Mumps Vaccination Policies and Epidemiological Consequences: A Comparative Review of the United States and Japan.

Mumps remains a public health challenge worldwide despite being a vaccine-preventable disease. This review outlines the historical development and contrasting mumps vaccination policies in the United States and Japan and their epidemiological consequences. In the United States, where the two-dose measles, mumps, and rubella (MMR) vaccine is routinely administered, the incidence of mumps has markedly decreased. Recently, however, sporadic outbreaks have emerged, driven by breakthrough infections among vaccinated individuals. These resurgences are driven largely by waning immunity, coupled with antigenic divergence between the vaccine strain and circulating wild-type viruses. In contrast, Japan vaccination the MMR vaccine was licensed in 2026 after being suspended in 1993 owing to concerns over aseptic meningitis as an adverse event following immunization but mumps vaccination remains voluntary. This has resulted in low vaccination coverage, a lack of herd immunity, and recurrent epidemics. This review also highlights the disease-modifying effect of mumps vaccination, including a reduced risk of complications such as meningitis and orchitis, even in cases of breakthrough infection. In Japan, irreversible sensorineural hearing loss (mumps deafness), continues to occur even following subclinical infection. To prevent these complications, continuous molecular surveillance, improved vaccine safety, and reintroduction of routine two-dose mumps vaccination in Japan are essential.