AbstractImportancePost-authorization monitoring of vaccines in a large population can detect rare adverse events not identified in clinical trials including Guillain-Barré syndrome (GBS). GBS has a background rate of 1-2 per 100,000 person-years.ObjectiveTo 1) describe cases and incidence of GBS following COVID-19 vaccination, and 2) assess the risk of GBS after vaccination for Ad.26.COV2.S (Janssen) and mRNA vaccines.DesignInterim analysis of surveillance data from the Vaccine Safety Datalink.SettingEight participating integrated healthcare systems in the United States.Participants10,158,003 individuals aged ≥12 years.ExposuresReceipt of Ad.26.COV2.S, BNT162b2 (Pfizer-BioNTech), or mRNA-1273 (Moderna) COVID-19 vaccine.Main Outcomes and MeasuresGBS with symptom onset in the 1-84 days after vaccination as confirmed by medical record review and adjudication. Descriptive characteristics of confirmed cases, GBS incidence rates during postvaccination risk intervals after each type of vaccine compared to the background rate, rate ratios (RRs) comparing GBS incidence in the 1-21 vs. 22-42 days postvaccination, and RRs directly comparing risk of GBS after Ad.26.COV2.S vs. mRNA vaccination, using Poisson regression adjusted for age, sex, race/ethnicity, site, and calendar day.ResultsFrom December 13, 2020 through November 13, 2021, 14,723,318 doses of COVID-19 vaccines were administered, including 467,126 Ad.26.COV2.S, 8,573,823 BNT162b2, and 5,682,369 mRNA-1273 doses. Eleven cases of GBS after Ad.26.COV2.S were confirmed. The unadjusted incidence rate of confirmed cases of GBS per 100,000 person-years in the 1-21 days after Ad.26.COV2.S was 34.6 (95% confidence interval [CI]: 15.8-65.7), significantly higher than the background rate, and the adjusted RR in the 1-21 vs. 22-42 days following Ad.26.COV2.S was 6.03 (95% CI: 0.79-147.79). Thirty-four cases of GBS after mRNA vaccines were confirmed. The unadjusted incidence rate of confirmed cases per 100,000 person-years in the 1-21 days after mRNA vaccines was 1.4 (95% CI: 0.7-2.5) and the adjusted RR in the 1-21 vs. 22-42 days following mRNA vaccines was 0.56 (95% CI: 0.21-1.48). In a head-to-head comparison of Ad.26.COV2.S vs. mRNA vaccines, the adjusted RR was 20.56 (95% CI: 6.94-64.66).Conclusions and RelevanceIn this interim analysis of surveillance data of COVID-19 vaccines, the incidence of GBS was elevated after Ad.26.COV2.S. Surveillance is ongoing.