What is the recommended acyclovir dosing for varicella (chickenpox) in healthy children, immunocompetent adolescents/adults, and immunocompromised patients?

Acyclovir Dosing for Chickenpox

For healthy children under 12 years, use oral acyclovir 20 mg/kg four times daily (maximum 800 mg per dose) for 5 days; for adolescents ≥12 years and adults, use 800 mg four times daily for 5 days; and for immunocompromised patients, use IV acyclovir 10 mg/kg (or 1500 mg/m²/day) every 8 hours for 7-10 days—all initiated within 24 hours of rash onset. 1, 2

Healthy Children (2-12 years, <40 kg)

• Dose: 20 mg/kg per dose orally, four times daily (total 80 mg/kg/day) 2, 3
• Maximum single dose: 800 mg 2, 3
• Duration: 5 days 2, 1
• Timing: Must be started within 24 hours of rash onset for clinical benefit 1, 4

Common pitfall: Do not apply the adult fixed dose of 800 mg to children under 40 kg—always calculate based on weight (20 mg/kg per dose). 3 Never exceed 800 mg in a single oral dose regardless of weight. 3

Indications in Healthy Children

The American Academy of Pediatrics does not recommend routine acyclovir for all healthy children with chickenpox. 3 However, treatment should be considered for:

• Children ≥12 years of age 5, 1
• Those with chronic cutaneous or pulmonary disorders 5, 1
• Those receiving long-term salicylate therapy 3, 1

Healthy Adolescents (≥12 years) and Adults (>40 kg)

• Dose: 800 mg orally, four times daily 2, 1
• Duration: 5 days 2, 1
• Timing: Must be started within 24 hours of rash onset 1, 6

Rationale: Varicella is significantly more severe in adolescents than younger children, with higher mean maximum lesion counts (421 vs 347), greater constitutional illness scores, and more residual lesions (92.7 vs 33.2). 6 Treatment in this age group reduces time to cessation of new lesions, maximum lesion count, fever duration, and residual hypopigmented lesions at 28 days. 6

Special Consideration for Pregnant Women

• For mild disease: Same 5-day oral regimen (800 mg four times daily) 1
• For serious complications (e.g., pneumonia): Consider IV acyclovir 1

Immunocompromised Patients

For all immunocompromised patients, IV acyclovir is mandatory regardless of timing from rash onset. 1

• Dose: 10 mg/kg IV every 8 hours OR 1500 mg/m²/day divided into three doses 1, 3
• Duration: 7-10 days OR until no new lesions for 48 hours 3, 1
• Route: Intravenous only for severe immunosuppression 3

Immunocompromised populations include:

• HIV-infected patients (especially CD4 <200 cells/µL) 1
• Chemotherapy recipients 1
• Organ transplant recipients 1
• Primary immunodeficiency disorders 1

Sequential IV-to-Oral Therapy Option

For immunocompromised children with less severe disease, a sequential approach may be considered:

• Start with IV acyclovir (1500 mg/m²/day in 3 divided doses) 7
• Switch to oral acyclovir 20 mg/kg four times daily after minimum 48 hours of IV therapy if: 7
  • Patient is afebrile
  • No new lesions for 24 hours
  • No internal organ involvement
  • Able to tolerate oral medications
• Continue oral therapy to complete 7-10 days total (IV + oral) 7

This approach reduces hospitalization duration while maintaining efficacy. 7

HIV-Infected Children Dosing

• Moderate immunosuppression: 20 mg/kg orally (maximum 800 mg/dose) four times daily for 7-10 days 3
• Severe immunosuppression: 10 mg/kg IV every 8 hours for 7-10 days, followed by oral therapy 3

Critical Timing Considerations

The 24-hour window is absolute for clinical benefit in immunocompetent patients. 1, 4 A controlled trial demonstrated a clear gradation in clinical response correlating with time from rash onset to treatment initiation. 4 Patients treated on day 1 had significantly better outcomes than those starting on day 2, who in turn had better outcomes than those starting on day 3. 4

• Treatment initiated >24 hours after rash onset provides minimal benefit in healthy patients 1, 4
• For immunocompromised patients, treatment should be given regardless of timing 1

Duration: 5 Days vs 7 Days

Five days of therapy is sufficient—a 7-day course provides no additional benefit. 4 A randomized controlled trial comparing 5-day versus 7-day regimens in patients treated within 24 hours of rash onset found equivalent efficacy. 4

Renal Dose Adjustments

For patients with renal impairment, modify dosing based on creatinine clearance: 2

• CrCl >25 mL/min: 800 mg every 4 hours (5 times daily)
• CrCl 10-25 mL/min: 800 mg every 8 hours
• CrCl 0-10 mL/min: 800 mg every 12 hours
• Hemodialysis: Administer additional dose after each dialysis session 2

Important Clinical Caveats

• Acyclovir does NOT prevent transmission or reduce school absence duration—isolation must continue until all lesions are crusted. 1
• Acyclovir does NOT interfere with immune response—antibody titers after treatment are equivalent to untreated infection, and future zoster risk is unchanged. 5, 3
• Acyclovir does NOT eradicate latent virus—the varicella-zoster virus still establishes latency in dorsal root ganglia despite treatment. 5
• Viral resistance does not develop with standard 5-day treatment courses—viruses shed during therapy retain normal thymidine kinase function and acyclovir susceptibility. 4

Hydration and Monitoring

At the higher doses used for varicella (especially IV dosing), ensure: 8

• Adequate hydration and urine flow
• Monitor mental status (neurotoxicity risk)
• Adjust for impaired renal function