What prophylaxis should be given to a 5‑month‑old infant who has been exposed to measles from his 6‑year‑old sibling?

Post-Exposure Measles Prophylaxis for a 5-Month-Old Infant

Direct Answer

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

Rationale for Immune Globulin Administration

Infants under 6 months of age cannot receive measles vaccination and require passive immunization with IG for post-exposure prophylaxis. 1, 2

• The Centers for Disease Control and Prevention specifically recommends IG at 0.25 mL/kg intramuscularly (maximum 15 mL) for infants under 6 months who are household contacts of measles patients 1
• This age group faces the highest risk for severe complications and mortality from measles infection, making prompt intervention critical 1
• IG can prevent or modify measles infection if administered within 6 days of exposure 1, 3

Why Not Vaccination?

Measles vaccine is contraindicated in this 5-month-old infant because it cannot be given before 6 months of age, even during outbreaks. 1

• Routine measles vaccination begins at 12 months due to poor immunogenicity and high vaccine failure rates in younger infants 1
• During outbreak situations, measles vaccine can be administered as early as 6 months of age, but this 5-month-old is below that threshold 1
• Maternal antibodies still present at 5 months would interfere with vaccine response, making IG the only viable option 1
• Post-exposure vaccination is only effective within 72 hours of exposure, and household contacts typically exceed this window before the index case is diagnosed 1

Dosing Specifications

Use the standard immunocompetent dose of 0.25 mL/kg IG, not the higher 0.5 mL/kg dose reserved for immunocompromised patients. 1, 3

• The 0.25 mL/kg dose is appropriate for healthy infants under 6 months 1
• The higher 0.5 mL/kg dose is specifically indicated only for immunocompromised individuals or preterm infants 4, 3
• Maximum total dose is 15 mL regardless of calculated dose 1, 3

Critical Timing Considerations

Administer IG as soon as the exposure is identified—do not wait for symptom development in the 5-month-old. 3

• The window for effective prophylaxis is narrow (6 days from exposure) 1, 2
• Household exposure represents high-risk contact requiring immediate intervention 1
• Clinical diagnosis of measles in the sibling is sufficient to warrant prophylaxis; laboratory confirmation is not required 4

Follow-Up Vaccination Strategy

After IG administration, the infant must receive MMR vaccine at 12 months of age (or 5-6 months after IG administration, whichever is later). 1, 4

• Passively acquired measles antibodies from IG interfere with vaccine immune response for 5-6 months 1
• Any measles vaccine given before 12 months does not count toward the routine two-dose series 4
• A second MMR dose should be administered before school entry 1

Common Pitfalls to Avoid

• Do not use "watchful waiting"—the absence of symptoms in the 5-month-old does not eliminate the need for prophylaxis given the known household exposure 3
• Do not assume maternal antibodies provide adequate protection—by 5 months, maternal antibody levels may have waned sufficiently to leave the infant vulnerable 1
• Do not confuse the dosing: 0.25 mL/kg for immunocompetent infants versus 0.5 mL/kg for immunocompromised patients 1, 3
• Do not forget delayed vaccination—failure to vaccinate 5-6 months after IG leaves the infant vulnerable to future exposures 1, 4

Why "Nothing" Is Wrong

Doing nothing is inappropriate because infants under 6 months are at highest risk for severe measles complications and mortality, and household exposure represents significant risk. 1

• The recommendation to provide IG prophylaxis for susceptible household contacts is explicit in CDC guidelines 1
• The risk-benefit analysis strongly favors intervention in this high-risk age group 2, 5

Recent Evidence on IG Effectiveness

Contemporary IG preparations remain effective for measles prophylaxis despite concerns about declining antibody titers in donor populations. 5

• A 2021 Austrian outbreak study demonstrated 99.3% effectiveness (95% CI: 88.7-100%) of IVIG at 400 mg/kg in preventing measles in exposed infants 5
• German recommendations updated in 2018 specifically address declining measles antibody concentrations in IG products by recommending higher doses 2
• Testing of current IG batches shows measles-neutralizing antibody capacity 1.57-2.26 fold higher than minimum protective levels 5