Post-Exposure Prophylaxis for a 5-Month-Old Infant After Measles Exposure
Administer immune globulin (IG) at 0.25 mL/kg intramuscularly (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, 2
Why Immune Globulin is the Correct Choice
This infant is too young for MMR vaccination and requires passive immunization. The 5-month-old falls below the minimum age threshold for measles vaccination, which is 6 months even during outbreaks. 1, 2 The Centers for Disease Control and Prevention explicitly recommends IG for infants under 6 months who are household contacts of measles patients. 1
Key Clinical Reasoning Points:
Infants under 12 months are at highest risk for severe complications and mortality from measles, making this intervention critical despite potential maternal antibody protection. 1
Maternal antibodies at 5 months are unreliable for protection. While some passive immunity may persist, it cannot be guaranteed, and household exposure to a symptomatic sibling represents high-risk contact requiring intervention. 1
The 6-day window for IG effectiveness is crucial. IG must be administered within 6 days of exposure to prevent or modify measles infection. 3, 1, 2 Given that the siblings "spent a lot of time together," this infant has had significant exposure and the clock is ticking.
Why Other Options Are Incorrect
MMR Vaccine (Option B) - Inappropriate
MMR can only be considered for infants ≥6 months during outbreaks, not for a 5-month-old. 1, 2
Post-exposure vaccination is only effective within 72 hours of initial exposure. 2 For household contacts, this window is typically exceeded before the index case is diagnosed, making IG the more reliable choice. 1
"Nothing" (Option C) - Dangerous
This represents a failure to provide indicated prophylaxis. The recommendation that infants under 6 months don't need intervention is categorically false for household contacts of active measles cases. 1, 2
Infants under 12 months face the highest mortality risk from measles, with complications occurring in 10-40% of cases including pneumonia, encephalitis (1 per 1,000), and death (1-2 per 1,000). 2, 4
Measles Antiviral Drug (Option D) - Does Not Exist
- There is no specific antiviral therapy for measles. Treatment is primarily supportive care with vitamin A supplementation. 2
Dosing Specifications
Standard IG dose for immunocompetent infants: 0.25 mL/kg IM (maximum 15 mL). 3, 1, 2
This differs from the 0.5 mL/kg dose reserved for immunocompromised patients. 3, 5
For a typical 5-month-old weighing approximately 7 kg, this would be approximately 1.75 mL IM.
Critical Follow-Up Care
After IG administration, this infant must receive MMR vaccination 5-6 months later (around 10-11 months of age), as passively acquired antibodies from IG interfere with vaccine response. 1, 6
This delayed MMR dose will NOT count toward the routine series. 1, 2
The infant will still require two additional MMR doses: first at 12-15 months and second at least 28 days later. 1, 2
Common Pitfalls to Avoid
Do not delay IG administration while waiting for antibody testing. Clinical diagnosis of measles in the sibling is sufficient to warrant immediate prophylaxis. 5
Do not assume maternal antibodies provide adequate protection. In poor socioeconomic conditions or with multiple early infections, passively acquired antibody is depleted more rapidly. 7
Do not forget the 6-day window. IG effectiveness drops significantly after this timeframe. 3, 1, 2
Do not confuse measles (rubeola) with rubella (German measles). IG is not indicated for rubella exposure. 1, 2