What is the recommended dose of acyclovir (antiviral medication) for varicella (chickenpox) prophylaxis?

Acyclovir Prophylaxis Dosing for Varicella

For varicella prophylaxis after exposure, the recommended dose is acyclovir 20 mg/kg (maximum 800 mg) orally four times daily for 5-7 days, initiated 7-10 days after exposure when varicella-zoster immune globulin (VZIG) is unavailable or if more than 96 hours have passed since exposure. 1

Primary Prophylaxis Strategy

VZIG remains the first-line prophylaxis for susceptible high-risk individuals (immunocompromised, pregnant women, neonates) after varicella exposure, administered within 96 hours of exposure at 125 units/10 kg body weight (maximum 625 units) intramuscularly. 1

Acyclovir as Alternative Prophylaxis

When VZIG is unavailable or the 96-hour window has passed, acyclovir prophylaxis becomes the recommended alternative:

• Dosing regimen: 20 mg/kg body weight (maximum 800 mg per dose) orally four times daily for 5-7 days 1
• Timing: Initiate 7-10 days after exposure, corresponding to the late incubation period when secondary viremia begins 1
• Target population: Susceptible immunocompromised patients and other high-risk individuals without evidence of immunity 1

Evidence Supporting This Approach

The 7-10 day post-exposure timing is critical because it targets the late incubation period:

• Research demonstrates that oral acyclovir administered 9 days after exposure prevented clinical varicella in 92.6% of exposed children, with seroconversion occurring in 63% 2
• Another study showed that starting acyclovir 7-9 days post-exposure reduced disease incidence from 100% to 16% in family contacts 3
• This late-incubation timing allows for natural immune response development while preventing clinical disease 3, 2

Special Populations

Immunocompromised Children

• Same dosing: 20 mg/kg (maximum 800 mg) four times daily 1
• Close physician observation is mandatory, as some patients may require conversion to intravenous therapy 4

Pregnant Women

• VZIG is strongly preferred within 96 hours of exposure 1, 5
• If VZIG unavailable: oral acyclovir 10 mg/kg four times daily for 7 days can be considered 1
• Acyclovir is FDA Category B with no increased birth defects in 596 first-trimester exposures 5

HIV-Infected Patients

• For prophylaxis after exposure: 20 mg/kg (maximum 800 mg) four times daily for 5-7 days 1
• Note: Data on acyclovir preventing chickenpox in HIV-infected individuals are limited 1

Important Clinical Caveats

Common pitfall: Starting acyclovir too early (immediately after exposure) may interfere with natural immunity development. The 7-10 day delay is intentional and evidence-based. 3, 2

Monitoring requirement: Patients on prophylaxis should be observed closely, as breakthrough disease can still occur and may require escalation to intravenous therapy. 4

Duration of protection: Unlike VZIG which provides approximately 3 weeks of protection, acyclovir prophylaxis only covers the treatment period. 1

Renal Dose Adjustment

For patients with renal impairment receiving the 800 mg four times daily regimen:

• Creatinine clearance >25 mL/min: 800 mg every 4 hours (5 times daily)
• Creatinine clearance 10-25 mL/min: 800 mg every 8 hours
• Creatinine clearance 0-10 mL/min: 800 mg every 12 hours
• Hemodialysis: Additional dose after each dialysis session 6