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, Rcov = 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 Rcov > 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 Rcov (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.