Once viewed as a convenient, one-dose alternative to the multi-shot regimens from Pfizer and Moderna, J&J’s shot has instead been marred by safety concerns that have curtailed its use in the U.S. and elsewhere.
Even before the vaccine was launched in the U.S., regulators were already keeping an eye on a rare syndrome characterized by an unusual combination of blood clots and low platelet counts. The condition, known as thrombosis with thrombocytopenia syndrome, or TTS, was first identified in people who’d received a vaccine developed by AstraZeneca and the University of Oxford that uses similar technology. Though very rare, the side effect had led to the deaths of a number of individuals.
