Today, smaller outbreaks of mumps continue to occur, especially among those living in group settings associated with educational institutions, even when these populations have very high rates of 2-dose MMR vaccine receipt. The annual number of cases is still far below the numbers that occurred during the prevaccine era, providing convincing evidence that the vaccine is quite efficacious. However, it is not well understood why these outbreaks continue to occur; some of the hypotheses that have been proposed include the following: Vaccine efficacy and corresponding antibody levels may be lower than expected, especially now that natural boosting due to disease exposure is very rare (estimates of the effectiveness of the mumps vaccine have varied but have been estimated to be as low as 64% after 1 dose and 79% percent after 2 doses). Over time, immunity may wane, even after receipt of 2 doses. This perhaps provides a reason for today\';s outbreaks occurring primarily among young adults. This has resulted in suggestions that a third dose may be needed or that the second dose of MMR vaccine should be given during adolescence rather than just before school entry at 4 or 5 years of age. One study showed that the 2-dose vaccine efficacy decreased from 99% among 5–6-year-olds to 86% among 11–12-year-olds. High population density in communal living situations, such as dormitories or boarding schools, may provide increased opportunities for close contact or saliva exposures and higher dose exposures to mumps virus when introduced, resulting in easier transmission and higher rates of disease than might occur in other parts of society. The vaccine strain (derived from genotype A) may be less effective against serogroup G wild strains (the type seen in the United States). The herd immunity threshold may need to be higher than the previously suggested 88%–92% to prevent community transmission and outbreaks of mumps.