The committee found five controlled studies (Blom et al., 1991; DeStefano et al., 2001; EURODIAB, 2000; Heijbel et al., 1997; Karvonen et al., 1999a) (Table 3) and three ecological studies (Classen, 1996; Hiltunen et al., 1999; Hyoty et al., 1993) that studied this relationship. The studies looked at the effects of the addition of one vaccine to an existing immunization schedule, of one vaccine consisting of antigens from more than one infectious agent or strain of virus (e.g., DTP, OPV, or MMR), or of several vaccines received at the same time. Despite these variations, the overall findings from the studies consistently demonstrated no effect of multiple immunizations on the incidence of type 1 diabetes. Therefore, the committee concludes that the epidemiological and clinical evidence favors rejection of a causal relationship between multiple immunizations and an increased risk of type 1 diabetes.
Anderson and colleagues (2001) examined the relationship between immunization and allergic disease by comparing trends in immunization rates with the prevalence of allergic disease symptoms. Allergy data, specifically for asthma, allergic rhinoconjunctivitis, and atopic eczema, were obtained for children ages 6 to 7 years and 13 to 14 years from centers participating in the International Study of Asthma and Allergies in Childhood (ISAAC). Those data were compared with national and local immunization rates for BCG, DTP, and measles.
For children age 6 to 7 years, allergy data were obtained from 91 centers, with a median of 2,996 children per center (range 1,104–6,533). Local immunization rates were available for 57 centers. Allergy data for children age 13 to 14 years were available from 154 centers with a median of 2,064 children (range = 1,046–11,400); 92 centers had local immunization data.
In the 13- to 14-year-old age group, the authors observed a significant negative association (rank correlations, p<.05, adjusted for socioeconomic factors) between local DTP rates and wheezing (-0.53, 95% CI, -1.49, 0.43), rhinoconjunctivitis (-0.60, -1.02, -0.19), and atopic eczema (-0.27, -0.76, 0.21). For measles vaccine, significant negative correlations were found for rhinoconjunctivitis (-0.47, -.98, 0.04) and atopic eczema (-0.42, -0.98, 0.13). No associations were observed for children age 6 to 7 years. The authors concluded that DTP vaccines are not risk factors for allergic disease at the population level. However, the authors noted that because immunization data were available only at the population level, associations at the individual level could not be excluded.