The FDA decision was based on a recommendation of the CDC Advisory Committee on Immunization Practices (ACIP) after it evaluated data related to a very rare event involving blood clots in combination with low platelet counts -- thrombosis with thrombocytopenia -- that had been observed within about one to two weeks after vaccination, J&J said.

The company has updated its emergency use authorization (EUA) fact sheets for the vaccine to include information about the diagnosis and treatment of thrombosis with thrombocytopenia. In addition, the CDC, FDA, and the American Society of Hematology have also published information on the proper recognition and management of this condition, as well as its unique treatment, according to J&J.

Recipients of the J&J COVID-19 vaccine should immediately contact their healthcare provider if they develop severe headache, chest pain, swelling in the leg, abdominal pain, tiny blood spots under the skin, or excessive bruising within two weeks after the vaccination, according to the company.