How do I check for immunity to varicella (chickenpox)?

Checking for Immunity from Varicella (Chickenpox)

Varicella immunity should be assessed using a varicella IgG antibody test, preferably a commercial ELISA, which offers high specificity for screening purposes. 1

Evidence of Immunity to Varicella

Immunity to varicella can be established through any of the following:

• Laboratory evidence: Positive varicella IgG antibody test
• Documentation of vaccination: Written record of 2 doses of varicella vaccine
• Healthcare provider verification: Documented history of varicella disease or herpes zoster
• Laboratory confirmation: Previous laboratory-confirmed varicella or herpes zoster 1

Serological Testing Methods

Recommended Testing Approach

1. Commercial ELISA (Enzyme-Linked Immunosorbent Assay)

   • Preferred for screening purposes due to higher specificity
   • Less sensitive than other tests but more specific, reducing false positives
   • Recommended particularly for healthcare personnel screening 1

2. Latex Agglutination (LA) Test

   • More sensitive but may produce false-positive results
   • Can be completed in 15 minutes
   • Uses latex particles coated with VZV glycoprotein antigens
   • Not recommended for routine screening due to documented cases of false positives leading to varicella infections in unvaccinated individuals 1

Test Performance Considerations

• Commercial assays can detect disease-induced immunity but may lack sensitivity for vaccine-induced immunity 1
• For quantitative determination of immunity status, glycoprotein-based ELISAs (gpELISA) have shown excellent sensitivity (100%) compared to the reference FAMA test 2
• Standard ELISAs using whole antigen extract have lower sensitivity (83% for natural infection, 78% for post-vaccination immunity) 2

Special Populations

Healthcare Workers

• All healthcare workers should be screened for varicella immunity
• Serologic screening before vaccination is cost-effective in healthcare settings 1
• Consider testing all unvaccinated healthcare personnel, regardless of disease history, as a small proportion with positive history may still be susceptible 1

Immunocompromised Patients

• Patients with inflammatory bowel disease or other immunocompromising conditions should be screened by:
  • History of chickenpox or shingles
  • Varicella zoster virus IgG testing if history is uncertain/negative 1
• Testing should be performed before initiating immunosuppressive therapy when possible

Post-Testing Management

• If positive for VZV IgG: Individual is considered immune to varicella
• If negative for VZV IgG:
  • In healthy individuals: Vaccination with 2 doses of varicella vaccine at least 4 weeks apart is recommended
  • In healthcare workers: Vaccination is strongly recommended to prevent nosocomial transmission 1
  • In immunocompromised patients: Consider varicella zoster immune globulin (VZIG) for post-exposure prophylaxis 1

Common Pitfalls

1. False-positive results with LA testing: Using only a single concentration of serum with LA testing can lead to false positives, leaving individuals vulnerable to infection 1

2. Misinterpreting negative results after vaccination: Routine testing after vaccination is not recommended as commercial assays may not detect vaccine-induced immunity despite protection 1

3. Relying solely on patient-reported history: Self-reported history without healthcare provider verification may be unreliable; serological testing is preferred when history is uncertain 1

4. Timing of sample collection: Avoid testing samples that may contain passively acquired VZV IgG (e.g., after blood transfusion) 1

5. Using inappropriate tests: Using tests not optimized to detect antibody to the vaccine virus may lead to false negatives in vaccinated individuals 1

By following these evidence-based guidelines for checking varicella immunity, clinicians can accurately identify susceptible individuals and implement appropriate preventive measures to reduce morbidity and mortality from varicella infection.