Post-Exposure Prophylaxis for Measles in Infants Under 6 Months
Administer immune globulin (IG) at 0.25 mL/kg intramuscularly (maximum 15 mL) as soon as possible, ideally within 6 days of exposure, to this infant under 6 months of age who is a household contact of a sibling with measles. 1
Why Immune Globulin is the Correct Choice
Infants under 6 months cannot receive MMR vaccine and require passive immunization with IG for measles protection. The standard immunization schedule establishes that MMR vaccine has a minimum age of 12 months for routine administration 2. Even during outbreaks or for international travel, measles vaccine can only be given as early as 6 months of age 2, making this infant below the threshold for any measles vaccination 1.
Key Physiologic Rationale
- Infants under 12 months are at highest risk for severe measles complications and mortality, making passive immunization critical 1
- Maternal antibodies may still be present at this age, which would interfere with vaccine response even if the vaccine could be given 1
- IG prevents or modifies measles infection when administered within 6 days of exposure 2
Dosing and Administration Details
The standard IG dose for immunocompetent infants is 0.25 mL/kg (maximum 15 mL) administered intramuscularly 2, 1, 3. This differs from the higher dose of 0.5 mL/kg reserved for immunocompromised patients 2, 4, 3.
Timing Considerations
- IG is particularly indicated for susceptible household contacts of measles patients because household contacts typically exceed the 72-hour window for post-exposure MMR vaccination before diagnosis is confirmed 2, 1, 3
- Post-exposure MMR vaccination is only effective within 72 hours of initial exposure, and by the time a sibling's rash is recognized as measles, this window has usually passed 2, 3
Essential Follow-Up Care
After IG administration, the infant must receive MMR vaccine starting at 12 months of age, with the first dose given 5-6 months after IG administration if this timing coincides with reaching 12 months 1, 3. This delay is necessary because passively acquired antibodies from IG interfere with vaccine immunogenicity 1, 3.
- The routine two-dose MMR series should begin at 12-15 months, with the second dose given at least 28 days later 2, 1
- Any measles vaccine given before 12 months (which would not apply to this infant) does not count toward the routine series 2, 4, 3
Common Pitfalls to Avoid
- Do not assume "no prophylaxis is needed" for infants under 6 months - while they may have some residual maternal antibodies, household exposure to measles requires IG prophylaxis 1
- Do not attempt to give MMR vaccine to an infant under 6 months - this is below the minimum age even for outbreak situations 2, 1
- Do not use the immunocompromised dose (0.5 mL/kg) for healthy infants - the standard 0.25 mL/kg dose is appropriate 1, 3
- Do not forget to vaccinate 5-6 months after IG administration - failure to do so leaves the infant vulnerable to future exposures 1, 3
Answer: A. Administer immune globulin