Is Dry (Crusted) Herpes Zoster Infectious?
No, dry (fully crusted) herpes zoster lesions are not infectious. Once all lesions have completely dried and crusted over, patients are no longer contagious and isolation precautions can be discontinued 1, 2.
Understanding the Infectious Period
The contagious period of herpes zoster follows a predictable timeline:
Patients remain infectious from 1-2 days before rash onset until all lesions are fully crusted, which typically occurs 4-7 days after the rash appears in immunocompetent individuals 2.
The crusting phase marks the end of active viral replication and infectivity, as viral titers peak in the first 24 hours after lesion onset and progressively decline as lesions convert to crusts 1.
Complete crusting is the key clinical milestone that determines when isolation precautions can be safely discontinued, according to CDC guidelines 1, 2.
Transmission Mechanism
Understanding how herpes zoster spreads clarifies why crusted lesions are not infectious:
Transmission requires direct contact with fluid from active vesicles or, rarely in healthcare settings, airborne exposure to viral particles from uncrusted lesions 2.
Vesicular fluid contains thousands of infectious viral particles, but once lesions crust over, this fluid is no longer present and viral shedding has essentially ceased 3.
Herpes zoster is approximately 20% as contagious as chickenpox and cannot spread through casual contact—only through direct contact with active vesicular fluid 2.
Special Considerations for Immunocompromised Patients
Immunocompromised patients require extended precautions because their disease course differs significantly:
New lesions may continue appearing for 7-14 days or longer (compared to 4-6 days in healthy individuals), extending the infectious period 1, 2.
Lesion healing is slower, which delays the time to complete crusting 1.
Some immunocompromised patients develop chronic ulcerative lesions with persistent viral replication, requiring prolonged isolation 1.
Practical Guidelines for Ending Isolation
For lesions that crust normally:
- Discontinue isolation when all lesions are completely dry and crusted, with no moist or weeping areas remaining 1, 2.
For non-crusting lesions (macules and papules):
For healthcare workers:
- Complete work clearance without restrictions is permitted once all lesions are crusted (or no new lesions in 24 hours for non-crusting presentations) 2.
- Healthcare workers with localized zoster must cover all lesions and avoid high-risk patients until fully crusted 2.
Common Pitfalls to Avoid
Do not assume antiviral therapy immediately renders patients non-contagious—viral shedding continues until lesions are fully crusted, regardless of treatment 2.
Ensure lesions are completely crusted, not just dry-appearing—partially crusted lesions with any moist areas can still transmit virus 1, 2.
Remember that immunocompromised patients may have prolonged viral shedding even after lesions appear to be healing 1, 2.
Avoid premature discontinuation of isolation in disseminated zoster—airborne and contact precautions must continue until all lesions are dry and crusted, regardless of immune status 2.