The study population was a geographically and socially clustered community of ≈20,000 persons, primarily Orthodox Jews, in Orange County, New York. A common feature of this community was its high household contact rates because of large family size (average, 6 members) and shared bedrooms (9). Most members of this community followed ACIP vaccination recommendations; 2-dose MMR vaccine coverage among school-aged children in the community was 94.3%, which was higher than the national average (8,10,11).

Mumps cases were classified according to the case definition for mumps of the Council of State and Territorial Epidemiologists in 2008 (12). Household members were considered secondary case-patients if mumps onset occurred 12–25 days (1 incubation period) after parotitis onset in the household’s index case-patient. Household members were considered to be co-primary case-patients if mumps onset occurred within 11 days after onset in the household index patient. PEP was defined as a dose of MMR vaccine given to a household contact within the first 5 days of another household member’s onset of provider-diagnosed mumps parotitis. Any dose administered earlier than this was not considered a PEP dose.

Suspected cases of mumps within the affected community were reported to the local health department, and parents were encouraged to contact one of the community’s 2 primary medical providers. The provider invited the parent to bring the case-patient and all other household members to the clinic and instructed the parent regarding routine measures required by the practice to prevent patients with respiratory illnesses from causing the other patients to be exposed to the virus. At the initial visit, the provider assessed the case-patient, and if a diagnosis of mumps was confirmed, the provider determined whether other household members had a history of mumps and their vaccination status. If the family visited the healthcare provider during the study period, February 24–April 24, 2010, then household members who met the eligibility criteria (i.e., had received 2 documented doses of MMR vaccine, had no contraindications for vaccination, had no history of mumps, and 5 days had not yet elapsed since onset of parotitis in the household index case-patient) were offered a third dose of MMR vaccine. Household members who were not up to date with their routine vaccinations were offered a first or second dose of MMR vaccine as PEP. Adult household members whose vaccination history was not documented were eligible to receive a dose.

Eligible family members who did not receive PEP either chose not to be vaccinated or lived in a household in which mumps had been diagnosed in a case-patient earlier in the outbreak, and it was too late for family members to receive PEP. Household members were not eligible for PEP and were excluded from the analysis if they had received a recent MMR vaccine dose within the past 60 days (i.e., either at their health care provider’s office or by participating in a recent school-based third-dose MMR intervention study [10]), if they had a history of mumps, if they were too young to be vaccinated (i.e., <1 year of age), or if they were a co-primary or index case-patient. Members of households who chose not to be vaccinated and members of households of mumps case-patients identified earlier in the outbreak who were not offered a third MMR dose were used as a comparison group.

Because the use of a third dose of MMR vaccine is not recommended by ACIP for PEP, a protocol was submitted and approved by the US Centers for Disease Control and Prevention and New York State Institutional Review Boards. Participants provided written consent or assent.

Baseline surveys captured demographic characteristics, MMR vaccination history, and mumps history of household members. Follow-up surveys were completed at least 60 days after the date of parotitis onset for the household mumps index case-patient; information gathered included any MMR doses received by family members since the baseline interview and whether mumps developed in any household members.

Vaccination status of study participants was assessed. Health care provider records were reviewed to verify this information.

All data were analyzed with SAS 9.3 (SAS Institute Inc., Cary, NC, USA). For each household, we added the number of family members eligible for PEP with MMR vaccine and the number who received PEP. χ² and Wilcoxon rank sum tests were used to compare 1) demographic characteristics and intervals since last MMR dose among index case-patients and 2) household members who received a third dose of MMR vaccine as PEP with persons who had 2 previous doses and did not receive PEP. Secondary mumps attack rates during the first incubation period after mumps onset in the index case-patient were calculated.

Of the 49 index case-patients, 25 were male (51.0%) (Table 1). The median age was 9 years (range 1–39 years). Thirty-two (65.3%) had received 2 doses of MMR vaccine. Eleven (22.4%) index case-patients were unvaccinated or had unknown MMR vaccination status. Among the 38 (77.6%) who reported receiving >1 doses of MMR vaccine, the median interval since their last dose was 47 months (range 3–170 months).

In 49 households, there were 365 household members, of whom 239 (65.5%) were eligible to receive PEP and 126 (34.5%) were deemed ineligible and excluded from further analysis of mumps risk factors. Those excluded were the following: 59 household members who had received a recent dose of MMR vaccine within 60 days before the intervention, 49 who were the household index patients, 15 who were <1 year of age, 2 who became co-primary case-patients, and 1 woman with a history of having had mumps in 1979 (Figure).

Forty-four (18.4%) of the 239 eligible household members received a postexposure dose of MMR vaccine; 28 (11.7%) received a third MMR vaccine dose, 6 (2.5%) received a second MMR vaccine dose, 2 (0.8%) received a first MMR vaccine dose, and 8 (3.3%) adults with unknown vaccination status received a dose. The age groups of household members who received a third dose of MMR vaccine as postexposure prophylaxis included 10 (27.8%) of 36 children aged 4–6 years, 17 (24.3%) of 70 children 7–17 years of age, and 1 (1.1%) of 88 adults >18 years. Of the 16 other household members who received PEP with MMR vaccine, 2 children 1 year of age received a first dose, 6 children 1–17 years of age received a second dose, and 8 adults with unknown MMR vaccination status received a dose (Table 2).

Postexposure vaccinations were not administered to 195 (81.6%) eligible household members. Of eligible persons who did not receive postexposure vaccine, 77 had previously received 2 doses (of whom 21 were 4–6 years of age, 50 were 7–17 years, and 6 were >18 years), 40 had previously received 1 dose (of whom 33 were 1–3 years, 3 were 4–6 years, 2 were 7–17 years, and 2 were >18 years), and 78 had unknown vaccination status (of whom 6 were 1–3 years of age, 71 were >18 years, and 1 was <18 years, but the exact age was not available).

Of the 9 household secondary cases that occurred during the first incubation period after the index patient’s mumps onset, 3 (33.3%) were in male patients. Only 1 (11.1%) case-patient received the MMR vaccine as PEP. He was a 27-year-old father with unknown vaccination status. Eight (88.9%) persons who did not receive PEP became infected with mumps during the first incubation period after their exposure (2 were from the same household).

The median age of the 8 secondary case-patients who did not receive PEP was 18.5 years (range 6─39 years). The ages and vaccination status of these 8 included the following: 1 child 6 years of age who had a history of 2 doses of MMR vaccine, 3 children 7–17 years of age who had a history of 2 doses of MMR vaccine, and 4 adults with unknown vaccination status. All household members >18 years of age who were infected were parents of index case-patients. The interval between the last MMR vaccine dose and reported mumps onset was 18 days for the father who received PEP and from 2 to 6 years for the 4 case-patients with known vaccination status who did not receive PEP.

Mumps also developed in 2 persons within the first 11 days of the onset of the index case; these patients were considered co-primary. No secondary cases developed during the first incubation period after the index patient’s mumps onset among remaining family members at risk for mumps in the 2 households with co-primary case-patients (5 members were at risk in each household).

The interval between receipt of the last dose of MMR vaccine and mumps onset among the index case-patients did not differ between households with a secondary case-patient and those without (median interval 3 years; both groups, p = 1.0). Additionally, the ages of the index case-patients did not significantly differ between households with a secondary case-patient and those without (median ages 7.5 years and 9 years, respectively; p = 0.21).

None of the 28 family members who received a third dose of MMR vaccine as PEP became infected with mumps virus in contrast with 4 (5.2%) of the 77 who had previously received 2 doses but did not receive PEP (Table 3). The difference in secondary attack rates between the 2 groups was not statistically significant (p = 0.57). Two of the 2-dose case-patients were male; the sex-specific attack rates were 6.9% for male and 4.3% for female patients (p = 0.62). The median age of those receiving a third dose was 8 years (range 5–20 years) and also 8 years (range 4–20 years) among those eligible who did not. The median number of years since the last MMR dose (before the PEP dose) was 10 years (range 2–39 years) among those who received a third dose, compared with 11 years (range 0–39 years) among those eligible who did not (p = 0.47).