Influenza vaccine and subsequent development of zoster

BACKGROUND: Herpes zoster (HZ), which is caused by reactivation of the latent varicella zoster virus, was not listed as a side effect of any vaccines until the introduction of coronavirus disease 2019 (COVID‐19) vaccine. This study used a nationwide population database to examine whether the HZ risk is increased after receiving the influenza vaccination. METHODS: This population‐based retrospective self‐controlled case series evaluated the association between influenza vaccine exposure and HZ risk. Data were collected from Taiwan's National Health Insurance Research Database between 2015 and 2017. Patients with HZ diagnosed within 6 months before and after receiving the influenza vaccination were included. After receiving the influenza vaccine, the first 15 and 30 days were defined as risk intervals, while the other periods were defined as control intervals. Poisson regression was used to compare the incidence rate ratio (IRR) for HZ during the risk interval vs. the control interval. RESULTS: In total, 13,728 patients were diagnosed with HZ before and after receiving the influenza vaccine. The IRR for days 1–15 was significantly higher (IRR = 1.11; 95% confidence interval [CI], 1.02–1.20), but insignificant for days 1–30 (IRR = 1.04; 95% CI, 0.98–1.10). In a subgroup analysis, the IRRs were significantly higher in participants, including 50–64 years old (1.16; 95% CI, 1.02–1.33), males (1.14; 95% CI, 1.01–1.28), and healthier individuals (i.e., no history of cancer or autoimmune diseases). CONCLUSIONS: There was a slight increase in risk of HZ in people receiving influenza vaccine in the first 1–15 days after vaccination.