Can Vaccinated Children Still Get Chickenpox?
Yes, children who have received the varicella vaccine can still contract chickenpox, but this "breakthrough" disease is typically much milder, less common, and occurs in only a small percentage of vaccinated children. 1, 2
Vaccine Effectiveness and Breakthrough Disease
The varicella vaccine provides strong but not absolute protection:
- After one dose, vaccine effectiveness is approximately 82-94% against any varicella disease and 97% against moderate-to-severe disease 1, 3
- After two doses, vaccine effectiveness increases significantly to 98.3%, with a 3.3-fold lower risk of breakthrough disease compared to one dose 1
- In a 14-year prospective study, vaccine effectiveness remained at 90% with no evidence of waning immunity over time 4
Characteristics of Breakthrough Varicella
When vaccinated children do develop chickenpox, the presentation is distinctly different:
- Fewer lesions: Typically fewer than 50 lesions compared to 250-500 in unvaccinated children 2
- Different rash appearance: More maculopapular (flat, raised bumps) rather than vesicular (fluid-filled blisters) 2, 3
- Milder symptoms: Fever is less common and systemic symptoms are reduced 2
- Still contagious: Despite being milder, breakthrough cases can transmit the virus to others 3
Risk Factors for Breakthrough Disease
The likelihood of breakthrough varicella increases with:
- Time since vaccination: Annual breakthrough rates increase from 1.6 cases per 1,000 person-years within 1 year of vaccination to 58.2 per 1,000 person-years at 9 years post-vaccination 5
- Single-dose vaccination: Children who received only one dose have significantly higher breakthrough rates than those who received two doses 1, 3
- Intense exposure settings: School outbreaks demonstrate that even 99% vaccination coverage may not prevent transmission in settings with close, prolonged contact 3
Current Two-Dose Recommendation
The Advisory Committee on Immunization Practices now recommends two doses of varicella vaccine to address breakthrough disease:
- First dose: Age 12-15 months 1
- Second dose: Age 4-6 years (before school entry), though it may be given earlier with a minimum 3-month interval between doses 1
- Catch-up vaccination: All children and adolescents who previously received only one dose should receive a second dose 1
Important Clinical Pitfalls
- Diagnosis difficulty: Breakthrough cases with atypical, mild rashes may be difficult to recognize, potentially delaying outbreak control measures 3
- False sense of security: High vaccination coverage does not guarantee outbreak prevention in school settings where exposure intensity is high 3
- Transmission potential: Even mild breakthrough cases are contagious and can sustain transmission chains, particularly in congregate settings 3