Post-Exposure Measles Prophylaxis for a 5-Month-Old Infant
Administer immune globulin (IG) intramuscularly at 0.25 mL/kg (maximum 15 mL) as soon as possible, ideally within 6 days of exposure. 1
Rationale for Immune Globulin Administration
Infants under 6 months of age are specifically indicated for IG prophylaxis when they are household contacts of measles patients, as they are at highest risk for severe complications and mortality from measles. 1, 2
The standard dose is 0.25 mL/kg intramuscularly (maximum 15 mL) for immunocompetent infants, though some recent evidence suggests 0.5 mL/kg may provide better protection given declining antibody concentrations in modern IG preparations. 1, 3, 4
IG can prevent or modify measles infection if administered within 6 days of exposure, making prompt administration critical. 1, 2
Why Not Vaccination at 5 Months?
Measles vaccine is not recommended below 6 months of age because maternal antibodies interfere with vaccine response, leading to poor immunogenicity and high vaccine failure rates. 1, 2
Even when vaccine can be given (≥6 months), it is only effective as post-exposure prophylaxis within 72 hours of initial exposure. 1, 2 In household settings, measles is typically not recognized until after this window has passed. 1
For household contacts aged ≥6 months, vaccine within 72 hours is acceptable, but if that window is missed, IG becomes the recommended intervention. 1
Why Not Acyclovir?
- Acyclovir has no role in measles prophylaxis or treatment—measles is caused by a paramyxovirus, not a herpesvirus. This option represents a fundamental misunderstanding of viral pathogens.
Critical Follow-Up
Any infant who receives IG must subsequently receive MMR vaccine starting at 12 months of age, administered no earlier than 5-6 months after IG administration. 1, 2
This delayed vaccination schedule is necessary because passively acquired measles antibodies from IG interfere with the immune response to live measles vaccination. 1, 2
The infant should receive the standard 2-dose MMR series: first dose at 12-15 months and second dose at 4-6 years before school entry. 2, 5
Important Caveats
Infants under 6 months are usually immune due to passively acquired maternal antibodies, but if the mother herself has measles, this protection cannot be assumed and IG should be given to all unvaccinated household contacts regardless of age. 1
The effectiveness of IG depends on the concentration of measles antibodies in the product, which has declined over time as more donors have vaccine-derived rather than natural immunity. 3, 4, 6 Recent studies show modern IG preparations remain effective when dosed appropriately. 7, 4
Do not delay IG administration while waiting for maternal antibody testing—the 6-day window is critical, and testing results may not return in time. 7
Answer: B. Immunoglobulins