In utero exposure to the 1918 pandemic influenza in Denmark and risk of dementia

Abstract

Background: Substantial but inconclusive evidence suggests in utero exposure to influenza infection may be linked with Alzheimer′s disease. Objectives: We examined whether individuals exposed in utero to the 1918 influenza pandemic are at increased risk of dementia. Patients/Methods In this cohort study, surveillance data were used to identify months when influenza activity was at its peak during the pandemic. Using birth dates, exposed and unexposed individuals were identified based on whether they were in utero during ≥1 of the peak months. The outcome, any type of dementia, was identified in population‐based medical registries. Time and age at risk were restricted so exposed and unexposed had equal time at risk; diagnoses for dementia were assessed between ages 62 and 92, with a maximum of 30 years at risk. Poisson regression was used to estimate sex‐adjusted incidence rate ratios (IRRs). Results: We identified 106 479 exposed and 177 918 unexposed persons. Using the cumulative risk function, there were similar proportions of exposed and unexposed with a dementia diagnosis at 11.9% and 11.7%, respectively. Across all ages, the IRR for the association between in utero influenza exposure and any dementia was 1.01 (95% CI 0.99‐1.04); for Alzheimer's disease, it was 0.97 (0.93‐1.01). When stratified by age and sex, and when dementia type was examined, estimates of association were also null or close to null. Conclusions: Our study suggests there is likely not an association between in utero exposure to the 1918 influenza pandemic and dementia among those 62 and older.