Incidence of ischemic stroke after COVID-19 bivalent booster vaccination in an integrated health system

We conducted a retrospective cohort study of Kaiser Permanente Northwest patients 18 years and older who were vaccinated with the COVID-19 bivalent vaccine between September 1, 2022 and March 1, 2023. We replicated the Vaccine Safety Datalink (VSD) rapid cycle analysis protocol to identify cases of ischemic stroke or transient ischemic attack (TIA) in the 21 days following vaccination using ICD-10-CM diagnosis codes in the primary position. The incidence of ischemic stroke or TIA was 34.3 per 100,000 (95 % CI, 17.7-59.9) in patients 65 years or older who received the bivalent Pfizer vaccine. Although a safety signal was detected in this study, further investigation is warranted to validate an association between COVID-19 vaccination and risk of ischemic stroke. Replication of the VSD case definition confirmed the exceptionally high positive predictive value in identifying ischemic stroke or TIA within 21 days of Pfizer bivalent vaccination in individuals 65 years and older. Two physician adjudication with chart review and confirmation of ischemic stroke cases allowed accurate absolute incidence estimates of stroke per 100,000 vaccine recipients and is helpful in calculation of net benefit for policy recommendations and shared decision-making.