Board-style review of measles post-exposure prophylaxis: when to give MMR within 72 hours vs immune globulin for high-risk patients.
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Key takeaway: After measles exposure, give MMR within 72 hours to eligible patients—even if they’ve already had 1 dose—while reserving immune globulin for high-risk groups.
You are evaluating a 3-year-old child after a measles outbreak was reported in the local school district. The child had been in contact with a child diagnosed with measles less than 24 hours prior to today's visit. The child had 1 dose of MMR vaccine at 13 months of age.
What would you recommend for this child?
Answer Options:
In board-style outbreak/exposure scenarios, test-takers often overthink whether a previously vaccinated child “needs anything.” The exam-reliable rule is that measles PEP is time-dependent: MMR can prevent or modify disease if given within 72 hours of exposure (CDC/ACIP; AAP Red Book). This child is eligible (age >12 months, not immunocompromised, exposed <24 hours ago), and they are also not fully immunized (needs 2 doses), so administering MMR now is both PEP and an accelerated second dose in the context of outbreak risk.
Immune globulin is a common trap: it is not first-line for healthy, immunocompetent children >12 months. Per CDC/ACIP and AAP, immune globulin PEP is reserved for higher-risk groups (e.g., infants <12 months, pregnant patients without evidence of immunity, and severely immunocompromised patients), and must be given within an appropriate timeframe (generally within 6 days of exposure).
| Option | What It Tests / Implies | Why It’s Wrong Here |
|---|---|---|
| 1 dose of measles, mumps, and rubella (MMR) vaccine | Knowledge that MMR within 72 hours can serve as measles PEP; accelerated completion of 2-dose series | Correct for an immunocompetent 3-year-old exposed <24 hours ago (CDC/ACIP; AAP Red Book). |
| Intravenous immune globulin at a dose of 0.5 mL/kg | Confusion that any exposure warrants immune globulin | IVIG is for high-risk exposed patients (e.g., severe immunocompromise) rather than a healthy 3-year-old. Dose/route also varies by product/indication. |
| close observation with measles, mumps, and rubella (MMR) vaccine dose given if any viral prodrome is appreciated | Misbelief that vaccination should wait for symptoms | PEP is most effective when given promptly; waiting for prodrome misses the 72-hour window for vaccine-based PEP. |
| no intervention indicated | Assumes 1 prior dose is “fully protected,” so nothing needed | Full evidence of immunity generally requires 2 documented MMR doses (and outbreaks warrant aggressive prophylaxis). |
Measles exposure: MMR ≤72 hours for eligible patients; immune globulin (≤6 days) only for infants <12 months, pregnant non-immune, or severely immunocompromised.
The question “feels” like a routine immunization schedule item (second dose at 4–6 years), but it is actually a PEP timing question: exposure <24 hours means you should act now, and in an eligible child the action is MMR, not observation and not immune globulin.
A 8-month-old infant is exposed to a confirmed measles case at daycare yesterday. What is the best prophylaxis?
A 6-year-old with acute lymphoblastic leukemia on chemotherapy is exposed to measles 2 days ago. Best management?
A 2-year-old received 1 MMR dose at 12 months and is exposed to measles today. What should you do?
A healthy 4-year-old (no MMR doses) is exposed to measles 5 days ago. Best prophylaxis today?
How would your recommendation change (MMR vs immune globulin vs no action) if the exposed patient were: (1) 10 months old, (2) pregnant and nonimmune, (3) a healthcare worker with 2 documented MMR doses, or (4) on high-dose steroids?
Q1: Why give a second MMR dose early during an outbreak/exposure?
A: The ABP/boards expect you to recognize that the second dose can be given earlier than age 4–6 years when indicated; exposure/outbreak risk shifts you from “routine schedule” to “risk-based dosing” (CDC/ACIP).
Q2: What’s the key timing difference between MMR and immune globulin for measles PEP?
A: MMR works as PEP if given within 72 hours; immune globulin can be given later (generally within 6 days) and is used for high-risk or vaccine-ineligible patients (CDC/ACIP; AAP Red Book).
Q3: Who should get immune globulin instead of MMR?
A: High-risk exposed patients—classically infants <12 months, pregnant nonimmune, and severely immunocompromised—because they either can’t receive live vaccine or are at high risk of severe measles.
Q4: If someone already has 2 documented MMR doses, do they need PEP after exposure?
A: Typically no; 2 documented doses is accepted as evidence of immunity for board purposes (CDC/ACIP), barring special institutional policies or unusual immunodeficiency contexts.
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