Pandemic Influenza A H1N1 Vaccination and Subsequent Risk of Type 1 Diabetes in Norway
We included all residents in the Norwegian National Registry ages 30 years and younger per 1 October 2009. Dates of vaccination were obtained from the Norwegian Immunization Register, in which Pandemrix registration was mandatory. We identified newly onset type 1 diabetes during the period 1 October 2009 to 30 June 2014 from combining information on antidiabetic drugs dispensed from pharmacies in Norway from the Norwegian Prescription Database, specialist care diagnosis, from the Norwegian Patient Registry and primary care diagnoses from the reimbursement database (eFigure 1; http://links.lww.com/EDE/B264). These nationwide databases are independently reported and mandatory with a high level of completeness.
Information from the different sources was linked using the personal identification number assigned to all residents in Norway. Further details of the Norwegian health registries and the pandemic are given elsewhere.6
Type 1 diabetes was defined as registration of at least one type 1 diabetes diagnosis in primary or specialist health care combined with continuation with insulin for at least 6 months and no use of oral antidiabetic drugs within 12 months after diagnosis. To avoid any prevalent cases of diabetes at baseline, individuals who had a diagnosis of diabetes or use of any glucose-lowering medication between 2004 and 1 October 2009 were excluded. Immigrants were included only if they immigrated to Norway more than 1 year before 1 October 2009, to ensure that immigrants with prevalent diabetes did not appear as incident cases. We used Cox regression with time-dependent exposures to estimate hazard ratios (aHRs) adjusted for age, sex, place of birth, and highest level of education (in 2013) for the individual or his/her parents. The study was approved by the Regional Committee for Medical and Health Research Ethics and the Norwegian Data Protection Authority.
Of the 1,825,117 individuals in the study 40% were vaccinated with Pandemrix and 2098 were diagnosed with type 1 diabetes (incidence rate 26.5/100,000 person-years). Risk of type 1 diabetes was not increased after vaccination (aHR = 0.97; 95% CI = 0.89, 1.07), consistently throughout the follow-up period and after restricting the age group to <15 years (1,301 cases; aHR = 0.99; 95% CI = 0.88, 1.11; Figure).
We found no evidence of an association between vaccination with Pandemrix and risk of developing type 1 diabetes in this large, registry-based study. The study strengths include the combination of several high-quality registers and a large sample size. As in any nonrandomized study, we cannot exclude unmeasured confounding. Our results are not consistent with the suggestion of increased risk observed in Sweden after 2 years postvaccination follow-up until 2011.