What is the most appropriate action to protect a 6-month-old infant living with a sibling who has a maculopapular rash, likely indicating a measles or rubella infection?

Post-Exposure Prophylaxis for 6-Month-Old Infant Exposed to Measles

Administer immune globulin (IG) at 0.25 mL/kg intramuscularly (maximum 15 mL) as soon as possible, ideally within 6 days of exposure, as this 6-month-old infant is too young for MMR vaccination and requires passive immunization to prevent severe measles infection. 1

Why Immune Globulin is the Correct Choice

The clinical scenario describes a sibling with a maculopapular rash starting behind the ears and spreading to the body—this is the classic presentation of measles (rubeola), not rubella. 2 At 6 months of age, this infant falls into a critical gap:

• Too young for MMR vaccination: The minimum age for routine MMR vaccination is 12 months, though it can be given as early as 6 months during outbreaks for post-exposure prophylaxis within 72 hours. 3, 1
• At highest risk for complications: Infants under 12 months have the highest risk for severe complications and mortality from measles, making passive immunization with IG the preferred intervention. 1
• Household exposure exceeds the 72-hour window: By the time the sibling's rash is recognized and diagnosed, the 72-hour window for effective post-exposure MMR vaccination has typically passed. 1

Dosing and Administration

• Standard dose: 0.25 mL/kg intramuscularly (maximum 15 mL) for immunocompetent infants. 1, 4
• Timing is critical: IG must be administered within 6 days of exposure to be effective in preventing or modifying measles infection. 1, 4
• Higher dose (0.5 mL/kg) is reserved for immunocompromised individuals, not healthy infants. 5

Why the Other Options Are Incorrect

Option B (No prophylaxis needed) is dangerous and incorrect. Infants under 6 months may have some residual maternal antibodies, but a 6-month-old infant is at the tail end of this protection and cannot be assumed to be immune. 1 The CDC explicitly recommends IG for infants aged birth to 6 months who are household contacts of measles patients. 4

Option C (Give MMR vaccine now) is problematic because:

• At exactly 6 months, the infant is at the minimum threshold where MMR could be considered during an outbreak, but only within 72 hours of exposure. 3, 1
• Household contacts typically exceed this 72-hour window before the index case is diagnosed. 1
• Maternal antibodies may still interfere with vaccine response at this age. 1
• Any MMR given before 12 months does not count toward the routine two-dose series and must be repeated. 1, 4

Essential Follow-Up Care

After IG administration, this infant requires:

• Delayed MMR vaccination: Administer MMR no earlier than 5-6 months after IG administration to avoid interference from passively acquired antibodies. 1, 4
• Complete routine series: The first official MMR dose at 12 months of age, followed by a second dose at 4-6 years before school entry. 3, 4
• Failure to provide this delayed vaccination leaves the infant vulnerable to future exposures. 1

Common Pitfalls to Avoid

• Do not confuse measles (rubeola) with rubella (German measles): IG is not indicated for rubella exposure, which causes a milder illness with different complications. 1, 6
• Do not delay IG while waiting for laboratory confirmation: Clinical diagnosis in the sibling is sufficient to warrant prophylaxis. 5
• Do not use the wrong dose: The 0.5 mL/kg dose is only for immunocompromised patients, not healthy infants. 1, 5