Which of the diseases prevented by the MMR vaccine is most rare in the U.S.?

Rubella, also known as German measles, is the disease prevented by the MMR vaccine that is most rare in the U.S. The introduction of the MMR vaccine has led to a significant decline in the incidence of rubella due to high vaccination coverage, effectively eliminating endemic transmission in many areas. As a result, rubella is now rarely reported, especially compared to the other diseases covered by the vaccine, such as measles and mumps, which still experience outbreaks and have higher incidences despite vaccination efforts. This rarity is a direct consequence of successful public health initiatives focused on vaccination, making rubella the least common of the diseases included in the MMR vaccine in the current U.S. context.