Mumps Control Today

Even though mumps was described by Hippocrates >2 thousand years ago and a vaccine has been available for decades, the disease continues to spread in our communities and act in unexpected ways. During the prevaccine era in the United States, ∼90% of all children had experienced mumps virus infection by 14 years of age and large outbreaks occurred regularly, especially when those raised in rural areas were brought together into crowded situations, such as military barracks. After a mumps vaccine was licensed in 1968 and recommended for all children in 1977, the reported incidence of mumps began to decline substantially. A resurgence of outbreaks of both mumps and measles in the mid-1980s, often among high school and college students, resulted in the Immunization Practices Advisory Committee recommendation in late 1989 of a second dose of measles-mumps-rubella (MMR) vaccine [1]. By the 2006–2007 school year, between 81% and 100% of children entering schools in the United States had received 2 doses [2]. Between 2000 and 2005, <350 cases of mumps were reported annually in the United States, and an elimination goal for mumps was set for the year 2010. In 2006, however, college campuses in Iowa were the initial foci of what became the largest epidemic of mumps in 2 decades; by the end of that year, 6584 cases of mumps had been reported in the United States [2]. Today, smaller outbreaks 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 [3] and 79% percent after 2 doses [4]). 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-yearolds [5]. 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) [6]. The herd immunity threshold may need to be higher than the previously suggested 88%–92% [7] to prevent community transmission and outbreaks of mumps. The most compelling and concerning of these hypotheses is that our most important public health tool against this disease—2 doses of MMR vaccine—is not providing the necessary levels of protection to control mumps in the US population. In an article in this issue of the Journal, Kutty et al [8] assist us in our understanding of the true levels of immunity against mumps in the United States. By using serologic data from a nationally representative sample of the US population (the National Health and Nutrition Examination Survey), mumps seroprevalence and the factors associated with the presence of antibodies to mumps virus were analyzed. As the study demonstrated, the calculated seroprevalence of antibodies to mumps virus in 6–49-year-old Americans is only ∼90%, which is below the estimated 92% needed for mumps control (ie, herd immunity). These levels are particularly important as our population changes—as those older Americans who have lifetime immunity due to childhood mumps disease age out of our population, a larger percentage of Americans will need to rely on vaccine for protection. Studies such as that of Kutty et al are useful for understanding the underlying immune status of a population and then extrapolating this knowledge to calculate the risk for the whole community. Thus, understanding the limitations of the MMR vaccine on a population level and how best to use this tool is critical to future mumps control. If 2 doses of MMR vaccine are not able to provide sufficient herd immunity to prevent outbreaks, the question is what to do? Choices include the following: Use a third dose of MMR vaccine (as may happen unintentionally in settings such as the military and as is being tried today in some New York communities) [9]. Delay the second dose until later in adolescence (which has coordination and verification barriers but would likely provide better levels of immunity during young adulthood). Decrease exposure among young adults (this measure has obvious problems with changing the behavior of young adults at all times for sporadic and rare exposures). Decrease introduction of virus (which is not feasible at this time, given the ease of transportation and levels of mumps activity elsewhere). Use a vaccine that provides higher efficacy and that is longer lasting (which is not available at this time and which may not be possible without tolerating the increased frequency of undesired adverse effects that often accompanies a more potent vaccine). Studies such as the one by Kutty et al allow us to better understand what level of protection the presently available mumps vaccine provides and to make the best possible use of this protection. We must use this information if we are to control mumps in today';s environment. Outbreaks and mumps transmission continue to occur around the United States (primarily in young adults and often in those connected to educational institutions), and attempts to control the spread of mumps virus are continuing. A large outbreak began in June 2009 in New York and New Jersey after the introduction of mumps into a camp, with continued spread among school-age males in the tradition-observant Jewish community [9]; in mid-May 2010, a small outbreak of 9 cases occurred in northwest Iowa, primarily on a college campus where only 7 of ∼1200 students had not received 2 doses of MMR vaccine (author';s unpublished data). Studies of the serological evidence of immunity (such as the one by Kutty et al) as well as studies that assess other control strategies (ie, the effect of a third dose of MMR vaccine) will help us to more fully understand and use the public health strategies that will best control mumps today.