# Estimating population immunity against serotype-two poliomyelitis from the inactivated polio vaccine in routine immunization across 112 countries: A modelling study

## TL;DR

This study models population immunity against type-2 polio from inactivated polio vaccine (IPV) in 112 countries to guide optimal vaccination schedules and catch-up campaigns.

## Contribution

A novel cohort model was developed to estimate and compare population immunity under different IPV schedules and interventions across 112 countries.

## Key findings

- Estimated median type-2 population immunity in 2025 is 61% among children under five, rising to 71% by 2031 under current schedules.
- Adopting the 14–39 week schedule could raise median immunity to 78% by 2031 if universally implemented.
- 25 countries or territories have <50% immunity in children under five, highlighting suboptimal schedules and coverage.

## Abstract

To mitigate the risk of outbreaks of serotype 2 poliomyelitis after withdrawal of this serotype from oral poliovirus vaccine (OPV) in 2016, inactivated poliovirus vaccine (IPV) was introduced into the routine immunization (RI) programmes of all countries using OPV. Since 2022, WHO has recommended a 2-dose schedule, with a first dose at 14 weeks of age followed by a second dose at least 4 months later (e.g., 14–39 week schedule), although an earlier schedule may be adopted, despite lower immunogenicity, if vaccine coverage is low at older ages.

We combined published data on type-2 IPV seroconversion with age, national RI coverage estimates, dose introduction dates, and country-specific schedules using a cohort model of population immunity to estimate IPV-induced immunity from 2024–2031 for 112 countries using either one or two doses of IPV. We projected immunity for current, 6–14, and 14–39 week schedules to find the optimal schedule and estimate the impact of interventions such as schedule changes and catch-ups. Under current schedules, estimated median serotype 2 population immunity in 2025 among children under five years of age is at 61% (IQR: 52%, 72%), rising to 71% (IQR: 57%, 80%) in 2031. The later 14–39 week schedule was optimal in all countries, with potential for the median immunity to rise to 78% (IQR: 66%, 85%) by 2031 if adopted by all countries in 2026. Eight countries would still have <50% immunity, rising to 65%−72% if catch-up campaigns with 80% coverage were implemented in 2030. The work is limited by the fact that IPV provides only a partial picture of total immunity where there has been emergency type-2 OPV use. Furthermore, national estimates may mask subnational coverage differences and pockets of extremely low immunity.

Under these estimates, IPV schedules and coverage are suboptimal in many countries. Those with a single dose should introduce a second on the 14–39 week schedule; those on early schedules would benefit from adopting the 14–39 week schedule. IPV catch-up campaigns are recommended where RI coverage is low.

Multiple vaccine-derived type-2 poliovirus outbreaks continue to circulate.

The inactivated polio vaccine (IPV) is the only vaccine delivered through routine immunization that protects against type-2 poliomyelitis. A minimum of two IPV doses is recommended to be given at 14 weeks of age and 4 months later, (e.g., 39 weeks), although 43 countries or territories administer an earlier week schedule, often at 6/14 weeks.

There is a need to quantify type-2 population immunity from IPV given differences in country schedules, national coverage and the timing of the second dose introduction.

We developed a model to estimate national population immunity against type-2 poliomyelitis across 112 countries in children <5 years old.

We projected immunity levels for current immunization schedules, alternative scenarios, and catch-up vaccination campaigns.

Current IPV-induced immunity levels are critically low in many locations, with 25 countries or territories having <50% population immunity in children under five years of age.

We found that the IPV schedules and coverage are suboptimal in many countries, and in many cases adopting the 14–39 week schedule would be beneficial.

The 43 countries or territories with an early immunization schedule, and the 23 countries using a single dose of IPV immunization are recommended to change to a 14–39 week schedule.

Catch-up campaigns could be considered to shorten the time to reach higher immunity in countries with recent introductions of a second dose of IPV, and in countries where IPV coverage is very low and immunity would otherwise remain critically low.

The work is limited by the fact that IPV provides only a partial picture of total immunity where there has been emergency type-2 live vaccine use. Furthermore, national estimates may mask subnational coverage differences and pockets of extremely low immunity.