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. 2023 Apr 6:41 Suppl 1:A93-A104.
doi: 10.1016/j.vaccine.2021.09.037. Epub 2021 Sep 25.

Modelling the spread of serotype-2 vaccine derived-poliovirus outbreak in Pakistan and Afghanistan to inform outbreak control strategies in the context of the COVID-19 pandemic

Natalia A Molodecky  1 , Hamid Jafari  2 , Rana M Safdar  3 , Jamal A Ahmed  4 , Abdirahman Mahamud  4 , Ananda S Bandyopadhyay  5 , Hemant Shukla  2 , Arshad Quddus  6 , Michel Zaffran  6 , Roland W Sutter  6 , Nicholas C Grassly  7 , Isobel M Blake  7
Affiliations

Modelling the spread of serotype-2 vaccine derived-poliovirus outbreak in Pakistan and Afghanistan to inform outbreak control strategies in the context of the COVID-19 pandemic

Natalia A Molodecky et al. Vaccine. .

Abstract

Background: Since July 2019, Pakistan and Afghanistan have been facing an outbreak of serotype-2 circulating vaccine derived poliovirus (cVDPV2) in addition to continued transmission of serotype-1 wild poliovirus (WPV1) and SARS-CoV-2 in 2020. Understanding the risks of cVDPV2 transmission due to pause of global vaccination efforts and the impact of potential vaccination response strategies in the current context of COVID-19 mitigation measures is critical.

Methods: We developed a stochastic, geographically structured mathematical model of cVDPV2 transmission which captures both mucosal and humoral immunity separately and allows for reversion of serotype-2 oral polio vaccine (OPV2) virus to cVDPV2 following vaccine administration. The model includes geographic heterogeneities in vaccination coverage, population immunity and population movement. The model was fitted to historic cVDPV2 cases in Pakistan and Afghanistan between January 2010-April 2016 and July 2019-March 2020 using iterated particle filtering. The model was used to simulate spread of cVDPV2 infection from July 2019 to explore impact of various proposed vaccination responses on stopping transmission and risk of spread of reverted Sabin-2 under varying assumptions of impacts from COVID-19 lockdown measures on movement patterns as well as declines in vaccination coverage.

Results: Simulated monthly incidence of cVDPV2 from the best-fit model demonstrated general spatio-temporal alignment with observed cVDPV2 cases. The model predicted substantial spread of cVDPV2 infection, with widespread transmission through 2020 in the absence of any vaccination activities. Vaccination responses were predicted to substantially reduce transmission and case burden, with a greater impact from earlier responses and those with larger geographic scope. While the greatest risk of seeding reverted Sabin-2 was predicted in areas targeted with OPV2, subsequent spread was greatest in areas with no or delayed response. The proposed vaccination strategy demonstrated ability to stop the cVDPV2 outbreak (with low risk of reverted Sabin-2 spread) by February 2021.

Conclusion: Outbreak response vaccination campaigns against cVDPV2 will be challenging throughout the COVID-19 pandemic but must be implemented urgently when feasible to stop transmission of cVDPV2.

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Conflict of interest statement

Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Figures

Fig. 1
Fig. 1
Model validation. (A) Monthly incidence of cVDPV2 observed cases (red line) in Pakistan and Afghanistan between 2010 and 2016 and simulated cases (boxplots, across 100 simulations). Inset maps display spatial distribution of observed and simulated cumulative cVDPV2 cases between 2010 and 2016. (B-D) Model calibration of Diamir, Pakistan cVDPV2 outbreak (between 01 July 2019 to 29 February 2020), including (B) log likelihood profile for R0 local and the factor determining the relative between- district coefficient compared to within-district transmission. (C) Monthly observed and simulated cumulative cVDPV2 cases and (D) spatial distribution of observed and simulated cases.
Fig. 2
Fig. 2
Forward simulation of cVDPV2 outbreak originating in Diamir, Pakistan, assuming no vaccination response (since March 2020) based on varying assumptions of the impact of population movement changes on poliovirus transmission due to COVID-19 lockdown. (A) Serotype-2 mucosal and humoral immunity in March 2020 based on simulations from the model. (B) Proportion of districts with > 400 cVDPV2 infections (corresponding to 1 cVDPV2 case) with no response (since March 2020) and various assumptions of poliovirus transmission from changes in transmission due to lockdown measures (typical, modest reduction, substantial reduction, complete halt) between March-June 2020 and March-November 2020). (C) Maps displaying the number of cVDPV2 infections over time under the various assumptions of movement patterns between March-June 2020. (D) Distribution of cumulative number of cVDPV2 cases between July 2019 to November 2020 across the assumptions of movement on transmission.
Fig. 3
Fig. 3
Forward simulation of cVDPV2 outbreak originating in Diamir, Pakistan, assuming no vaccination response (since March 2020) based on 50% reduction in routine immunization (RI) coverage (between March-June 2020) due to COVID-19 lockdown. (A) Estimated RI coverage. (B) Absolute increase in median cumulative number of cVDPV2 cases by 1 November 2020 due to 50% reduced RI during March–June 2020 compared to normal RI coverage (assuming typical poliovirus transmission). (C) Cumulative number of cVDPV2 cases from March 2020 across the assumptions of changes in poliovirus transmission and with or without 50% reduction in RI coverage.
Fig. 4
Fig. 4
Forward simulation of cVDPV2 outbreak originating in Diamir, Pakistan, considering impact of different vaccination strategies. (A) Impact of number, timing and geographic scope (i.e., National and Sub-National) of SIAs based on assumption of modest reduction in transmission between Mar-Jun 2020. (B) Distribution of cumulative number of cVDPV2 cases between July 2019 to November 2020 based on the different assumptions of the impact of population movement on poliovirus transmission and vaccination strategies.
Fig. 5
Fig. 5
Forward simulation of cVDPV2 outbreak originating in Diamir, Pakistan, considering impact of proposed vaccination response. (A) Estimated SIA coverage. (B) Geographic scope and number of proposed SIAs. (C) Impact of proposed SIAs in proportion of districts with > 400 cVDPV2 infections under-estimated and reduced SIA coverage and considering responses with and without NIDs. (D) Maps displaying the number of cVDPV2 infections over time under the proposed response and modest reduction in transmission between March-June 2020. (E) Distribution of cumulative number of cVDPV2 cases between July 2019 to November 2020 (top panel) and July 2019 to February 2021 (bottom panel) based on the different assumptions of movement and vaccination strategies.
Fig. 6
Fig. 6
Risk of emergence and spread of reverted Sabin-2 from proposed SIA response (based on modest reduction in transmission between March-June 2020). (A) Proportion of districts with any (i.e. > 0) and > 100 reverted Sabin-2 infections over time under proposed response, reduced SIA coverage, and without NIDs. (B) Geographic distribution of number of reverted Sabin-2 infections over time based on proposed response with and without NIDs.
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