Post-Exposure Prophylaxis for 5-Month-Old Infant
Administer immune globulin (IG) at 0.25 mL/kg intramuscularly (maximum 15 mL) as soon as possible, ideally within 6 days of exposure. 1, 2
Rationale for Immune Globulin Over Vaccination
For infants under 6 months of age who are household contacts of measles patients, passive immunization with IG is the preferred intervention because:
- The child is likely exposed more than 72 hours before the sibling's measles diagnosis was confirmed, making vaccine less effective 1
- Infants under 12 months are at highest risk for severe complications and mortality from measles 1
- Maternal antibodies may still be present at 5 months, which would interfere with vaccine response 1
- IG can prevent or modify measles if given within 6 days of exposure 1, 2
Why Measles Vaccine Is Not Appropriate Now
- Routine measles vaccination is not recommended before 12 months of age due to poor immunogenicity and high vaccine failure rates 1
- While measles vaccine can be given as early as 6 months during outbreaks, this 5-month-old is below even that threshold 1
- Post-exposure vaccination is only effective within 72 hours of initial exposure, and household contacts typically exceed this window before diagnosis 1
Dosing Specifications
The standard IG dose for immunocompetent infants is 0.25 mL/kg (maximum 15 mL) administered intramuscularly 1. Recent evidence suggests considering 0.5 mL/kg for infants under 6 months if injection volume is not a concern, given declining antibody concentrations in modern IG products 3.
Follow-Up Vaccination Plan
- After IG administration, the child should receive MMR vaccine at 12-15 months of age, but not earlier than 5-6 months after IG administration to avoid interference from passively acquired antibodies 2
- A second MMR dose should be given before school entry 1
Critical Pitfall to Avoid
Do not wait for symptom development before administering IG 1. The goal is prevention of disease, not treatment after onset. IG is most effective when given promptly after exposure, and waiting for symptoms defeats the purpose of post-exposure prophylaxis 1, 2.