From Infectious Disease Special Edition

Why Do Vaccine-Preventable Disease Outbreaks Occur in the US?

James D. Cherry MD, MSc; Kathleen H. Harriman PhD, MPH

In This Article


Although mumps has not officially been eliminated in the United States, there appears to be limited endemic transmission. The R 0 for mumps is estimated to be between 4 and 12, and the level of community immunity needed to interrupt its transmission is estimated at 75% to 92%.[1,2,3] The mumps vaccine, however, may not be as efficacious as has been previously estimated; therefore, achieving the elimination of mumps may be more difficult than anticipated.

Post-licensure studies conducted in the United States between 1973 and 1989 determined that the efficacy of a single dose of the MMR vaccine was between 75% and 91%. A study in the United Kingdom reported that the efficacy of 2 doses of the MMR vaccine was 88%. Protection was considered to be long-term;[21] however, outbreaks of mumps recently have occurred among populations in which most people received 2 doses of the MMR vaccine, thus calling into question the longterm effectiveness of the vaccine.

In 2006, the United States experienced a multistate outbreak of mumps that resulted in 6,584 reported cases. This outbreak primarily affected college students in the Midwest, with the highest incidences occurring within dormitories.[19] Studies conducted on the campuses involved in the outbreak found that high rates of 2-dose MMR vaccine coverage were not sufficient to prevent the outbreak. In addition to primary vaccine failure, the protective effect afforded by the vaccine may wane over time.[22,23]

Another large outbreak occurred in the northeastern United States during 2009 and 2010. The index case was an 11-year-old boy who was infected with mumps in the United Kingdom. There were 3,502 reported cases, primarily within Orthodox Jewish communities in New York, where prolonged, close contact between individuals in congregate settings facilitated its transmission. Among patients for whom vaccination status was reported, 90% had received at least 1 dose of mumpscontaining vaccine, whereas 76% had received 2 doses.[21]

In 2011, a mumps outbreak that involved 29 people occurred on a university campus in California. This outbreak was sparked by an unvaccinated student who also had been infected in the United Kingdom. One of the infected individuals had received 1 dose of MMR, 22 had received 2 doses, 2 had been vaccinated with 3 doses of MMR, and 1 was unvaccinated. One person had received a single dose of mumps-rubella vaccine and had documented serologic evidence of prior mumps immunity; 2 of the other infected individuals were unsure of their vaccination status.[8]

Similar to measles, cases of mumps also have increased in frequency throughout Europe, secondary to declining rates of MMR vaccinations. In the United Kingdom, 3,965 cases of laboratory-confirmed mumps were reported by the Health Protection Agency in 2010.[24] As mumps cases continue to be imported, outbreaks are likely to occur in US settings where large numbers of adolescents and young adults have very close and prolonged contact, which facilitates transmission. In the prevaccine era, mumps typically affected young children, a population in which complications are less common. A transition to cases of mumps in older individuals is likely to result in a greater percentage of complicated cases.[21]Although the Centers for Disease Control and Prevention has not yet made this recommendation, a third dose of MMR was implemented in 2010 during outbreaks in both the northeastern United States and in Guam. The additional dose appeared to aid the control of these outbreaks.[25,26]