What is the risk of contracting chicken pox (varicella) despite receiving the varicella vaccine?

Risk of Contracting Chickenpox Despite Vaccination

The risk of breakthrough varicella (chickenpox) after vaccination is approximately 7-23% with one dose and 2% with two doses, though breakthrough cases are typically mild with significantly reduced severity compared to unvaccinated individuals. 1, 2

Vaccine Effectiveness by Dosing Regimen

One-Dose Vaccination

• Vaccine effectiveness against any varicella disease: 33-82% depending on the outbreak setting and time since vaccination 3, 2
• Vaccine effectiveness against moderate-to-severe varicella: 82-97% 3, 2
• The 10-year cumulative risk of developing varicella after one dose is approximately 7.3% 1
• Attack rates during school outbreaks among one-dose vaccinated children range from 18-23% 3, 2

Two-Dose Vaccination

• Vaccine effectiveness: 98.3% over a 10-year observation period 1
• The cumulative risk of breakthrough varicella is only 2.2% over 10 years 1
• Two doses provide 3.3-fold lower risk of developing varicella compared to one dose 1
• Approximately 99% of children achieve protective antibody levels after two doses, compared to 86% after one dose 3

Key Risk Factors for Breakthrough Disease

Time Since Vaccination

• Children vaccinated ≥5 years before exposure have 3.5 times higher risk of breakthrough disease compared to those vaccinated more recently 2
• This waning immunity explains why outbreaks continue to occur in highly vaccinated populations 3, 2

Age at Vaccination

• The median age at vaccination in outbreak studies was 18 months, though age at vaccination itself was not independently associated with breakthrough risk 3, 2

Clinical Characteristics of Breakthrough Varicella

Breakthrough cases in vaccinated individuals are significantly milder but remain contagious: 3

• Vaccinated cases typically present with fewer lesions (often <50 lesions vs. hundreds in unvaccinated) 3
• The rash may be atypical and maculopapular with few or no vesicles 3
• Despite mild presentation, vaccinated individuals can transmit disease and sustain outbreaks 3, 2
• Outbreaks can persist for 2+ months even in populations with 99% vaccination coverage 3

Antibody Response and Immunity

Serologic Testing Limitations

• Commercial ELISA assays lack sensitivity to detect vaccine-induced antibody in all cases 4, 5
• More sensitive tests (latex agglutination, fluorescent antibody to membrane antigen) show that 99% of adults develop antibodies after two doses 4
• Seroconversion does not always equal full protection, as cell-mediated immunity plays a critical role 4
• Approximately 25-31% of adult vaccine recipients lose detectable antibodies within 1-11 years, yet remain protected through cell-mediated immunity 4

Duration of Protection

• Vaccine-induced cell-mediated immunity persists in 94% of adults at 1 and 5 years post-vaccination 4
• Measurable antibody persists for at least 9 years in all subjects in long-term follow-up studies 1
• Disease remains mild in vaccinated persons even when antibodies are undetectable, suggesting robust cellular immunity 4

Clinical Implications

Common Pitfalls

• Do not rely solely on commercial ELISA testing to determine immunity in vaccinated healthcare workers, as false-negatives are common 4, 5
• Early outbreak recognition is challenging due to mild, atypical presentations in vaccinated individuals 3
• One-dose vaccination does not provide sufficient herd immunity to prevent school outbreaks where exposure intensity is high 3, 2

Current Recommendations

• Two-dose vaccination is now standard to achieve optimal protection and outbreak control 4, 3
• Healthcare personnel should receive two doses administered 4-8 weeks apart 4
• During outbreaks, a second dose should be offered to previously one-dose vaccinated individuals, with 3 months minimum interval from the first dose 4