A recent resurgence in mumps cases in the U.S. may be due to weakening immune protection from the mumps vaccine, researchers report. They say the results of their modeling studies suggest that a booster dose at age 18 may help control outbreaks of the disease. Before the mumps vaccine was developed in 1967, more than 90% of children and youth in the United States experienced this painful and highly-contagious viral infection prior to the age of 20. After decades of declining mumps incidence due to the introduction of widespread vaccination, an increase in mumps cases was observed in 2006 in adolescents and young adults in the U.S. Seeking to determine if the mumps virus had evolved to escape the vaccine or if immunity from the vaccine naturally decreased over time - a distinction that helps to inform whether new vaccines are needed to control transmission - Joseph Lewnard and Yonatan Grad closely examined epidemiological data from six mumps vaccine effectiveness studies conducted in the United States and Europe. The scientists concluded that the mumps vaccine protects people for an average of 27 years (with a range of 16 to 51 years), and did not seem to be less effective against emerging mumps strains. Transmission models indicated that routine use of a booster dose at age 18 could help to maintain population immunity. The authors say their findings emphasize a need to conduct clinical trials that would measure the benefit of administering this booster dose.