Post-Exposure Prophylaxis for a 3-Month-Old Infant Exposed to Shingles
Administer varicella-zoster immune globulin (VariZIG) 125 IU intramuscularly as soon as possible, ideally within 96 hours but up to 10 days after exposure to the person with shingles. 1
Rationale for VariZIG Administration
A 3-month-old infant meets the high-risk criteria for post-exposure prophylaxis because:
- Infants under 1 year of age are considered high-risk for severe varicella complications and should receive VariZIG after exposure to either chickenpox or shingles 1
- The infant is too young to receive varicella vaccination (which is only given starting at 12-15 months of age) 1
- Premature infants requiring hospital care for prematurity should receive VariZIG for any VZV exposure during their entire hospitalization period 1
Dosing and Administration
- Dose: 125 IU (one vial) administered intramuscularly for this 3-month-old infant 1
- The standard dosing is 125 IU per 10 kg body weight, with a maximum of 625 IU 1
- Timing is critical but flexible: While ideally given within 96 hours of exposure, CDC guidelines now recommend administration up to 10 days post-exposure 1
Evidence Supporting Extended Administration Window
Recent data demonstrates that VariZIG effectiveness is similar whether administered within 96 hours or between 96 hours and 10 days post-exposure 2:
- In a large expanded-access program of 507 high-risk participants, varicella incidence was 6.2% when VariZIG was given ≤96 hours versus 9.4% when given >96 hours (up to 10 days) 2
- Among 105 infants in this study, the overall varicella incidence was 11.5%, with most cases being mild 2
- Only 1 of 34 participants who developed varicella despite prophylaxis had severe complications (pneumonia and encephalitis) 2
Important Caveats
- Determine the infant's immune status first: If the mother had chickenpox or received varicella vaccination, the infant may have maternal antibodies providing some protection 1
- Shingles exposure carries the same risk as chickenpox exposure for susceptible individuals, as both involve active VZV shedding from lesions 1, 3
- VariZIG may not prevent infection entirely but significantly reduces disease severity and complications in breakthrough cases 1, 2
Alternative Considerations
If VariZIG is unavailable or >96 hours have elapsed since exposure, some experts recommend prophylactic acyclovir, though this is less well-established in infants 1. However, VariZIG remains the preferred and guideline-recommended approach for this age group 1.
Monitoring After Prophylaxis
- Observe the infant for 42 days post-exposure for development of varicella rash 2
- If breakthrough varicella occurs despite VariZIG, treat immediately with acyclovir 20 mg/kg orally four times daily (or intravenous formulation if severe) 4
- The infant should avoid contact with other susceptible high-risk individuals during the incubation period 1