What is the recommended antiviral medication for a patient with chickenpox (varicella), considering factors such as age, weight, and immunocompromised status?

Antiviral Treatment for Chickenpox

Acyclovir is the recommended antiviral medication for chickenpox, with route and dosing determined by immune status, age, and disease severity. 1, 2, 3

Treatment Algorithm by Patient Population

Immunocompromised Patients (Highest Priority)

Initiate intravenous acyclovir immediately upon diagnosis, ideally within 24 hours of rash onset. 1, 2

• Dosing: 10 mg/kg IV every 8 hours for 7-10 days 1
• This population faces significant risk of dissemination and death without prompt antiviral therapy 1
• Treatment should continue until all lesions have crusted over, not just for an arbitrary duration 4
• High-risk groups include HIV-infected patients, transplant recipients, cancer patients on chemotherapy, and those receiving high-dose corticosteroids (>2 mg/kg or >20 mg/day prednisone equivalent) 5, 1

Pregnant Women

Pregnant women with serious varicella complications (especially pneumonia) require intravenous acyclovir. 1, 2

• Acyclovir is FDA Category B in pregnancy with reassuring safety data showing no increased birth defect rates 1, 2
• For pregnant women at increased risk for moderate-to-severe disease without complications, oral acyclovir may be considered if initiated within 24 hours of rash onset 1

Adolescents and Adults (≥13 Years)

Oral acyclovir 800 mg four times daily for 5 days if initiated within 24 hours of rash onset. 1, 3

• Adults face significantly higher complication rates than children, with case-fatality rates 21.3 per 100,000 cases versus 0.8 per 100,000 in children aged 1-4 years 1
• Critical pitfall: Acyclovir loses efficacy when initiated >24 hours after rash onset 1

Children (2 to <13 Years)

Otherwise healthy children require supportive care only. 1, 2

However, oral acyclovir should be initiated within 24 hours for children with:

• Chronic cutaneous disorders (e.g., eczema) 1
• Chronic pulmonary disorders 1
• Long-term salicylate therapy 1

Dosing for children requiring treatment: 20 mg/kg orally four times daily (maximum 800 mg per dose) for 5 days 3

Neonates (Special High-Risk Population)

Newborns whose mothers developed varicella from 5 days before to 2 days after delivery require immediate intervention. 1, 2

• Administer Varicella-Zoster Immune Globulin (VZIG) 125 units/10 kg body weight (maximum 625 units) within 96 hours of exposure 5, 2
• If varicella develops despite VZIG, institute antiviral therapy immediately 1
• These neonates should not be discharged without VZIG due to high mortality risk 1

Premature Infants

• <28 weeks gestation or <1,000g with postnatal exposure: VZIG indicated regardless of maternal immunity 2
• ≥28 weeks gestation with postnatal exposure: VZIG indicated if mother lacks immunity 2

Alternative Antiviral: Valacyclovir

Valacyclovir 20 mg/kg orally three times daily for 5 days (maximum 1 gram three times daily) is FDA-approved for chickenpox treatment in children aged 2 to <18 years. 6

• Valacyclovir is a prodrug of acyclovir with improved bioavailability 6
• Not recommended for children <2 years as efficacy and safety have not been established 6
• Based on pharmacokinetic data showing adequate acyclovir exposures comparable to standard acyclovir regimens 6

Renal Dosing Adjustments

For patients with renal impairment, acyclovir dosing must be modified: 3

• 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 3

Post-Exposure Prophylaxis (Not Treatment)

Varicella vaccine within 3 days (up to 5 days) of exposure is >90% effective at preventing disease in susceptible immunocompetent individuals. 1, 2

• Vaccination is the method of choice for post-exposure prophylaxis in healthy individuals 1, 2
• Acyclovir is NOT indicated for prophylactic use in otherwise healthy individuals after exposure 1

VZIG is reserved for high-risk individuals who cannot receive vaccine: 1, 2

• Immunocompromised patients without evidence of immunity
• Pregnant women without evidence of immunity
• Newborns with maternal varicella exposure (as detailed above)
• Dosing: 125 units/10 kg body weight (maximum 625 units) within 96 hours of exposure 5, 2

Critical Clinical Pitfalls

Timing is everything: Treatment must be initiated within 24 hours of rash onset for maximum effectiveness in most populations 1, 2

Do not use acyclovir prophylactically: Acyclovir does not decrease transmission or reduce duration of school absence 2

Monitor for dissemination in immunocompromised patients: These patients require treatment regardless of timing and should receive IV therapy 1

Isolation requirements: Isolate patients until all lesions are crusted, typically 5-7 days after rash onset 1