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Mumps Outbreak -- New Jersey

From October 19, through December 14, 1983, 63 cases of mumps were reported from six schools in a school district in Atlantic County, New Jersey. These 63 cases are a 40% increase over the previous year's total of mumps among schoolchildren in the state. Before 1978, mumps vaccination was not required for school entry in New Jersey; however, beginning in 1978, mumps vaccination was required for school entry for children 7 years of age or younger. By the fall of 1983, students in kindergarten (K) through grade 5 would have been covered by the law. An investigation was undertaken to determine the possible reasons for the outbreak and to control its spread. In particular, this marked increase in reported mumps presented an opportunity to: (1) determine the compliance with the 1978 school immunization law; (2) investigate the effect of the law on the pattern of the outbreak; (3) estimate the efficacy of mumps vaccine; and (4) quantitate the economic impact of the outbreak on the community.

A case of mumps was defined as unilateral or bilateral swelling of the parotid or other salivary gland for 2 or more days as reported by a physician, nurse, or parent. Acute mumps infection was serologically confirmed (i.e., a fourfold rise in complement fixation antibodies between acute- and convalescent-phase sera) in four cases. No viral cultures were attempted.

The index patient was a 12-year-old, unvaccinated, sixth-grade boy attending Elementary School A. The source of his infection was not identified. All cases in the five other elementary, middle, and high schools involved could be epidemiologically linked to cases in this elementary school. When cases were plotted by date of onset, two distinct peaks of disease were identified that occurred 14-18 days apart (Figure 1). Of the 63 students who met the case definition, 37 (59%) were girls. Ill students ranged in age from 6 years to 17 years (mean 11 years). Thirty-six (57%) cases occurred among children in grade 6 or higher. None of the patients developed complications, and none were hospitalized.

Forty-eight (76%) cases occurred in Elementary School A, for an overall attack rate of 5% (48/933). Twenty-five (52%) of these 48 cases occurred among the sixth grade students (Table 1). The attack rate in grade 6 (15%) was five times that in grades K-5 (3%) (p 0.001) (Table 1).

School vaccination records were reviewed for evidence of previous mumps vaccination or mumps disease to determine student susceptibility. A student with (1) a history of physician-diagnosed mumps; (2) serologic evidence of mumps immunity; or (3) a dated parental, school, or physician record of vaccination with live mumps vaccine on or after 12 months of age was considered immune to mumps (1). Vaccine coverage rates were greater than 95% in grades K-5 but fell to 68% in the sixth grade. Overall, records showed grade-specific susceptibility rates (Table 2) parallelled grade-specific attack rates (Table 1). Sixth graders were nearly seven times more likely to be susceptible to mumps than students in other grades (p 0.001) (Table 2).

Since initial reports suggested that many mumps cases occurred in children known to have been vaccinated, a vaccine efficacy study was done.* The sixth grade was used to estimate vaccine efficacy, because it represented 52% of the school's cases and had enough unvaccinated and vaccinated students to make calculation of attack rates in these two groups meaningful. Vaccination status was verified for both vaccinated and unvaccinated students using a dated parental record. If unavailable, a physician record was then obtained. Studies relying solely on school records for determination of immunization status and casefinding may provide misleadingly low estimates of vaccine efficacy (2). Vaccine efficacy was estimated to be 91% for sixth graders, with a 95% confidence interval of 77%-93% (Table 3). The attack rate in the vaccinated children in the sixth grade was 4% and fell within the 5%-10% primary vaccine failure rate observed in clinical trials (3).

Outbreak control began with a review of all student immunization records in the school district to identify students lacking mumps vaccination. Students deficient in measles, rubella, diphtheria, and tetanus vaccinations were also identified. Vaccines were offered free to all susceptible schoolchildren in three state-run vaccination clinics held for 2 days in early December. Of 4,188 students in the district, 945 (23%) were identified as lacking mumps immunity, based on criteria of the Immunization Practices Advisory Committee (ACIP) (1). Of the 945 susceptible students, approximately 75% received vaccines containing mumps antigen at the state-run clinics. An unknown number of students was vaccinated by private physicians.

Telephone questionnaires administered to the parents of ill students were used to collect information on the economic impact of the outbreak. The 63 cases of mumps occurred in children from 46 different households. The total cost to households was $4,687 for an average cost of $102 per household. The average number of school days missed by each child was 5.7. Of the 46 households, 16 (35%) had at least one parent miss 1 day of work to take care of a child, with a mean of 3 days of work missed. Day-care services were used by six (13%) households. Medical services were utilized by 33 (72%) households. All these households consulted with their physicians by telephone, and three (7%) households took a child to a hospital emergency room. No persons were hospitalized.

The cost associated with the emergency vaccination clinics was $6,250, which included costs for clinic supplies, personnel, transportation, and vaccine. The cost of vaccine alone was $3,100, or 50% of the total clinic cost. The total direct cost of the outbreak was $10,937 (this includes clinic costs plus the total costs to households). Even though 362 pupil school days were lost because of illness associated with this outbreak, there was no loss of state school reimbursement aid, since New Jersey does not consider absenteeism when providing aid to local school districts. Reported by J Aiello, Atlantic County Health Dept, R Altman, L Dimasi, C Kauffman, T Ksell, R McCready, S Sloane, WE Parkin, DVM, State Epidemiologist, New Jersey State Dept of Health; Div of Field Svcs, Epidemiology Program Office, Div of Immunization, Center for Prevention Svcs, CDC.

Editorial Note

Editorial Note: Although mumps has never attained the same notoriety as measles or rubella in the public or medical community, mumps virus was the leading cause of viral encephalitis of known etiology in this country until 1975 (4). The routine use of combined measles-mumps-rubella (MMR) vaccine in recent efforts to increase protection rates for measles and rubella has had a beneficial effect on the reported mumps incidence. A provisional total of 3,285 mumps cases were reported nationally in 1983--the lowest reported incidence since mumps became a nationally notifiable disease in 1968. Peak mumps reporting occurred in 1967, the year of vaccine licensure, with 185,691 cases. Cases in 1982 decreased by 38% from 1982 (5,270 cases) and by 98% from 1967.

Age-specific data are not yet available for 1983. Data for 1982 indicate the risk of infection has declined by more than 90% for all age groups. However, the reported incidence rate for 10- to 14-year-olds in 1982 was higher than that for any other age group (5). In the years immediately following vaccine licensure, the highest incidence rates occurred in 5- to 9-year-olds, followed by children under 5 years of age. The age-specific changes in mumps infection rates are similar to those noted for measles and rubella and would be expected with any vaccination policy oriented towards schoolchildren (1). Thus, based on mumps epidemiology alone, the outbreak in New Jersey involving largely sixth grade and older children was not unexpected.

Mumps immunity was not required for school entry in New Jersey until 1978. Vaccination of this group with the highest susceptibility rates (5) allowed the most efficient allocation of limited resources. In New Jersey, immunity could be proven by evidence of appropriately administered mumps vaccine, parental,** or medical provider history of mumps or positive mumps serology. In this outbreak, the immunization law established two cohorts of students varying in their degrees of mumps immunity divided at the fifth- and sixth-grade levels--thus affecting the pattern of this outbreak.

Since the attack rate for each grade was directly proportional to the percentage of unvaccinated students, the higher attack rate for sixth graders most likely reflected the fact that sixth graders were not covered by the law. Compliance with the law, as reflected in vaccine coverage rates, was greater than 95%. Thus, poor compliance with the school vaccination law did not lead to this outbreak. Rather, it was those grades not covered by the school law that provided the susceptibles that allowed the disease to spread. A more comprehensive immunization law might have further limited both the size and scope of the outbreak.

Twenty states currently do not require proof of mumps immunity for school entry. Of the 30 states that do require mumps immunization, 15

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