It is unlikely to be a direct reaction to the measles, mumps, or rubella components, as those typically cause fever 5–12 days after vaccination. A fever within 24 hours is more commonly due to a coincidental viral infection or a reaction to other vaccines given during the same visit. Regardless of the cause, treat the symptoms by keeping your baby hydrated and comfortable, and consult your pediatrician if the fever persists or is accompanied by warning signs.