If I give MMR to an infant traveler younger than age 1 year, will that dose be considered valid for the U.S. immunization schedule?

No. A measles-containing vaccine administered more than 4 days before the first birthday should not be counted as part of the series. MMR should be repeated when the child is age 12 through 15 months (12 months if the child remains in an area where disease risk is high). The second dose should be administered at least 28 days after the first dose.