On February 27, 2021, the Food and Drug Administration (FDA) issued an Emergency Use Authorization (EUA) for the adenovirus-vectored COVID-19 vaccine (Janssen Biotech, Inc., a Janssen Pharmaceutical company, Johnson & Johnson), and on February 28, 2021, the Advisory Committee on Immunization Practices (ACIP) issued an interim recommendation for its use as a single-dose primary vaccination in persons aged ≥18 years (1,2). On April 13, 2021, CDC and FDA recommended a pause in the use of Janssen COVID-19 vaccine after reports of thrombosis with thrombocytopenia syndrome (TTS), a rare condition characterized by low platelets and thrombosis, including at unusual sites such as the cerebral venous sinus (cerebral venous sinus thrombosis [CVST]), after receipt of the vaccine.* ACIP rapidly convened two emergency meetings to review reported cases of TTS, and 10 days after the pause commenced, ACIP reaffirmed its interim recommendation for use of the Janssen COVID-19 vaccine in persons aged ≥18 years, but included a warning regarding rare clotting events after vaccination, primarily among women aged 18–49 years (3). In July, after review of an updated benefit-risk assessment accounting for risks of Guillain-Barré syndrome (GBS) and TTS, ACIP concluded that benefits of vaccination with Janssen COVID-19 vaccine outweighed risks. Through ongoing safety surveillance and review of reports from the Vaccine Adverse Event Reporting System (VAERS), additional cases of TTS after receipt of Janssen COVID-19 vaccine, including deaths, were identified. On December 16, 2021, ACIP held an emergency meeting to review updated data on TTS and an updated benefit-risk assessment. At that meeting, ACIP made a recommendation for preferential use of mRNA COVID-19 vaccines over the Janssen COVID-19 vaccine, including both primary and booster doses administered to prevent COVID-19, for all persons aged ≥18 years. The Janssen COVID-19 vaccine may be considered in some situations, including for persons with a contraindication to receipt of mRNA COVID-19 vaccines. Since June 2020, ACIP has convened 23 public meetings to review data on the epidemiology of COVID-19 and the use of COVID-19 vaccines, including nine during which Janssen COVID-19 vaccine-related data were reviewed. The ACIP COVID-19 Vaccines Work Group, comprising experts in infectious diseases, vaccinology, vaccine safety, public health, and ethics, has held weekly meetings to review COVID-19 surveillance data, evidence for vaccine efficacy and safety, and implementation considerations for COVID-19 vaccines. In addition, the COVID-19 Vaccines Safety Technical Work Group (VaST), consisting of independent vaccine safety experts and established to provide expert consultation on COVID-19 vaccine safety issues, has reviewed safety data from the COVID-19 vaccination program during weekly meetings. After TTS was first identified in the United States in April 2021, a benefit-risk assessment for the use of the Janssen COVID-19 vaccine was presented to ACIP using an adapted Evidence to Recommendations (EtR) framework.† In the setting of limited COVID-19 vaccine supply in the United States at that time, ACIP reaffirmed its interim recommendations for the use of the Janssen COVID-19 vaccine in persons aged ≥18 years under FDA’s EUA, which was updated to include a warning that rare clotting events might occur after vaccination, primarily among women aged 18–49 years (3). Updates to the benefit-risk assessment were also reviewed by ACIP in June 2021, after an increased risk for myocarditis, particularly in males aged 12–29 years, was observed after receipt of mRNA COVID-19 vaccines; and again, in July 2021, after an increased number of cases of GBS were identified following administration of Janssen COVID-19 vaccine (4,5). After each review, ACIP determined that the benefits of COVID-19 vaccination in preventing COVID-19 morbidity and associated mortality outweighed the risks for these rare, but serious adverse events; however, the balance of benefits and risks varied by age and sex. Ongoing postauthorization safety surveillance identified additional TTS cases and associated deaths after Janssen COVID-19 vaccination, and updated safety data were reviewed by VaST in December 2021. The COVID-19 Vaccines Work Group also reviewed an updated benefit-risk assessment of COVID-19 vaccines in the setting of new safety findings and sufficient COVID-19 vaccine supply in the United States. In addition, FDA updated the EUA fact sheets with additional TTS data in December 2021.§ A summary of the data reviewed and discussions from both VaST and the ACIP COVID-19 Vaccines Work Group were presented to ACIP during their emergency meeting on December 16, 2021.