When is chickenpox (varicella) contagious?

Chickenpox Contagious Period

Chickenpox is contagious from 1-2 days before the rash appears until all lesions have completely crusted over, which typically occurs 4-7 days after rash onset. 1, 2

Timeline of Contagiousness

Pre-Rash Period

• Infected individuals become contagious 1-2 days before any visible rash develops 1, 2
• This pre-symptomatic transmission window makes containment challenging, as patients appear healthy while already infectious 1
• The virus spreads through respiratory secretions and aerosolized particles during this early phase 1

Active Rash Period

• Peak contagiousness occurs during the vesicular stage when fluid-filled blisters contain high viral loads 1
• Transmission occurs via direct contact with vesicular fluid, inhalation of aerosols from skin lesions, or infected respiratory secretions 1
• The average incubation period after exposure is 14-16 days (range: 10-21 days) before the exposed person develops symptoms 1

End of Contagious Period

• Contagiousness ends when ALL lesions are completely crusted over, typically 4-7 days after rash onset 1, 2
• For non-crusting lesions (macules and papules only), the patient is no longer contagious when no new lesions appear within a 24-hour period 2
• This crusting criterion is absolute—even a single uncrusted lesion means the patient remains infectious 2

Special Populations and Breakthrough Disease

Vaccinated Individuals with Breakthrough Varicella

• Breakthrough cases in vaccinated children are still infectious, though typically less so than unvaccinated cases 1
• Vaccinated children with <50 lesions are approximately one-third as infectious as unvaccinated children 1
• Those with ≥50 lesions remain as infectious as unvaccinated children despite prior vaccination 1
• The same contagious period applies: 1-2 days before rash until all lesions crust 2

Immunocompromised Patients

• May experience prolonged viral shedding with lesions continuing to develop for 7-14 days or longer 3, 4
• Require extended isolation precautions until all lesions are completely crusted 3
• Progressive varicella (new lesions appearing >7 days) indicates continued viral replication and extended contagiousness 3

Transmission Risk and Secondary Attack Rates

• Varicella has an extremely high secondary attack rate of up to 90% among susceptible household contacts 1
• Secondary cases in household settings are typically more severe than the primary case due to higher viral inoculum from intense exposure 1
• Airborne transmission can occur in healthcare settings, with documented spread to individuals who had no direct contact with the index patient 1, 3

Critical Infection Control Implications

Healthcare Personnel Exposure Management

• Unvaccinated healthcare workers without immunity must be furloughed from days 10-21 after exposure, as they are potentially infectious during this period 2
• Fully vaccinated healthcare personnel (2 doses) require daily monitoring for fever and skin lesions during days 10-21 post-exposure 2
• Partially vaccinated personnel (1 dose) should receive a second dose within 3-5 days of exposure if ≥4 weeks have elapsed since the first dose 2

Post-Exposure Prophylaxis Window

• Varicella vaccine administered within 3 days (up to 5 days) of exposure is >90% effective at preventing disease 2
• VariZIG (varicella-zoster immune globulin) should be given within 96 hours (extended to 10 days per CDC) for high-risk individuals who cannot receive vaccine 2

Common Pitfalls to Avoid

• Do not assume patients are non-contagious once the rash appears—they remain highly infectious throughout the vesicular stage 1
• Do not clear patients to return to school/work based on time alone—all lesions must be completely crusted regardless of how many days have passed 2
• Do not overlook the 1-2 day pre-rash contagious period when identifying exposed contacts, though evidence for pre-rash transmission is limited and variable 5, 6
• Do not underestimate breakthrough varicella infectiousness—even mild cases with few lesions can transmit disease 1
• Do not allow immunocompromised patients to discontinue isolation prematurely—they may shed virus for weeks beyond the typical 4-7 day period 3, 4