which assumes response in the outbreak subpopulation and 4 subpopulations in the same block at highest-risk (1 + 4) in blocks with high transmissibility that starts 45 days after case detection with 2 rounds with immunization intensity consistent with recent experience in preventive campaigns, and target children < 5 years of age; A1 reduces the scope of the response to just the outbreak subpopulation (1 + 0) in blocks with high transmissibility; A2 increase the scope to all 10 subpopulations in the block (1 + 9) in blocks with high transmissibility; A3 assumes a shorter delay of 30 days to the first round; A4 assumes a longer delay of 60 days to the first round; A5 assumes three rounds; A6 assumes four rounds; A7 assumes high intensity rounds (i.e., 80% immunization coverage, 70% repeated miss probability) for all response rounds; A8 assumes an expanded target age group of children < 10 years of age for all responses; and A9 assumes an expanded target age group of children < 10 years of age for outbreaks in subpopulations with no OPV2 use since 2016.

Fig. 3.

Expected global value of polio cases by year for 100 stochastic iterations of the combination of SIA options for 2022–2026 compared to the reference case (see main text for descriptions). Notes: All scenarios use the best combination of univariate changes (see Table 1, bottom) and either monovalent Sabin OPV2 (Table 1, top, row A), novel OPV2 (nOPV2) with the most favorable properties (Table 1, top, row B), or the use of mOPV2 for six months followed by nOPV2 with different properties (G, H, I, and J). Abbreviations: cVDPV(1,2), circulating vaccine-derived poliovirus (type 1 or 2); RC (A0) indicates the reference case.

Fig. 4.

Expected global value of polio cases by year for 100 stochastic iterations of some nOPV2 oSIA options with larger (blockwide) response and some other improvements characteristics that exclude improvement of quality for 2022–2026 compared to the reference case (see main text for descriptions). Notes: All scenarios use the best combination of univariate changes (see Table 1, bottom) and novel OPV2 (nOPV2) with the most favorable properties (Table 1, row B), or the use of mOPV2 for six months followed by nOPV2 with the most favorable properties (Table 1, row G). Abbreviations: cVDPV(1,2), circulating vaccine-derived poliovirus (type 1 or 2); RC (A0) indicates the reference case.