Modelling Impact of Different Varicella Immunisation Strategies Upon Introduction in the Swedish National Programme.

Abstract

Aim: Countries with varicella immunisation programmes have seen major reductions in disease burden. While two-dose schedules are now universally adopted, dosing ages vary, and some countries initially include catch-up vaccination of older children to speed impact. This modelling study assessed three two-dose schedules in the Swedish setting (1) 12 and 18 months, (2) 18 months and 5 years, and (3) 18 months and 7 years, with and without catch-up of susceptible children based on their medical history.

Methods: A deterministic, compartmental, age-structured, dynamic transmission model was used to evaluate the different strategies.

Results: With coverage similar to other childhood vaccinations in Sweden (≥ 95%) but no catch-up, varicella cases are projected to decrease significantly within four years of programme start for all schedules modelled. However, a temporary rebound may occur around years 4-7 due to accumulating susceptibles before subsiding. Adding catch-up vaccination in this high-coverage setting accelerates and sustains the decline, with varicella disease burden significantly reduced within 2-3 years.

Conclusion: A national childhood varicella programme in Sweden could cut transmission and disease burden within 4 years, with catch-up vaccination speeding this to a few years and lowering rebound risk. The schedule should be guided by programmatic considerations.