Qualifying the benefits of improved operations for polio outbreak response: A model-based analysis
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Background
Novel type 2-containing oral poliovirus vaccine (nOPV2) was introduced in 2021. Since then, over 1 billion doses of nOPV2 have been used in outbreak response globally. Despite the reduced risk of virus reversion with nOPV2, stopping ongoing circulating vaccine-derived poliovirus type 2 (cVDPV2) outbreaks in some countries and achieving global cVDPV2 elimination have proved challenging, which highlights the potential needs to update outbreak response operations. In this study, we adapted and extended an existing poliovirus transmission model to evaluate the impact of two operational changes in outbreak response: prioritizing under-vaccinated individuals to receive nOPV2 in each vaccination round and increasing the number of vaccination rounds in each response.
Methods
We tested several outbreak response scenarios, assuming a limited nOPV2 stockpile. These scenarios varied in the strategy of allocating nOPV2 doses among target individuals of a response: (1) Baseline allocation where each target individual had equal chance of receiving nOPV2; (2) Priority allocation where “under-vaccinated individuals” were prioritized to receive nOPV2; Under-vaccinated individuals were identified based on the history of, either type 2-containing OPV doses received through outbreak response or inactivated poliovirus vaccine (IPV) doses received through essential immunization; (3) Idealized allocation where “low-immunity individuals” were prioritized to receive nOPV2; Low-immunity individuals were identified based on true immunity level. These scenarios also varied in the number of vaccination rounds (i.e., 3 to 5 rounds) in the response to each detection of cVDPV2 transmission. Outcome measures included outbreak size (i.e., number of cVDPV2 paralytic cases), time to outbreak interruption, and total nOPV2 doses used in outbreak response. We tested these scenarios by predicting cVDPV2 transmission in Nigeria for the study period of 2024-2028.
Results
Interruption of cVDPV2 transmission in Nigeria was only achieved with idealized allocation and 5 vaccination rounds in each response. In all other scenarios, cVDPV2 transmission continued at the end of the study period. Compared to the baseline allocation, with the same number of vaccination rounds, priority allocation based on the history of type 2-containing OPV doses received through outbreak response resulted in smaller outbreak sizes using fewer nOPV2 doses; priority allocation based on the history of IPV doses received through essential immunization resulted in larger outbreak sizes using more nOPV2 doses.
Conclusions
Prioritizing under-vaccinated individuals based on type 2-containing OPV vaccination history along with increasing the number of vaccination rounds can reduce cVDPV2 transmission. However, reactive use of nOPV2 in outbreak response may not suffice to eliminate cVDPV2 outbreaks in high-risk countries such as Nigeria, as the required operational changes (e.g., prioritization of low-immunity individuals and 5 rounds) might supersede the response capacity in these countries. Incorporating nOPV2 into essential immunization or other preventive vaccinations need to be considered to stop cVDPV2 transmission in high-risk countries and achieve global cVDPV2 elimination.
