The odds ratios for all nine vaccines were nonsignificant using either a Mantel Haenszel analysis stratified by center, or a logistic regression analysis with adjustment for center, age group, breast feeding, birth weight, maternal age, jaundice at birth, asthma, and vitamin D supplementation. There was no evidence, the authors concluded, that vaccinations increase the risk of type 1 diabetes. The study may have been compromised by ascertainment bias. About 75% of responders had validated vaccination records available. Validation was based on either a review of official records or on parental recall of exact vaccination dates, even if the investigator did not see a record. The latter may have contributed to imprecision in assigning vaccine status.

Finland. Karvonen and colleagues (1999a) studied the relationship between multiple vaccines and type 1 diabetes by examining the effect of adding Hib vaccine to the routine childhood immunization schedule. Incidence of type 1 diabetes was compared in cohorts of Finnish children born before or after a Hib vaccine efficacy trial and followed for 10 years. One cohort of 128,936 children was born between October 1983 and September 1985, prior to the Hib vaccine trial and thus was not exposed to the vaccine. Children born between October 1985 and August 1987 participated in the Hib vaccine efficacy trial. These children were divided into two cohorts: 59,238 children who were born on odd days were vaccinated with Hib at 3, 4, 6, and 14 to 18 months; 57,114 children born on even days were vaccinated at 24 months only. All children were assumed to have received BCG, diphtheria-tetanus-pertussis, polio, and measles-mumps-rubella vaccines. Newly diagnosed cases of diabetes among all three cohorts were ascertained from a national hospital discharge registry (1983–1986) or a nationwide prospective childhood diabetes registry (1987–1997).

There was no significant difference in the risk of diabetes by age 10 between the children who did not receive the Hib vaccine and children who were vaccinated at 24 months of age. Similarly, no difference in risk was found between the children first vaccinated at 3 months of age and those vaccinated at 24 months. For each of the comparisons, the relative risk was near 1.0. The authors concluded that neither the addition of Hib vaccine to the immunization schedule nor the timing of Hib vaccine increased the risk of type 1 diabetes in children. Estimates of both vaccine exposure and diabetes cases were based on aggregate data from three cohorts and from the population as a whole. Thus, interpreting the results at the level of the individual is difficult.

Sweden. Heijbel and colleagues (1997) examined the effect of pertussis vaccination in infancy on the risk of developing type 1 diabetes. Cumulative incidence of type 1 diabetes at ages 0 to 12 years was compared in cohorts of children born before or after the pertussis vaccine was removed from the routine immunization schedule in Sweden. Specifically, cohorts of children born in 1977 (96,057 children) and in 1978 (93,248 children) received pertussis vaccine

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