WASHINGTON (AP) — U.S. regulators on Thursday strictly limited who can receive Johnson & Johnson’s COVID-19 vaccine due to the ongoing risk of rare but serious blood clots.

The Food and Drug Administration said the shot should only be given to adults who cannot receive a different vaccine or specifically request J&J’s vaccine. U.S. authorities for months have recommended that Americans starting their COVID-19 vaccinations use the Pfizer or Moderna shots instead.

FDA officials said in a statement that they decided to restrict J&J’s vaccine after taking another look at data on the risk of life-threatening blood clots within two weeks of vaccination.

J&J’s vaccine was initially considered an important tool in fighting the pandemic because it required only one shot. But the single-dose option proved less effective than two doses of the Pfizer and Moderna vaccines.

In December, the Centers for Disease Control and Prevention recommended prioritizing Moderna and Pfizer shots over J&J’s because of its safety issues. Previously, U.S. officials had treated all three vaccines similarly because they’d each been shown to offer strong protection.