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

Area of Science:

• Epidemiology
• Immunization Programs
• Public Health Modeling

Background:

• Varicella (chickenpox) immunization programs have proven effective in reducing disease burden globally.
• Two-dose varicella vaccination schedules are standard, but optimal dosing ages and the utility of catch-up campaigns vary.
• Sweden's context requires evaluation of different two-dose schedules and catch-up strategies.

Purpose of the Study:

• To model and compare the impact of three distinct two-dose varicella vaccination schedules in Sweden.
• To assess the effect of including catch-up vaccination for susceptible children on disease reduction.
• To evaluate the projected timelines for disease burden reduction and potential rebound effects.

Main Methods:

• Utilized a deterministic, compartmental, age-structured, dynamic transmission model.
• Simulated three two-dose varicella vaccination schedules: (1) 12 and 18 months, (2) 18 months and 5 years, (3) 18 months and 7 years.
• Incorporated scenarios with and without catch-up vaccination based on medical history.

Main Results:

• All modeled schedules, with ≥95% coverage and no catch-up, projected significant varicella case reduction within four years.
• A potential temporary rebound in cases was observed between years 4-7 without catch-up.
• Adding catch-up vaccination accelerated and sustained disease reduction, achieving significant burden decrease within 2-3 years.

Conclusions:

• A national childhood varicella vaccination program in Sweden is projected to reduce transmission and disease burden within four years.
• Catch-up vaccination can expedite disease control to a few years and mitigate rebound risks.
• The optimal vaccination schedule should be determined by programmatic considerations and public health goals.