For neonatal varicella with clinical disease, treat with IV acyclovir for at least 7 days, commonly 7-10 days. UK guidance specifies 20 mg/kg IV every 8 hours for at least 7 days, started promptly. UK neonatal varicella guidance

Do not confuse this with post-exposure prophylaxis for a baby whose mother develops varicella around delivery: antiviral prophylaxis may be continued through day 21 after exposure, depending on the protocol and use of VZIG/IVIG.

The exact duration should be individualized with neonatal/infectious-disease input, especially if there is pneumonitis, hepatitis, encephalitis, ongoing new lesions, or immunodeficiency.