Quantifying the benefits of improved operations for polio outbreak response: A model-based analysis

Novel type 2-containing oral poliovirus vaccine (OPV2) was introduced in 2021, and over two billion doses have since been used globally for outbreak response. Despite its reduced risk of virus reversion, stopping circulating vaccine-derived poliovirus type 2 (cVDPV2) outbreaks remains challenging in some countries, highlighting the need to improve outbreak response operations. This study adapted and extended a poliovirus transmission model to evaluate two operational changes in outbreak response: prioritizing under-vaccinated individuals to receive novel OPV2 (nOPV2) and increasing the number of vaccination rounds.

We simulated cVDPV2 transmission in Nigeria from 2024 to 2028 under multiple outbreak response scenarios. These scenarios varied by vaccine allocation strategy: (1) baseline allocation where all target individual had equal chance of receiving nOPV2; (2) priority allocation where under-vaccinated individuals were prioritized based on their vaccination histories of OPV2 doses received through outbreak response or of inactivated poliovirus vaccine (IPV) doses received through routine immunization; (3) idealized allocation where individuals with the lowest immunity were prioritized. Scenarios also varied in the number of vaccination rounds (3 to 5 rounds per outbreak response). Outcome measures included outbreak size (cumulative cVDPV2 paralytic case counts), time to transmission interruption, and total nOPV2 doses used.

Transmission interruption was only achieved under the idealized allocation with five vaccination rounds per response. In all other scenarios, cVDPV2 transmission continued through the end of 2028. Compared with baseline allocation, priority allocation based on OPV2 vaccination history resulted in smaller outbreak sizes with fewer nOPV2 doses for the same number of vaccination rounds, whereas priority allocation based on IPV vaccination history led to larger outbreak sizes and higher nOPV2 use.

Both prioritizing under-vaccinated individuals based on OPV2 vaccination history and increasing the number of vaccination rounds reduce cVDPV2 transmission. However, reactive outbreak response alone may be insufficient to stop cVDPV2 outbreaks in high-risk settings such as Nigeria, as the required operational improvements may exceed programmatic capacity. Future work should evaluate the role of expanded nOPV2 use in other vaccination programs, such as routine immunization, to support cVDPV2 elimination.