Treatment of Varicella in Adults
Oral acyclovir 800 mg five times daily for 5 days is the recommended treatment for uncomplicated varicella in adults, but must be initiated within 24 hours of rash onset to provide meaningful clinical benefit. 1, 2, 3
Critical Timing Window
Treatment initiated within 24 hours of rash onset reduces time to complete crusting from 7.4 to 5.6 days, decreases fever duration by half a day, reduces maximum lesion count by 46%, and lessens symptom severity 3, 4
Treatment initiated after 24 hours (25-72 hours post-rash) provides no measurable benefit in uncomplicated adult varicella and should not be routinely administered 3, 5
The narrow therapeutic window exists because oral acyclovir only impacts active viral replication, which peaks early in the disease course 6, 5
Standard Dosing Regimen
Oral acyclovir 800 mg orally 4 times daily for 5 days is the FDA-approved dosing for adults and children over 40 kg 2
Some protocols use 800 mg five times daily (every 4 hours while awake), which was the regimen studied in pivotal trials 3, 7
The higher dosing compared to herpes simplex treatment is necessary because varicella-zoster virus is less sensitive to acyclovir than HSV 7
Immunocompromised Patients
Immunocompromised adults should receive intravenous acyclovir regardless of timing since diagnosis, as they face substantially higher risk of disseminated infection and visceral complications. 1, 6
IV acyclovir 10 mg/kg every 8 hours for 7-10 days is the standard regimen for immunocompromised patients 7
Adequate hydration must be maintained, mental status monitored, and dosing adjusted for renal impairment 2, 7
Complicated Varicella
Adults with varicella pneumonia or other serious complications require IV acyclovir at 10 mg/kg every 8 hours, initiated as early as possible 6, 5
Varicella pneumonia occurs more frequently in adults than children and represents the most common serious complication 3
Renal Dose Adjustments
For creatinine clearance 10-25 mL/min: 800 mg every 8 hours 2
For creatinine clearance 0-10 mL/min: 800 mg every 12 hours 2
Hemodialysis patients require an additional dose after each dialysis session due to 60% reduction in plasma concentrations during 6-hour dialysis 2
Pregnancy Considerations
Acyclovir is FDA Pregnancy Category B with no increased birth defects documented in 596 first-trimester exposures 1
The decision to treat pregnant women with active varicella should weigh potential benefits against theoretical risks, though the safety profile is reassuring 5
Post-Exposure Prophylaxis (Different from Treatment)
For susceptible adults exposed to varicella, varicella-zoster immune globulin (VZIG) administered within 96 hours of exposure is first-line prophylaxis, not acyclovir. 8, 1, 9
VZIG dosing is 125 units/10 kg body weight (maximum 625 units) intramuscularly 8, 9
If VZIG is unavailable or >96 hours have elapsed, acyclovir 20 mg/kg (maximum 800 mg) four times daily for 5-7 days can be initiated 7-10 days after exposure 1, 9
The 7-10 day delay corresponds to the late incubation period when secondary viremia begins 9
For pregnant women specifically, VZIG is strongly preferred over acyclovir for post-exposure prophylaxis 8, 1, 9
Common Pitfalls to Avoid
Do not confuse varicella treatment (24-hour window) with herpes zoster treatment (72-hour window) - these are distinct conditions with different therapeutic windows 1
Do not prescribe oral acyclovir for uncomplicated adult varicella if the patient presents more than 24 hours after rash onset, as no clinical benefit has been demonstrated 3, 5
Do not use oral acyclovir in immunocompromised patients - they require IV therapy regardless of timing 1, 6
Do not confuse post-exposure prophylaxis (acyclovir started 7-10 days after exposure) with treatment of active disease (started within 24 hours of rash) 9