Post-Exposure Prophylaxis for 5-Month-Old Infant After Measles Exposure
Give immunoglobulin (IG) to this 5-month-old infant immediately, as MMR vaccine is not routinely recommended before 6 months of age and the window for effective post-exposure vaccination has likely passed.
Rationale for Immunoglobulin Administration
For infants under 6 months who are household contacts of measles patients, immune globulin is the appropriate post-exposure prophylaxis. 1 The standard dose is 0.25 mL/kg IM (maximum 15 mL) for immunocompetent persons, and must be administered within 6 days of exposure to be effective. 1
- Infants aged less than 6 months are typically protected by maternally derived antibodies, but this protection is not guaranteed and ordinarily they do not require additional protection unless exposed to measles. 2
- At 5 months of age, this infant falls below the minimum age for routine MMR vaccination (6 months for high-risk situations, 12 months routinely). 2
Why MMR Vaccine Is Not the Primary Choice Here
MMR vaccine is only considered for infants aged 6-11 months in outbreak or high-risk exposure situations, not for 5-month-olds. 2, 1
- Even when MMR is given to infants aged 6-11 months during outbreaks, children are less likely to develop serologic evidence of immunity compared to older children, with seroconversion rates around 80% at 9 months versus >95% at 12 months or later. 3, 4
- Post-exposure MMR vaccination is most effective within 72 hours of initial exposure. 2, 1, 5 Given that the sibling already has developed the characteristic measles rash (which appears 14 days after exposure on average), the 5-month-old was likely exposed days ago, making vaccine administration too late to prevent infection. 1
Clinical Context Supporting IG Use
The sibling's presentation is classic for measles: conjunctivitis, fever, and rash starting behind the ear (retroauricular) then spreading—this represents the pathognomonic Koplik spots prodrome followed by the characteristic cephalocaudal rash progression. 1
- Measles patients are contagious from 4 days before rash onset to 4 days after, meaning this household has been an active exposure environment. 1
- The 5-month-old infant is at high risk for severe complications, as measles mortality occurs in 1-2 per 1,000 cases with encephalitis in 1 per 1,000 cases. 1
Important Caveats
The 6-day window for IG effectiveness is critical—if more than 6 days have elapsed since the infant's first exposure to the symptomatic sibling, IG may have reduced efficacy, though it should still be considered. 1
This infant will still require routine MMR vaccination starting at 12 months of age (or 6 months if in a high-risk area or outbreak), as IG provides only temporary passive immunity. 2 Any MMR dose given before 12 months must be followed by two additional doses: the first at 12-15 months and the second at least 28 days later. 2