Is the varicella vaccine effective when administered post-exposure?

Varicella Vaccine Post-Exposure Prophylaxis

Varicella vaccine should be administered to unvaccinated persons without evidence of immunity within 3 days of exposure for optimal protection (>90% efficacy), and can still provide benefit up to 5 days post-exposure (approximately 70% efficacy in preventing disease and 100% efficacy in preventing severe disease). 1

Timing and Efficacy

The effectiveness of post-exposure varicella vaccination is critically time-dependent:

• Within 3 days of exposure: >90% effective in preventing varicella entirely 1
• Within 5 days of exposure: Approximately 70% effective in preventing varicella and 100% effective in modifying severe disease 1
• Mechanism of benefit: If exposure does not result in infection, vaccination provides protection against future exposures; if infection occurs, the vaccine does not increase adverse event risk during the presymptomatic or prodromal stage 1

Target Populations

Healthy Persons (Primary Candidates)

All unvaccinated persons without evidence of immunity should receive post-exposure vaccination, including: 1

• Children: Most data supporting post-exposure use comes from pediatric studies 1
• Adults: Should be offered post-exposure vaccination despite higher rates of non-response to first dose compared to children 1
• Healthcare workers: Post-exposure vaccination is appropriate, though routine pre-exposure vaccination is the preferred prevention method 1

Previously Vaccinated Persons

For individuals who previously received one dose of varicella vaccine, consider administering a second dose post-exposure to bring them up-to-date, though specific efficacy data for this strategy in post-exposure settings is limited in the ACIP guidelines 1. However, recent research demonstrates that second-dose vaccination as post-exposure prophylaxis has 77% effectiveness overall, with higher effectiveness (87%) when administered within 3 days of exposure 2.

Alternative for High-Risk Immunocompromised Patients

For persons with contraindications to vaccination who are at risk for severe disease (immunocompromised patients, pregnant women, neonates), varicella-zoster immune globulin (VZIG/VariZIG) should be administered within 96 hours of exposure 1. VZIG provides passive immunity and is most effective when given as soon as possible after exposure 1.

Important Clinical Caveats

• MMRV combination vaccine: No data available on its use for post-exposure prophylaxis; use single-antigen varicella vaccine 1
• Salicylate avoidance: Vaccine recipients should avoid salicylates for 6 weeks post-vaccination due to Reye syndrome risk 1
• Immune globulin interference: Do not administer varicella vaccine for 3-11 months after receiving blood products or immune globulin, as passively transferred antibodies may inhibit vaccine response 1
• Safety profile: No evidence indicates increased adverse events when vaccine is given during presymptomatic or prodromal stages of infection 1

Real-World Effectiveness

Post-licensure studies confirm the pre-licensure data, demonstrating that varicella vaccine remains highly effective in preventing or modifying disease when administered within 3 days of exposure 1. A prospective cohort study showed 62.3% effectiveness in preventing any disease and 79.4% effectiveness in preventing moderate-to-severe disease when given within 5 days 3.