Outbreak–Related Questions and Answers for Healthcare Providers

A: During mumps outbreaks in highly vaccinated communities, the proportion of cases that occur among people who have been vaccinated may be high (see example below). This should not be interpreted as meaning that the vaccine is ineffective. The way to assess the effectiveness of the vaccine is by comparing the attack rate in people who are vaccinated with the attack rate in those who have not been vaccinated. In outbreaks in highly vaccinated populations, the relatively few people who have not been vaccinated against mumps usually have a much greater mumps attack rate than those who have been fully vaccinated. During the outbreak in 2006, most of the mumps cases occurred in those who had received 2 doses of the MMR vaccine because most of the affected population had received 2 vaccine doses. However, the attack rate was much higher in the unvaccinated people, and 2 doses of the vaccine were estimated to be 88% (range: 66-95%) effective in preventing mumps.

A: The MMR vaccine is very effective against measles, mumps, and rubella and has made these three once-common childhood diseases very rare in the United States.

Before there was a vaccine against mumps, mumps was a common disease in the United States and caused complications such as permanent deafness in children, and it occasionally caused encephalitis, which can result in death, although very rarely. Before vaccine was used, over 150,000 mumps cases were reported every year, although many more unreported cases occurred. Now, a few hundred cases of mumps are reported every year on average. However, outbreaks still occasionally occur. In 2006, there was an outbreak affecting more than 6,584 people in the United States, with many cases occurring on college campuses. In 2009, an outbreak started in close-knit religious communities and schools in the Northeast, resulting in more than 3,000 cases. These outbreaks have shown that when people who are sick with mumps have close contact with a lot of other people (such as among students living in dormitories and students and families in close-knit communities) mumps can spread even among vaccinated people. However, outbreaks are much larger in areas where vaccine coverage rates are lower.

The MMR vaccine is also effective against measles and rubella. Prior to a vaccine against measles, measles infected millions of people and caused approximately 500 deaths every year in the United States. Now, on average, fewer than 100 cases are reported every year and almost all can be linked to importation of measles from other countries and occur in individuals who have not been completely vaccinated.

And before a vaccine against rubella (German measles) was widely used, rubella and rubella-associated birth defects (congenital rubella syndrome or CRS) were common. In the global epidemic from 1962 to 1965, an estimated 12.5 million rubella cases occurred in the United States and 20,000 infants were born with CRS. Now, approximately 10 cases of rubella are reported each year and CRS is very rare. These cases are usually the result of importation from other countries and almost always occur in individuals who have not been vaccinated against rubella.

A: The vaccine against mumps is effective, with the 2-dose effectiveness estimated at 80% to 90%. The 10% to 20% of people who have received 2 doses of MMR but are still susceptible to mumps is sufficient to sustain occasional mumps outbreaks, especially in population settings where people have a high number of close contacts with others (e.g., school and college settings). In the 2009-10 mumps outbreak in the northeastern United States, a boy traveled to the United Kingdom, where mumps is more prevalent and cases were ongoing. From there, he returned to the United States and went to a summer camp without knowing he was infected with mumps. The virus spread within the camp, and after camp ended, the disease spread primarily in schools and then into households.

Similar outbreaks have occurred in the past. In 2006, a mumps outbreak occurred in the midwestern United States, with the highest incidence of disease among young adults, many of whom were college students. The spread of virus was likely facilitated by close contact in certain settings, such as dormitories. The strain of mumps virus that caused this outbreak was the same strain that was circulating at the time in the United Kingdom, where most cases were occurring in unvaccinated persons.

An outbreak also occurred in 1997-98 among a tradition-observant Jewish community in Brooklyn. Most cases occurred among school-aged children attending local yeshivas (traditional Jewish schools).