A 37‑week pregnant, unvaccinated woman with unknown varicella immunity was exposed to varicella from her child who received the live attenuated varicella vaccine after exposure; what prophylaxis and management should be provided?

Management of Unvaccinated Pregnant Woman Exposed to Varicella via Child's Post-Exposure Vaccination

The pregnant woman requires immediate varicella immunity testing, and if seronegative or results unavailable within 96 hours, she must receive VariZIG within 96 hours (ideally) or up to 10 days post-exposure to prevent severe maternal complications. 1, 2

Critical Reassurance Regarding Vaccine Exposure

• Exposure to a child who received the live attenuated varicella vaccine poses minimal to no risk for transmission to the pregnant mother, as vaccine-strain virus transmission is extremely rare and the attenuated vaccine virus exhibits significantly lower virulence than wild-type varicella. 3
• In a prospective registry of 131 live-born infants whose mothers inadvertently received varicella vaccine during pregnancy (including 82 in first/second trimester), zero cases of congenital varicella syndrome occurred (95% CI: 0-6.7%), with birth defect rates (2.3%) comparable to the general population (3.2%). 3
• The primary concern is whether the mother has immunity to varicella from natural infection or prior vaccination, not the child's recent vaccine administration. 3

Immediate Assessment and Prophylaxis Protocol

Step 1: Determine Maternal Immunity Status

• Immediately obtain varicella zoster IgG serology to document immunity status, as history alone may be unreliable. 4
• Evidence of immunity includes: documented two-dose varicella vaccination, laboratory-confirmed immunity, or healthcare provider-verified history of varicella or herpes zoster. 1

Step 2: Administer VariZIG if Seronegative or Unknown Immunity

• Administer VariZIG within 96 hours of exposure for maximum benefit (though FDA now permits administration up to 10 days post-exposure with potentially reduced effectiveness). 1, 2
• VariZIG reduces infection rates in exposed pregnant women from >70% to approximately 30%, and substantially decreases severe maternal complications including pneumonia. 1, 3
• Critical caveat: VariZIG prevents severe maternal disease but does NOT prevent viremia, fetal infection, or congenital varicella syndrome—its purpose is maternal protection. 3, 5
• If serology confirms immunity, no prophylaxis is needed and the patient can be reassured. 4

Step 3: Extended Monitoring Period

• Monitor the mother for prodromal symptoms (fever, malaise, headache) followed by characteristic vesicular rash for 28 days post-exposure (extended from 21 days due to VariZIG's effect on incubation period). 3, 5
• If VariZIG was not administered, monitor for 21 days. 1

Management if Maternal Varicella Develops

Antiviral Treatment

• Initiate oral acyclovir 800 mg five times daily within 24 hours of rash onset for pregnant women who develop varicella, as efficacy diminishes significantly after this window. 3, 4
• For severe complications (e.g., pneumonitis), consider hospital admission and intravenous acyclovir 10-15 mg/kg or 500 mg/m² every 8 hours for 5-10 days. 4
• Acyclovir is FDA Category B in pregnancy with strong safety data from 596 first-trimester exposures showing no increased birth defect rates. 3

Fetal Surveillance

• Perform detailed ultrasound and appropriate follow-up to screen for fetal consequences if maternal infection occurs, particularly if infection happens in first or second trimester (congenital varicella syndrome risk 0.5-1.5%). 4, 6

Neonatal Considerations if Delivery is Imminent

• If maternal varicella rash appears between 5 days before delivery and 2 days after delivery, the neonate is at highest risk (historical mortality 31% without intervention). 1, 5
• Administer VariZIG to the neonate immediately after birth regardless of whether the mother received VariZIG, as neonates lack sufficient transplacentally acquired maternal antibody during this critical window. 1, 5
• Neonatal VariZIG dosing: 125 units per 10 kg body weight (maximum 625 units). 5
• If neonatal varicella develops despite prophylaxis, initiate intravenous acyclovir 10 mg/kg every 8 hours for 10 days within 24 hours of rash onset. 5
• Inform neonatal healthcare providers of peripartum varicella exposure to optimize early care. 4

Critical Pitfalls to Avoid

• Do not delay serology testing or VariZIG administration—the 96-hour window is critical for maximum effectiveness, though benefit may extend to 10 days. 1, 2
• Do not assume the child's vaccine poses significant transmission risk—focus on the mother's baseline immunity status. 3
• Do not withhold neonatal VariZIG if maternal rash occurs in the critical peripartum window (5 days before to 2 days after delivery), even if the mother received VariZIG. 1, 5
• Do not delay acyclovir if maternal varicella develops—efficacy is time-dependent and greatest within 24 hours of rash onset. 3, 4

Postpartum Vaccination

• Administer varicella vaccine postpartum to non-immune women as part of routine postpartum care to prevent future exposures in subsequent pregnancies. 4