Efficacy of Mumps Vaccine -- Ohio

From February 5 through April 23, 1982, 110 cases of mumps were reported among 357 sixth-, seventh-, and eighth-grade students in a middle school in Ashtabula County, Ohio. Because the overall attack rate was 31%, the efficacy of the mumps vaccine was investigated.

Vaccine efficacy (VE) was determined using the standard formula*, and was calculated using a variety of case definitions, case surveillance systems, and vaccination-status ascertainment methods to evaluate their effects on the estimated VE. Three studies were performed. Study 1: Using data collected at the school for case ascertainment and vaccination status, clinical VE was initially estimated at 37% (Table 1). Mumps vaccination was not required for school entry, and vaccination-status records were incomplete. Case ascertainment relied on either the school nurse's diagnosis or a parental history of mumps illness and, therefore, lacked a uniform case definition. Study 2: Using a uniform case definition (parotitis lasting 2 or more days) and only cases and vaccination status ascertained from parental questionnaires, estimated VE increased to 70% (Table 1). Ill and well children with histories of mumps disease or unknown vaccination histories or dates were excluded from the parental history of mumps illness and, therefore, lacked a uniform case definition. Study 2: Using a uniform case definition (parotitis lasting 2 or more days) and only cases and vaccination status ascertained from parental questionnaires, estimated VE increased to 70% (Table 1). Ill and well children with histories of mumps disease or unknown vaccination histories or dates were excluded from the analyses.

Because the Study 2 estimate of VE was still lower than expected (1), a third study was conducted. Since VE depends on the assumptions that all children are equally exposed--and that exposure is more likely to be uniform among household members than among schoolmates--and that records of vaccination are valid indicators of vaccination status, a study of household members with provider-verified vaccination status was performed. Study 3: Of 99 household members whose vaccine status was verified, 32 secondary cases met the uniform case definition and were included in the calculation of VE**. The VE estimate was 85% (Table 1). Age at vaccination, duration of time since vaccination, type of vaccine (combined or single antigen), and provider of vaccine were not found to be significantly different in a grade- and sex-matched study comparing 17 vaccinated patients with 17 vaccinated controls. Reported by JW Nye, Grand Valley School District, Orwell; JM Kettunen, C Hart, Ashtabula County Health Department, Jefferson; KM Sullivan, TJ Halpin, MD, State Epidemiologist, Ohio State Dept of Health. Div of Immunization, Center for Prevention Svcs, CDC.

Editorial Note

Editorial Note: The methods used to ascertain cases and determine vaccination status greatly affect estimates of VE. Studies relying solely on school records for case-finding and determination of immunization status may provide misleadingly low estimates of efficacy. In this study, the calculated VE improved with progressively more rigorous methods of case-finding, application of a uniform case definition, better documentation of vaccine history, and use of a setting with presumed more uniform exposure.

Serologic studies have generally shown that mumps vaccine induces antibodies--which have been correlated with protection--in greater than 90% of individuals. Clinical evaluations have noted VE ranging from 75% to 90% (1). Estimates of clinical VE in this study are consistent with these earlier studies. Efficacy calculations could have been affected by lack of serologic confirmation of prior immune status and by inapparent mumps disease. Field trials of the vaccine noted that protective efficacy dropped from 97% using laboratory-proven mumps cases to 88% using clinically diagnosed cases (2).

Although mumps vaccine is considered one of the safest of the childhood immunizing agents and is efficacious (1), 19 states, including Ohio, do not require proof of mumps immunity as a condition for school entry. From 1967, the first year of licensure, through 1982, approximately 59 million doses of vaccine were distributed, and the number of reported mumps cases dropped from 152,209 in 1968 to 5,196 in 1982 (a 97% decrease). The 1982-1983 school-enterers survey indicated a nationwide mumps vaccine coverage rate of 95% (range 69-99%). Older students, such as those involved in this outbreak, may have substantially lower levels of vaccine coverage. Recent increases in mumps vaccine coverage, even in states that do not require mumps vaccine as a condition for school entry, is attributed to the use of combined measles-mumps-rubella vaccine--the vaccine of choice for the routine immunization of children 15 months of age or older.

An evaluation of the costs of the outbreak in case families is shown in Table 2. Sixteen percent of persons with mumps visited a physician at least once. The one hospitalization was for an adverse reaction to drug treatment to control vomiting reportedly due to mumps. The total direct cost of the outbreak for the middle-school students and their families was estimated at $900. However, if indirect costs, such as loss of time from work, are included, the estimate would exceed $20,000 (3).

As indicated in this outbreak and in a recent study of benefit-cost analysis of mumps vaccine (3), the considerable medical and economic costs associated with mumps morbidity can best be averted by including mumps immunization as part of state compulsory school immunization laws. CDC has previously documented that comprehensive and strictly enforced school immunization laws can result in lower incidence of vaccine-preventable diseases (4). Provisional 1982 mumps-incidence data demonstrate that the incidence of mumps disease in states without a compulsory school mumps immunization law (34.7 per million population) was twice as high as in states that have such a law (17.5 per million population).

References

1. Immunization Practices Advisory Committee. Mumps vaccine. MMWR mumps-incidence data demonstrate that the incidence of mumps disease in states without a compulsory school mumps immunization law (34.7 per million population) was twice as high as in states that have such a law (17.5 per million population). References

2. Immunization Practices Advisory Committee. Mumps vaccine. MMWR 1982;31:617-20,625.

3. Hilleman MR, Weibel RE, Buynak EB, et al. Live attenuated mumps-virus vaccine. IV. Protective efficacy as measured in a field evaluation. N Engl J Med 1967;276:252-8.

4. Koplan JP, Preblud SR. A benefit-cost analysis of mumps vaccine. Am J Dis Child 1982;136:362-4.

5. Robbins KB, Brandling-Bennett AD, Hinman AR. Low measles incidence: association with enforcement of school immunization laws. Am J Public Health 1981;71:270-4.

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