Even though it requires only one dose and is easier to store than Pfizer’s and Moderna’s shots, Johnson & Johnson’s has played only a minor role in the nation’s inoculation campaign. That is partly because the Baltimore plant that was supposed to supply most of the doses here has been shut down for three months because of regulatory violations. The factory, operated by Emergent BioSolutions, a subcontractor, has been forced to throw out the equivalent of 75 million doses because of suspected contamination, significantly delaying deliveries to the federal government.

At the same time, demand for the shot plummeted after the April safety pause. At that time, 15 women in United States and Europe who had received the Johnson & Johnson shot had been diagnosed with the clotting disorder. Three had died. The C.D.C. has now confirmed 38 cases of the disorder.

Regulators and federal health officials warned that the women younger than 50, in particular, should be aware of the “rare but increased” clotting risk. But in the nearly three months since they ended the pause, only about five million people in the United States have taken Johnson & Johnson’s shot, and state officials report that people are much more wary of it. Millions of doses that have been distributed by the federal government are sitting unused and will expire this summer.

Alex Gorsky, Johnson & Johnson’s chief executive, said last month that he was still hopeful that the vaccine, which has been used in 27 countries so far, would help contain the pandemic overseas. The company has promised up to 400 million doses to the African Union. Separately, Covax, the global vaccine-sharing program, is supposed to receive hundreds of millions of doses.

Studies have showed that the Johnson & Johnson shot protects people against more contagious virus variants, including the Delta variant, and is highly effective at preventing severe Covid-19, hospitalizations and death.

The F.D.A. shares jurisdiction over vaccines with the C.D.C., but is solely responsible for issuing product warnings. The Guillain-Barré cases will be discussed in an upcoming meeting of a committee of outside experts who advise the C.D.C., the agency said.

The F.D.A. has also attached a warning to the Pfizer-BioNTech and Moderna vaccines, but some government health officials described that as less serious than the warnings about Johnson & Johnson. Last month, the agency pointed to an increased risk of inflammation of the heart or the tissue surrounding it — diseases known as myocarditis and pericarditis — particularly among adolescents and young adults who had received Pfizer-BioNTech or Moderna shots.

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