In their article, Sangli and colleagues described catastrophic thrombosis after the second dose of the SARS-CoV-2 messenger RNA–1273 vaccine from Moderna. The editorialists discuss vaccine-induced thrombosis and thrombocytopenia and thrombosis with thrombocytopenia syndrome and why this case cannot be definitively attributed to the vaccine. They also note that the remarkable speed with which clinicians and scientists have recognized this rare entity and developed evidence-based diagnosis and treatment guidelines should bolster public confidence in postlicensure vaccine safety monitoring.