Chickenpox and risk of stroke: a self-controlled case series analysis.

BACKGROUND: There is good evidence that respiratory and other infections that cause systemic inflammation can trigger strokes; however, the role of specific infections is unclear. Case reports have highlighted chickenpox as a possible risk factor for arterial ischemic stroke, particularly in children, but rigorous studies are needed to determine and quantify any increased risk. METHODS: We used anonymized electronic health records totaling >100 million person-years of observation from 4 UK primary care databases to identify individuals who had documented clinical chickenpox and a stroke or transient ischemic attack (TIA). Self-controlled case series methods were used to quantify any increased risk of first stroke or TIA in the 0-6 and 7-12 months following chickenpox compared to other observed time periods. We analyzed data within each database, and performed meta-analyses to obtain summary age-adjusted incidence ratios (IRs) separately for adults and children. RESULTS: Five hundred sixty eligible individuals (including 60 children) were identified who experienced chickenpox and a stroke or TIA during follow-up. Among children, there was a 4-fold increased risk of stroke in the 0-6 months after chickenpox (summary IR = 4.07; 95% confidence interval [CI], 1.96-8.45; I(2) = 0%). Among adults, there was a less marked increased risk with moderate between-database heterogeneity (random-effects summary IR = 2.13; 95% CI, 1.05-4.36; I(2) = 51%). There was no significant increased risk of stroke in the 7-12 months after chickenpox in children or adults, nor was there evidence of increased risk of TIA in either time period. CONCLUSIONS: Our study provides new evidence that children who experience chickenpox are at increased risk of stroke in the subsequent 6 months.