In the 4-month period, September 9-December 27, 1981, 111 cases of measles were reported among school children in Warren County, Pennsylvania (Figure 3). Nine of these cases were confirmed by serologic tests. An additional 45 related cases were reported in the rest of the community. Health authorities were first notified 6 weeks after the first patient had rash onset; they promptly initiated school-record reviews in preparation for mass vaccination clinics and exclusion from school of noncompliant susceptible students. Of the county's 8,315 students, 39% (3,210) were identified as lacking adequate proof of immunity.* At the time of the school exclusion order, 22% (728) of the students identified as being potentially susceptible furnished an updated record of vaccination, 73% (2,336) were vaccinated (most of them in control-program school clinics on October 28 and 29) and the other 5% (146) were excluded from school as of October 29.

Of the 111 cases among school children, 46 occurred more than 2 weeks after the control program was implemented, even though school records for these students indicated that all 46 had been vaccinated

2 weeks before becoming ill. Thirteen of these children from 3 schools had been vaccinated at control-program clinics; 10 of the 13 were from the same school. At these 3 schools, the measles attack rate was significantly higher for students vaccinated in control-program clinics (13 of 267, 5%) than for students who already had records of adequate vaccination before the outbreak-control program began (19 of 990, 2%)(p0.01). The 13 children who had been vaccinated during the control program but had measles disease during the outbreak were among 362 students of a high school and 4 elementary schools who had been vaccinated by a single jet-injector team (team A) at control-program clinics on October 28. No cases were reported among the 1,771 students vaccinated on that date by 4 other jet-injector teams or by several needle-and-syringe teams.

A seroprevalence survey was done in order to investigate the extent of apparent vaccination failures. Blood samples were collected from 115 of the 128 clinic vaccinees who were attended by team A at the 4 elementary schools but did not become ill and from 85 of the 101 vaccinees at selected elementary schools each attended by 1 of 3 other jet-injector teams. Students vaccinated by team A had a significantly higher rate of vaccination failure** than did children vaccinated by the other jet-injector teams (37% vs 6%, p0.001).

Control-program vaccination teams had all used vials from the same vaccine lot except team A, which had used some vials of vaccine of another lot as well; these particular vials from the other lot had been handled differently from other control-program vaccine. Records were not available on vaccine vial usage by team A in each school.

To evaluate possible differences in vaccination technique among jet-injector teams, students participating in the serosurvey from 2 schools at which vaccinations were given by team A and from 3 comparison schools were questioned about their experiences with vaccination in the control program. Students vaccinated by team A stated that they had experienced significantly less pain (36% vs 75%, p0.001) and bleeding at the site of vaccination (6% vs 62%, p0.001) than did students vaccinated by other teams.

To assess other factors (among the remaining 33 of the 46 patients) that might have been associated with students' susceptibility to measles after the control-program was implemented, a case-control study was done to examine such variables as age at vaccination and whether school records could be documented by the purported sources of vaccination. Cases were defined as measles illness occurring 2 weeks after the control program among the 33 students who had not been vaccinated at control-program clinics; controls were randomly chosen from classmates of ill students who also had not been vaccinated during the control program and who had not become ill. Three times as many children who contracted measles had school records of measles vaccination that could not be documented by physicians or clinics than did their well classmates. Of the students with physician-documented vaccination records, relatively more of those who contracted measles disease than well classmates had been vaccinated before 12 months of age (with school records in error) or at 12 months of age, rather than at greater than or equal to 15 months of age.

Thus, of the 46 students who became ill 2 weeks after the control-program was implemented, 13 (28%) had been vaccinated by team A, 13 (28%) did not have physician-documented records of vaccination, 2 (5%) had been vaccinated before 12 months of age, and 11 (24%) had been vaccinated at 12 months of age; no specific reason for remaining susceptible to measles could be identified for the other 7 (15%).

The Pennsylvania Department of Health has taken the necessary steps to provide live-measles vaccine to those students found to be seronegative in the seroprevalence survey. Reported by C Butler, RN, L Deiter, P Spencer, RN, C Stading, P Strickland, B White, RN, R Gens, MD, Acute Infectious Disease Div, EJ Witte, VMD, State Epidemiologist, Pennsylvania Dept of Health; Surveillance, Investigations, and Research Br, Immunization Div, Center For Prevention Svcs, CDC.

Editorial Note

Editorial Note: In school settings, the recommended measures for controlling measles outbreaks include the rapid identification and vaccination of susceptible individuals at risk and exclusion from school of students who lack adequate evidence of immunity.***

Generally, reports of new cases of measles decline markedly within 2 weeks of implementing a school exclusion order and transmission among students ends shortly thereafter (1,2). The outbreak in Pennsylvania was unusual in that transmission persisted for more than 8 weeks after control-program clinics were held and exclusion of noncompliant students was implemented.

There appeared to be at least 3 reasons for the continued transmission. First, vaccination by team A was significantly less effective than vaccination by the other teams. This lower efficacy may have been caused by administration of impotent vaccine, poor vaccine administration technique (e.g., unrecognized jet-injector malfunction), or some combination of the two. Proper handling of vaccine should not be taken for granted, and persons using vaccine should follow the manufacturer's instructions carefully. It appears unlikely that measles transmission would have continued in the county's schools for such an extended period if these operational problems had not occurred.

Second, some school records were found to be inaccurate. Ideally, vaccination records should be verified by health-care providers when students first enter school. Third, these data indicate that persons vaccinated at 12 months of age are at higher risk of contracting measles than those who are older when vaccinated. These results are consistent with those of previous studies (3). Persons vaccinated at l2 months are not routinely revaccinated because their estimated level of protection (80%-95%) has been considered adequate.

References

1979;28:488,493-4. 2 CDC. Multiple measles importations--New York. MMWR 1981;30:288-90. 3 CDC. Measles--Florida. MMWR 1981;29:625-8. *Pennsylvania State Health Department criteria for adequate proof of immunity to measles consist of either a documented history of live-measles vaccine received on or after the first birthday or of detectable measles-specific antibody on serologic testing. **Vaccination failures were defined as measles cases among clinic vaccinees or the absence of detectable hemagglutination-inhibition antibody in blood specimens collected from clinic vaccinees. ***Generally considered a documented history of live-measles vaccine on or after the first birthday or of physician-diagnosed measles disease.

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