Measles outbreak risk in Afghanistan: provincial immunity gaps and epidemic potential following immunisation system disruption

Background Following the 2021 political transition, Afghanistan's routine immunisation system experienced disruption. The 2022–23 Afghanistan Multiple Indicator Cluster Survey (MICS) estimated national first-dose measles-containing vaccine (MCV1) coverage at 51.2% (95% CI 49.1–53.2). Provincial heterogeneity in coverage-derived measles transmission potential has not been quantified using these household survey data. Methods Provincial MCV1 coverage among children aged 12–23 months was obtained from the 2022–23 MICS (unweighted n = 6177; weighted n = 6383). We calculated a coverage-adjusted transmission-potential index, R cov = R₀ × (1 − MCV1 × VE₁), using R₀ = 15 and one-dose vaccine effectiveness (VE₁) = 0.93. Deterministic SEIR scenarios and complementary stochastic tau-leaping simulations were run using an 8-day mean latent period and 4-day infectious-compartment duration. Survey uncertainty, upper-bound routine-MCV1 protection gaps, and preferential- and random-targeting SIA requirements were quantified. Results Provincial MCV1 coverage ranged from 8.6% to 88.7%, and all 34 provinces had R cov > 1 (range 2.63–13.80; U5-weighted aggregate 7.95). The theoretical one-dose coverage requirement was 100.36%, whereas the requirement for a complete two-dose series using empirical VE₂ = 0.97 was 96.22%. The standardized 3.5-year upper-bound accumulation of children remaining unprotected by routine MCV1 was 3.02 million (conditional 95% interval 2.92–3.12 million). The province-summed SIA requirement was 4.06 million children (49.8% of U5) under idealized preferential targeting and 7.53 million (92.4%) under random targeting. Single-importation establishment probability ranged from 65.9% to 93.9% across nine illustrative provinces. WHO week-14 surveillance showed positive ordinal geographic concordance with R cov (Kendall τb = 0.331; Spearman ρ = 0.430; both p = 0.011). Conclusions Routine MCV1 alone is mathematically insufficient to reach the population-immunity threshold under the central assumptions. High complete two-dose-series coverage remains theoretically capable of reaching the threshold but is operationally demanding. Sustained routine two-dose delivery, high-reach SIAs, and restoration of immunisation-system capacity are likely to be required. The surveillance comparison is retrospective and supports geographic concordance rather than external model validation.