Assessment of pre-specified adverse events following varicella vaccine: A population-based self-controlled risk interval study

Abstract Background Clinical trials and spontaneous reporting systems have revealed rare but biologically plausible adverse events following varicella immunization. Few post-marketing controlled studies have been conducted to assess the relationship between the varicella vaccine and these outcomes. Objectives To evaluate the risk of pneumonia, idiopathic thrombocytopenic purpura (ITP), meningitis, encephalitis and ischemic stroke following varicella immunization. Materials and methods This nationwide observational study was based on Taiwan National Health Insurance data and National Immunization Information System from 2004 through 2014. Primary analysis included children aged 12–35 months who received the single varicella vaccine on the date of administration. The self-controlled risk interval design compared the incidence of pre-specified outcomes during a risk interval of 1–42 days post-vaccination and a control interval of 43–84 days. The outcomes of interest were defined as admitted pneumonia, ITP, meningitis, encephalitis, and ischemic stroke, as well as fracture as a negative control. Conditional Poisson regression was used to assess the incidence rate ratio (aIRR) with adjustments for age and seasonal effects. Results Among 1,194,189 children, who receiving the varicella vaccine, there was no observed increase in the risk for ITP (aIRR 1.00; 95% CI, 0.76–1.33), meningitis (aIRR 1.21; 95% CI, 0.49–2.95), encephalitis (aIRR 1.00; 95% CI, 0.62–1.60), or ischemic stroke (aIRR 1.24; 95% CI, 0.31–4.95). A clustering feature with pneumonia occurred during days 36–42 post-vaccination (aIRR 1.10; 95% CI, 1.02–1.18). An increase in the risk for ITP was observed in children receiving the varicella and MMR vaccines concomitantly (aIRR 1.70; 95% CI, 1.19–2.43), but not among those receiving the varicella vaccine only. Conclusions We detected a small risk of incidental pneumonia associated with varicella vaccine in the 6th week after immunization. There was no increase in the risk of other pre-specified adverse events.