There were no significant differences in other clinical characteristics. Vaccinated cases clustered in the period from late October through late December and peaked at 2 to 3 weeks after vaccination. The incidence of GBS in the vaccinated group The authors also compared the rates of GBS for the vaccine from the four separate manufacturers and their individual vaccine lots.
The differences among manufacturers were not significant. Using case rates calculated with person-weeks of surveillance, one vaccine lot had a significantly higher GBS rate 10 cases per 4. The authors noted several limitations of the study in assessing causality.
No standard diagnosis for GBS was available, but they found little clinical difference between cases diagnosed in vaccinated and unvaccinated persons. Case ascertainment may have been more complete in the vaccinated group, but doubling the number of cases among the unvaccinated group would still have left the rate significantly lower. With little previous epidemiologic data on GBS, it was difficult to assess the rates found in the study.
The higher rate of GBS associated with a single vaccine lot might have been attributable to random variation in an analysis that included 47 different lots or to errors related to the 15 percent of vaccine without a known disposition. Breman and Hayner examined the incidence of GBS in Michigan between July 1, , and April 30, , a period that included the swine influenza vaccination program.
To identify cases, neurologists, neurosurgeons, hospital record room librarians, and physical therapists were contacted via mailed questionnaire or telephone. Information on cases was also requested from primary care physicians and local health departments. The authors noted that their methods allowed for more thorough ascertainment of GBS cases in Michigan than was achieved on the national level. Swine influenza vaccination were based on state and national data. Age- and sex-specific population estimates for the period were developed using data from the state and the U.
Bureau of Census. Incidence rates were compared using chi-square statistics. GBS cases were included in the study if they met the following criteria: diagnosis by a physician, bilateral muscle weakness of lower motor neuron type with or without cranial nerve or sensory abnormalities , acute or subacute onset and evolution of signs and symptoms, and absence of other conditions e.
Information was recorded on the form developed by CDC for the national surveillance program. The patient's primary care physician or consulting neurologist was consulted to confirm the diagnosis. A total of possible cases were identified, but over half failed to meet the inclusion criteria. Vaccination status was not considered when cases were selected, but it was known to the reviewers of the records—who were epidemiologists and physicians—once review began.
The analysis was confined to cases with onset before May 1, , to avoid the problem of under-ascertainment of cases with onset in May and June Overall, 79 cases were unvaccinated, 8 cases had been vaccinated after the onset of GBS, 31 cases were vaccinated and had onset of GBS within 6 weeks after vaccination, and 7 cases were vaccinated and had GBS onset 7 weeks or more after vaccination. No cases occurred in vaccinated children under 18 years of age , but 16 cases occurred in unvaccinated children.
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For the vaccinated population aged 18 years or older who had onset of GBS within 6 weeks after vaccination, the incidence of GBS was 2. The incidence rates were 0. Using the incidence rate in the unvaccinated group as the baseline, the attributable risk for acquiring GBS within 6 weeks of vaccination was calculated to be The authors also compared the vaccinated and unvaccinated groups in terms of the numbers of cases occurring in November and December and the number of cases from January to April The ratio of cases in these two time periods 30 cases in November—December to 7 cases in January—April 4.
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The authors noted that this indicated that the association between vaccination and GBS onset was limited to November—December and did not extend to January—April In other analyses, the authors found no statistically significant difference in incidence rates between the men and women, or among the vaccines from different manufacturers or with different formulations split versus whole-cell; monovalent versus bivalent.
The authors noted that the study showed an increased incidence of GBS during the swine influenza vaccination program and that the increased risk occurred for only 6 weeks after vaccination. Michigan and Minnesota. To address persistent questions about previous analyses that had found an association between GBS and receipt of the swine influenza vaccine, Safranek and colleagues conducted a detailed review of GBS cases that occurred in Michigan and Minnesota between October 1, , and January 31, About 10 percent of the cases analyzed by Schonberger and colleagues were from those two states.
The analysis by Safranek and colleagues was limited to cases in persons who lived in Michigan or Minnesota and who were 18 years of age or older. Cases were identified in two stages. First, GBS cases from Michigan and Minnesota that were reported to CDC and state health departments as part of the national surveillance effort were identified, and medical records relevant to neurological diagnosis were obtained.
Second, the acute care hospitals and rehabilitation facilities in the two states were asked to identify all patients discharged from October 1, , through June 30, , with an ICDA-8 diagnosis code a category that includes GBS. After a preliminary review, medical records were obtained for cases of possible GBS that had not been included in the original reports to CDC.
To confirm the diagnosis of GBS, the medical records for each case were reviewed by at least two neurologists from a six-member expert panel. Information on vaccination status and prior illness was masked. Diagnoses were based on standard criteria. Of cases identified from the CDC reports, 7 were excluded from further review because the person was less than 18 years of age or because onset of GBS fell outside the reference period.
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The remaining 95 cases included 67 with a GBS diagnosis 48 definite, 19 probable and 28 cases for whom the diagnosis of GBS was rejected. From the inquiries to Michigan and Minnesota hospitals and rehabilitation facilities, responses from institutions 93 percent identified patients. Of these, were eliminated because they did not meet clinical or nonclinical inclusion criteria.
The remaining 17 were reviewed by the expert panel, and 6 were included the analysis 1 definite GBS, 2 probable, 3 possible. Thus a total of 73 cases were included in the study, of whom 67 92 percent had been included in the CDC analysis Schonberger et al. Of the 73 cases, 45 62 percent received the swine influenza vaccine before onset of GBS. Four cases that had been classified as unvaccinated in the CDC records were reclassified as vaccinated. For Michigan, the number of vaccinations administered each week during the immunization program was determined using estimates from the National Center for Health Statistics.
For Minnesota, the numbers were based on data released as part of the court order that prompted the reanalysis of the national data Langmuir et al.
Overall, there were about 2. In Minnesota, there were 1. The incidence of GBS in the unvaccinated population in Michigan was 0. In the vaccinated population, the rate for the week surveillance period was 0. The relative risk for GBS for the full week period was 4. Looking only at the first 6 weeks following vaccination, however, the relative risk was 7.
For the two states combined, the relative risk for 6 weeks post-vaccination was 7. For the vaccinated group, incidence peaked in the third week after vaccination and then declined. In contrast, the weekly incidence rate for GBS in the unvaccinated population was stable over the entire observation period. The analysis showed that Michigan had 8.
The findings in this study were consistent with findings of earlier studies. The incidence of GBS among the vaccinated populations in Michigan and Minnesota was significantly higher than among the unvaccinated population, with a nonrandom clustering of cases in the 6 weeks following vaccination. These results were similar to the findings of Schonberger and colleagues The results of the study also showed that the CDC surveillance efforts were sensitive in detecting cases without regard to vaccination status and that over-reporting was similar both for the vaccinated and unvaccinated groups.
The incidence rate seen in the unvaccinated population of these two states was similar to that in the CDC analysis. Since cases were identified if they sought medical attention or if they had a discharge diagnosis of ICD-8 , the authors noted ascertainment bias was possible if cases in either the vaccinated or unvaccinated group were more likely to seek medical attention or to be labeled with the discharge code of ICD-8 Limitations in the data did not allow the authors to evaluate the extent of this bias.
Military Personnel. Kurland and colleagues examined data on GBS among military personnel but did not describe their methods in detail. Their analysis was based on hospitalization records of GBS cases diagnosed from to among active duty personnel serving in the U. Although it was not clear what sources were used to obtain the number of vaccinations administered and the number of active duty personnel serving in the military, the authors calculated that for this period an average of In , military personnel received only whole-virus vaccine.
The dose was twice the amount given to civilians. They calculated that The average number of cases in military personnel in any other 4-month period between and was The average annual rate of GBS in military personnel for the — period was 2. No statistical analysis of the data was discussed by the authors, and the committee notes possible information bias since it was unclear what data sources were used for the number of vaccinations administered or for the number of active duty personnel serving in the military and whether these data were validated.
A previous study had examined the incidence of GBS in U. Army active-duty personnel, but data for through were pooled and analyzed only by month of diagnosis, not by year of diagnosis or year of vaccination.