The Food and Drug Administration narrowed its use authorization for Johnson & Johnson’s single-dose COVID-19 vaccine due to risks of severe blood clotting, a move that comes months after an initial warning from health officials.

The vaccine, produced by J&J subsidiary Janssen, should only be administered when an mRNA vaccine from Pfizer-BioNTech or Moderna is unavailable due to the risk of severe blood clotting known as thrombosis with thrombocytopenia syndrome, or TTS, the FDA announced Thursday.

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