~50% neonates exposed to maternal chicken pox will become infected despite VZIG prophylaxis.
In up to 2/3 of these, infections are mild or asymptomatic but rare fatal cases have been reported in the period four days before to two days after delivery. Early treatment with
IV aciclovir is recommended for neonates in this exposure category who develop varicella infection despite VZIG prophylaxis.
Reference: Chapter 34, Immunisation against infectious disease (The Green Book) Public Health England, 2013 (last updated Aug 2015)..