Post-Exposure Prophylaxis for a 5-Month-Old Exposed to Measles
Administer intramuscular immune globulin (IG) at 0.25 mL/kg (maximum 15 mL) to this 5-month-old infant as soon as possible, ideally within 6 days of exposure to the measles-infected sibling. 1
Why Immune Globulin is the Correct Choice
Infants under 6 months of age cannot receive measles vaccine and require passive immunization with IG for post-exposure prophylaxis. 1 The CDC specifically recommends IG administration for infants younger than 6 months who are household contacts of measles patients, as this age group faces the highest risk for severe complications and mortality from measles infection. 1
Key Rationale Points:
Vaccine is not an option: MMR vaccine can only be considered for infants ≥6 months of age during outbreaks, and even then, it must be given within 72 hours of exposure to be effective as post-exposure prophylaxis. 1, 2 This 5-month-old is below the minimum age threshold.
Maternal antibodies interfere with vaccination: Infants under 6 months typically have maternal antibodies that would interfere with vaccine response, leading to poor immunogenicity and high vaccine failure rates. 1 This is why routine measles vaccination begins at 12 months of age. 1
IG provides immediate passive protection: Unlike vaccination, which requires time to generate an immune response, IG provides immediate passive antibodies that can prevent or modify measles infection if given within 6 days of exposure. 1, 2
Dosing and Administration
The standard dose for immunocompetent infants is 0.25 mL/kg administered intramuscularly, with a maximum total volume of 15 mL. 1, 2
Important Dosing Considerations:
Some recent evidence suggests that higher doses (0.5 mL/kg) may improve protection with modern IG preparations that have lower measles antibody concentrations due to vaccine-derived immunity in blood donors. 1, 3, 4 However, the official CDC recommendation remains 0.25 mL/kg for immunocompetent infants. 1
The higher 0.5 mL/kg dose is specifically reserved for immunocompromised patients, not healthy infants. 5, 6
Critical timing: IG must be administered as soon as possible and no later than 6 days after exposure for effectiveness. 1, 2 Efficacy declines sharply after this window.
Common Pitfalls to Avoid
Do not delay IG administration while awaiting maternal antibody testing—the 6-day window is critical and testing results may not be available in time. 1
Do not confuse this with a preterm infant born to a mother with active measles, which would require the higher 0.5 mL/kg dose. 5 This is a term 5-month-old exposed to a sibling.
Do not assume maternal antibodies provide adequate protection—even though most infants under 6 months have maternal antibodies, IG should be given to all unvaccinated household contacts in this age group regardless. 1
Follow-Up Vaccination Plan
After receiving IG, this infant must receive MMR vaccine at ≥12 months of age, scheduled at least 5-6 months after IG administration. 1, 5 This delay is necessary because passively acquired antibodies from IG interfere with the immune response to measles vaccination. 1
The standard two-dose MMR series should then be completed:
- First dose at 12-15 months (at least 5-6 months post-IG) 1
- Second dose at 4-6 years before school entry 1
Why Other Options Are Incorrect
MMR vaccine (Option B): Not indicated for infants <6 months due to poor immunogenicity and maternal antibody interference. 1, 2
Nothing (Option C): Incorrect—infants under 6 months are at highest risk for severe measles complications and mortality, making prophylaxis essential. 1
Antiviral medication: There is no specific antiviral therapy for measles; treatment is supportive only. 2 Acyclovir has no role in measles prophylaxis or treatment as measles is caused by a paramyxovirus, not a herpesvirus. 1