Does a child who received only the first dose of measles‑mumps‑rubella (MMR) vaccine at 15 months need a booster before traveling to the United States?

MMR Booster Before Travel to USA

Your child does NOT need a booster dose before traveling to the United States—they need their routine second dose of MMR vaccine, which can be given now since they are already past 15 months of age. 1, 2

Standard Two-Dose MMR Schedule

• The CDC and FDA recommend a two-dose MMR series: first dose at 12-15 months and second dose at 4-6 years before school entry. 3, 1, 2
• However, the second dose can be administered any time after the first dose, provided there is a minimum 28-day interval between doses. 1, 4
• Since your child received their first dose at 15 months (which is the appropriate age), they are eligible to receive the second dose now if desired for travel protection. 1, 2

Why Early Second Dose Is Acceptable

• Approximately 5% of children fail to develop immunity after the first MMR dose (primary vaccine failure). 3
• The second dose primarily serves to protect those who didn't respond to the first dose, not because immunity wanes significantly. 3, 5
• Studies demonstrate that children achieve 100% seropositivity after the second dose regardless of whether it's given at 4-6 years or 11-13 years, meaning earlier administration is equally effective. 5

Travel Considerations for the USA

• The United States does not have endemic measles transmission and maintains high vaccination coverage. 3
• There is no specific CDC requirement for accelerated MMR vaccination before travel to the USA, unlike travel to measles-endemic regions. 6, 2
• The recommendation for early MMR (before 12 months) applies only to high-risk situations: active outbreaks, travel to endemic areas, or known measles exposure—none of which apply to routine USA travel. 6

Practical Recommendation

Administer the second MMR dose now if:

• You want maximum protection during travel (reasonable approach)
• The child will be in crowded settings (airports, public venues)
• You prefer not to wait until age 4-6 years

Alternatively, you can safely wait until age 4-6 years if:

• The USA destination has no current measles activity
• Travel is brief and low-risk for exposure
• You prefer to follow the standard schedule

Important Caveats

• Any MMR dose given before 12 months does NOT count toward the two-dose series and must be repeated. 6, 1 This does not apply to your child since their first dose was appropriately given at 15 months.
• If you choose to give the second dose now, ensure at least 28 days have elapsed since the first dose. 1
• Document the vaccination in your child's permanent medical record for future school entry requirements. 3, 2