How does chicken pox spread and what are the preventive measures for high-risk populations, including children and individuals with compromised immune systems?

Chickenpox Transmission and Prevention

Chickenpox spreads through airborne respiratory droplets and direct contact with vesicular fluid, with over 90% of unvaccinated exposed individuals becoming infected, making vaccination the cornerstone of prevention for all eligible children and high-risk populations. 1, 2

Mechanism of Transmission

Varicella-zoster virus (VZV) is highly contagious and spreads through two primary routes 1:

• Airborne transmission: Respiratory droplets from infected individuals can remain suspended in air and transmit infection even without direct contact 1
• Direct contact: Touching fluid from vesicular lesions causes transmission 1
• Contagious period: Patients are infectious from 1-2 days before rash onset until all lesions have crusted (typically 5-7 days after rash appears) 3, 4

High-Risk Exposure Settings

Household exposure carries the highest transmission risk, with approximately 85% (range 65-100%) of susceptible household contacts developing chickenpox after exposure 5. This attack rate is 5 times higher than school or hospital exposure 5.

Direct face-to-face contact with an infectious person while indoors qualifies as substantial exposure requiring intervention 5.

Preventive Measures for Children

Primary Prevention Through Vaccination

All healthy children should receive two doses of varicella vaccine 6:

• First dose: 12-15 months of age 6
• Second dose: 4-6 years of age 6
• Minimum interval: At least 3 months between doses for children 12 months through 12 years 5, 6
• Efficacy: 98% against any varicella disease and 100% against severe disease 6

Post-Exposure Prophylaxis for Healthy Children

For susceptible children exposed to chickenpox, immediate vaccination is the intervention of choice 5:

• Within 3 days of exposure: >90% effective in preventing disease 5, 6
• Within 5 days of exposure: 70% effective in preventing disease and 100% effective in modifying severe disease 5, 6
• No contraindications exist for vaccinating healthy children after household exposure 5

Critical action point: Household contacts of active chickenpox cases should receive immediate vaccination if they lack immunity, given the 85% attack rate in this setting 5.

School and Childcare Requirements

States should require varicella vaccination or evidence of immunity for child care and elementary school entry, as children aged 1-6 years have the highest incidence 1. Evidence of immunity includes 1:

• Documentation of varicella vaccination
• Physician-diagnosed varicella
• Reliable history of disease
• Serologic evidence of immunity

Preventive Measures for Immunocompromised Populations

Vaccination Considerations

VZV vaccination should be strongly considered in varicella-naïve patients on methotrexate 1. The Task Force emphasizes that in VZV-naïve patients, it is preferable to immunize in a controlled manner rather than waiting for inevitable exposure to wild-type virus, which carries risk of severe disseminated disease 1.

VZV vaccination can be considered in varicella-naïve patients on 1:

• Low-dose glucocorticosteroids
• TNF inhibitors
• Anti-IL1 therapy
• Anti-IL6 therapy

Important caveat: No treatment pause is recommended due to risk of disease worsening or flares 1. In 78 pediatric patients with autoimmune inflammatory rheumatic diseases receiving VZV vaccination, there were no complicated or disseminated infections, and only three patients developed mild, transient VZV-like rash 1.

Post-Exposure Prophylaxis for High-Risk Patients

Varicella-zoster immune globulin (VZIG) should be given to susceptible high-risk patients as soon as possible after exposure, effective up to 10 days post-exposure 3. VZIG is reserved for 5:

• Immunocompromised patients
• Neonates with specific maternal exposure timing
• Premature infants
• NOT for healthy children 5

If VZIG is unavailable, prophylactic oral acyclovir (10 mg/kg four times daily for 7 days) within 7-10 days of exposure is recommended 3.

Contraindications to Live Vaccine

Immunocompromised children do not routinely receive varicella vaccine, except for certain HIV-infected children (CDC class 1 with CD4 ≥15%) who may be vaccinated with expert guidance 3, 6.

Isolation Precautions

Keep infected individuals isolated until all lesions have crusted over (typically 5-7 days after rash onset) to prevent spread 3. This is critical because transmission can occur 1-2 days before rash onset through the crusting phase 4.

Common Pitfalls to Avoid

• Do not delay post-exposure vaccination: The 3-day window for maximum effectiveness is narrow; act immediately upon exposure notification 5
• Do not confuse VZIG indications: Healthy children receive vaccine, not VZIG, even after high-risk household exposure 5
• Do not assume prior infection without documentation: Require physician diagnosis, reliable history, or serologic confirmation as evidence of immunity 1, 6
• Do not withhold vaccination from children on methotrexate: The controlled vaccine exposure is safer than inevitable wild-type exposure 1