When not given on the same day, is the interval between yellow fever and MMR vaccines 4 weeks (28 days) or 30 days? I have seen the yellow fever and live virus vaccine recommendations published both ways.

The CDC’s General Best Practice Guidelines for Immunization generally recommends that live parenterally or nasally administered vaccines not given on the same day should be separated by at least 28 days. Despite this general principle, limited data suggest that coadministration of yellow fever vaccine and MMR may diminish the immune response. The CDC travel health website recommends that yellow fever vaccine and other parenteral or nasal live vaccines should be separated by at least 30 days, if possible. If yellow fever vaccine and another injectable live-virus vaccine are not administered either simultaneously or at least 30 days apart, CDC advises that providers might consider measuring the patient’s neutralizing antibody response to vaccination before travel. CDC recommends contacting the state health department or the CDC Arboviral Disease Branch (970-221-6400) to discuss serologic testing. For details, see the 2024 Yellow Book section on spacing of vaccines and immunobiologics: wwwnc.cdc.gov/travel/yellowbook/2024/preparing/vaccination-and-immunoprophylaxis-general-principles#spacing.