People born before what year received only one dose of the mumps vaccine?

The correct answer is based on the historical context of the mumps vaccination program in the United States. The first effective mumps vaccine was licensed in 1967, which means that individuals born before this pivotal year would not have received the mumps vaccine in their childhood vaccinations, as it was not yet available.

However, looking at specific vaccination practices and recommendations, in 1977, the Advisory Committee on Immunization Practices (ACIP) recommended that children receive two doses of the mumps vaccine as part of the measles, mumps, and rubella (MMR) vaccine schedule. Thus, those born before 1957 would have only received one dose, as the two-dose recommendation was not implemented until much later.

Being born before 1957 indicates that these individuals would have been unlikely to receive a comprehensive vaccination strategy that included the multiple doses which became standard in later years. This is the critical aspect that clarifies why those born before this year only received one dose of the mumps vaccine.