Although the attack rate among persons who received a third dose of MMR vaccine as PEP was 0%, compared with a 5.2% attack rate for those with 2 doses who did not receive PEP, the difference was not statistically significant. Nonetheless, MMR vaccine administered because PEP might offer some benefits. If the exposure did not result in infection, the vaccine should boost antibody titers high enough to induce protection against subsequent infection (13
). Such boosting of antibody titers would be useful during an outbreak in which the virus continues to circulate and future exposures are likely. If infection does occur, the postexposure vaccine dose may lead to milder clinical manifestations, lower complication rates, and shorter duration of virus shedding (15
Although a third dose of MMR vaccine has been previously administered for outbreak control (10
), to our knowledge, a third dose of MMR vaccine has never previously been administered in a study to assess its efficacy as PEP. In 1986, a first dose of MMR vaccine was given as PEP in a Tennessee public high school to 53 of 178 students with no presumptive evidence of immunity. During the Tennessee outbreak, in 15 (28.3%) of 53 students who received a first dose of MMR vaccine as PEP, mumps developed between 1 and 21 days (1 incubation period) after they visited the clinic compared with mumps developing in 51 (40.8%) of 125 nonvaccinated students who did not receive PEP (6
In addition to the outbreak in the northeastern United States, other large mumps outbreaks have occurred among highly vaccinated US populations in recent years. In 2006, a total of 6,584 reported cases occurred, primarily in college students in the midwestern United States. Standard control measures (e.g., isolation and vaccine catch-up campaigns) were implemented for outbreak control (17
) with modest effectiveness. The outbreak did not subside until summer break when the students left their college campuses. During 2009-2010, a total of 505 mumps cases were reported in the US Territory of Guam, primarily among school-aged children 9–14 years of age, 96% of whom had received 2 doses of MMR vaccine. In addition to application of standard control measures, a third dose of MMR vaccine was administered to the most affected age group for outbreak control, not as PEP. The effectiveness of the intervention was inconclusive (16
). Outbreaks have also been reported in other industrialized countries among populations in which the proportion who received 2 doses of vaccine was high (18
Two MMR vaccine doses provide 66%–95% effectiveness against mumps (22
), and the 2-dose policy has reduced mumps incidence by >99% compared with incidence during the prevaccine era (24
). Nonetheless, mumps outbreaks in well-vaccinated populations continue to occur, posing challenges for outbreak control. Current public health measures for preventing the spread of mumps during outbreaks, including isolation, quarantine, contact tracing, and increasing vaccine coverage have had limited effect (17
). When schools follow public health guidance and send infected students home for 5 days, the intervention may be too late. Mumps can spread from symptomatic persons before parotitis onset. Mumps can also spread from persons who have asymptomatic infections, which can be as high as 15%–27% of infected persons (4
). In addition, isolating patients and quarantining contacts may be ineffective when infected persons live in large households with many other susceptible persons. Finally, raising vaccine coverage is also difficult in contexts where 2-dose vaccine coverage is already high, because current policy does not recommend a routine third MMR vaccine dose (5
In the 2 households with co-primary cases in this study, no additional cases occurred during the first incubation period. This finding suggests that those households were not more infectious than households with only 1 index patient.
This study was subject to limitations, however. Household members may have been exposed to mumps by a contact outside the home. Although our method might have been more robust if we could have randomly selected household contacts to receive PEP, because of ethical considerations, it was necessary to offer PEP to all eligible household contacts. Some household members had received the third dose during a school intervention a couple of months before this study. In addition, some members received either a first, second, or third dose during the outbreak but not as part of the study. Although these persons were excluded from the analysis because their doses were not administered as PEP, these doses outside the study may have limited the effect of the study doses because additional family members were protected. This could have lowered mumps attack rates in the households by reducing the number of susceptible persons. When the risk for mumps among persons potentially susceptible was assessed, the limited sample size and low attack rates resulted in large confidence intervals. Finally, the power of the study to detect a significant difference was extremely low because of the small number of study households, the relatively late implementation of the study during the outbreak, and the low number of mumps cases that occurred in the study population.
Although 2 MMR doses are sufficient for preventing mumps in most settings, administering a third MMR dose may be worthwhile in specific outbreak contexts, even if it does not offer protection as PEP. Our findings support the need for additional evaluations in which third doses of MMR vaccine are used as PEP in outbreaks among populations with 2-dose vaccination coverage. Future studies on administering any dose of MMR vaccine for mumps PEP during mumps outbreaks are also warranted.