Post-Exposure Prophylaxis for a 6-Month-Old Infant with Household Measles Exposure
Administer intramuscular immune globulin (IG) at 0.5 mL/kg (maximum 15 mL) immediately to this 6-month-old infant, as this is the only effective intervention for infants under 6 months exposed to measles within the household. 1
Why Immune Globulin is the Correct Choice
Age-Specific Considerations
- Infants younger than 6 months who are household contacts of confirmed measles cases should receive measles-specific immune globulin because they have the highest risk of severe disease and mortality from measles. 1
- At 6 months of age, this infant falls into a critical window where maternal antibodies have waned (92% of infants lack protective antibodies by 3 months, and 100% by 6 months in elimination settings), yet the infant is too young for reliable vaccine response. 2
- Measles vaccine is not recommended for infants younger than 6 months because maternal antibodies interfere with vaccine response, leading to poor immunogenicity and high failure rates. 1
Dosing and Timing
- The recommended dose is 0.5 mL/kg intramuscularly (maximum 15 mL), which is higher than the older standard of 0.25 mL/kg. 1, 3
- This higher dose reflects updated evidence showing that modern immune globulin preparations have lower measles antibody concentrations due to vaccine-derived immunity in blood donors rather than natural infection-derived immunity. 3
- IG must be administered as soon as possible and no later than 6 days after exposure to prevent or modify measles infection; effectiveness declines sharply after this window. 1
Why Other Options Are Incorrect
Option A: Acyclovir
- Acyclovir has no role in measles prophylaxis or treatment because measles is caused by a paramyxovirus, not a herpesvirus. 1
Option C: Measles Vaccine
- Although measles vaccine can be given as early as 6 months during outbreaks, it is only effective as post-exposure prophylaxis if administered within 72 hours of initial exposure. 1, 3
- For household contacts, the exposure is typically continuous and ongoing, making the 72-hour window impractical. 1
- Even if vaccine were given at 6 months, it would not count toward the routine two-dose series and would need to be repeated. 1, 4
Option D: No Intervention
- This is incorrect because infants under 12 months are at highest risk for severe complications and mortality from measles, making passive immunization with IG the preferred and necessary intervention. 1
- The assumption that maternal antibodies provide protection cannot be relied upon, especially since most infants lose protective antibody levels by 3-6 months of age. 2
Follow-Up Vaccination Strategy
Delayed MMR Schedule After IG
- Any infant who receives IG must receive the first dose of MMR vaccine at ≥12 months of age, scheduled at least 5-6 months after IG administration to avoid interference from passively acquired antibodies. 1, 5
- This delayed vaccination schedule is necessary because passively acquired measles antibodies from IG can interfere with the immune response to measles vaccination. 1
Complete Vaccination Series
- The standard two-dose MMR series (first dose at 12-15 months, second dose at 4-6 years) should be followed after the delayed start. 1
- Do not forget to vaccinate the infant 5-6 months after IG administration—failure to do so leaves the infant vulnerable to future measles exposures. 5
Critical Pitfalls to Avoid
- Do not delay IG administration while awaiting laboratory confirmation or maternal antibody testing—the 6-day window is critical and clinical diagnosis in the household contact is sufficient to warrant prophylaxis. 1, 5
- Do not use the outdated 0.25 mL/kg dose—current evidence supports 0.5 mL/kg for optimal protection with modern IG preparations. 1, 3
- Do not confuse measles (rubeola) with rubella (German measles)—IG is not indicated for rubella exposure, and the clinical presentations differ significantly. 6, 7