How is chickenpox (varicella) transmitted and what are the risks for individuals, particularly children under 10 and those with weakened immune systems, such as cancer or HIV/AIDS patients?

Chickenpox Contagiousness

Chickenpox is extremely contagious, with infected persons transmitting the virus from 1-2 days before rash onset until all lesions are completely crusted (typically 4-7 days after rash appears), and secondary attack rates reach 80-90% among susceptible household contacts. 1

Transmission Mechanisms

Varicella-zoster virus (VZV) spreads through multiple routes that make it highly infectious:

• Direct contact with vesicular fluid from skin lesions is the primary transmission route 1
• Airborne transmission occurs through inhalation of aerosolized respiratory secretions and aerosols from vesicular fluid, allowing spread even without direct contact 1
• The virus enters through the upper respiratory tract or conjunctivae after exposure 1

Contagious Period Timeline

The infectious period creates significant challenges for containment because transmission begins before the disease is clinically apparent:

• Pre-rash contagiousness: Infected persons can transmit VZV 1-2 days before any visible rash appears 1, 2
• Active disease period: Contagiousness continues throughout the vesicular stage when fluid-filled lesions are present 2
• End of infectivity: The person remains contagious until all lesions have completely crusted over, typically 4-7 days after rash onset 1, 2

Special Populations with Extended Infectious Periods

Immunocompromised patients present unique challenges and prolonged risk:

• Healing may take 7-14 days or longer, significantly extending the contagious period 2
• Persons with progressive varicella (new lesions developing for >7 days) remain contagious longer due to persistent viral replication 1

Infectivity Rates

The extraordinarily high transmission rates distinguish chickenpox from most other infectious diseases:

• Household secondary attack rate: 80-90% of susceptible contacts will develop infection after exposure 1
• More specifically, the household attack rate is approximately 85% (range 65-100%) among susceptible contacts 2
• Nosocomial transmission: Airborne spread in healthcare settings has resulted in infection of patients and staff with no direct contact with the index case 1

High-Risk Populations

Certain groups face life-threatening complications from chickenpox exposure:

• Pregnant women are at risk for severe disease and fetal complications 1
• Premature infants born to susceptible mothers face severe disease risk 1
• Infants born at <28 weeks gestation or weighing ≤1,000 grams regardless of maternal immune status 1
• Immunocompromised persons of all ages, including those undergoing immunosuppressive therapy, with malignant disease, or who are immunodeficient 1
• Neonates whose mothers develop varicella from 5 days before to 2 days after delivery face 17-30% risk of severe infection 2
• Infants under 1 year are 6 times more likely to require hospitalization than older children 2

Breakthrough Varicella in Vaccinated Persons

The vaccine era has changed disease presentation but not eliminated transmission risk:

• Breakthrough varicella (occurring >42 days after vaccination) is typically mild with <50 lesions, predominantly maculopapular rather than vesicular 1
• Despite milder presentation, breakthrough varicella remains infectious 1
• Vaccinated children with <50 lesions are approximately one-third as infectious as unvaccinated children, while those with ≥50 lesions are as infectious as unvaccinated children 1

Isolation Requirements

Strict isolation protocols are essential to prevent transmission:

• Isolate infected persons until all lesions have completely crusted, typically 5-7 days after rash onset 2, 3
• Healthcare workers with chickenpox must be excluded from duty until all lesions are completely crusted 2
• In healthcare settings, both airborne and contact precautions are required for patients with varicella 1

Critical Pitfalls to Avoid

Do not assume non-contagiousness based on rash appearance alone—the patient was already infectious 1-2 days before the rash appeared and remains so until complete crusting 2, 3

Do not permit return to school or work based on symptom improvement—wait until all lesions are fully crusted, not just improved 2

Do not underestimate transmission risk in immunocompromised patients, who may shed virus for weeks longer than immunocompetent individuals and require extended isolation 2

Do not overlook the pre-rash infectious period when conducting contact tracing—exposures occurred 1-2 days before the index case developed visible symptoms 1, 2

Post-Exposure Management

For susceptible exposed individuals, time-sensitive interventions can prevent or modify disease:

• Varicella vaccine administered within 3 days (up to 5 days) of exposure is >90% effective at preventing disease 3
• VariZIG (varicella-zoster immune globulin) should be administered within 96 hours (extended to 10 days per CDC guidelines) for high-risk individuals who cannot receive the vaccine 3