Contagiousness Duration of Herpes Zoster After Starting Antivirals
Patients with herpes zoster remain contagious until all lesions have completely crusted over, regardless of when antiviral therapy was initiated. 1, 2
Transmission Risk and Timeline
The key clinical endpoint for contagiousness is complete crusting of all lesions, not the duration of antiviral therapy. 1, 2 Antivirals do not immediately eliminate viral shedding—they reduce viral replication and accelerate lesion healing, but transmission risk persists until lesions are fully scabbed.
Viral shedding peaks in the first 24 hours after lesion onset when most lesions are vesicular. 1 This means patients are most contagious early in the disease course, before many even start treatment.
Patients should avoid contact with susceptible individuals (those who have not had chickenpox or vaccination) until all lesions have crusted. 1 This is the CDC's explicit recommendation for infection control.
Effect of Antivirals on Healing Time
Antiviral therapy accelerates lesion healing but does not eliminate contagiousness immediately. 3 In clinical trials, famciclovir reduced the time to full crusting from 7 days (placebo) to 5 days (famciclovir 500 mg three times daily). 4, 3
Treatment should continue until all lesions have scabbed, typically 7-10 days in immunocompetent patients. 1, 2 However, the actual duration varies by individual healing response.
In immunocompromised patients, lesions may continue to develop for 7-14 days and heal more slowly, extending the contagious period significantly. 1 These patients may require treatment extension well beyond the standard 7-10 days and remain contagious throughout this prolonged healing period.
Practical Isolation Guidance
Isolation precautions should remain in place until complete crusting occurs, not for an arbitrary number of days after starting antivirals. 1 The lesion appearance—not the calendar—determines when transmission risk ends.
Lesions can be covered with a dressing to reduce transmission risk, but this does not eliminate contagiousness if vesicular fluid is present. 1 Direct contact with lesions or contaminated materials can transmit varicella-zoster virus to susceptible individuals.
Critical Pitfall to Avoid
- Do not assume patients become non-contagious after a fixed number of days on antivirals (e.g., 24-48 hours). Unlike some other infections where antimicrobial therapy rapidly reduces transmission risk, herpes zoster remains contagious based on lesion status, not treatment duration. The virus is present in vesicular fluid until lesions crust completely, regardless of antiviral therapy. 1, 3