But manufacturing problems at a factory in Baltimore run by Emergent BioSolutions, Johnson & Johnson’s subcontractor, have had serious consequences for the vaccine. Because of a major production mishap that resulted in a two-month shutdown in operations, Johnson & Johnson has essentially been forced to sit out the brunt of the pandemic in the United States while Pfizer and Moderna, the other federally authorized vaccine makers, provided almost all the nation’s vaccine stock.

Johnson & Johnson has had to throw out the equivalent of 75 million doses, and the regulatory authorities in Canada, South Africa and the European Union also decided to pull back millions more doses made at the Baltimore plant. The company has been able to deliver only one-fourth of the 100 million doses it promised the federal government by the end of this month.

Dr. Anne Zink, Alaska’s chief medical officer, said that in her state, Johnson & Johnson’s shot had become a victim of its own timing. By late February, when it was authorized by the Food and Drug Administration, Alaska had figured out how to get two-dose vaccines to remote areas, leaving the one-shot regimen less crucial than she had initially imagined.

Dr. Clay Marsh, West Virginia’s Covid-19 czar, said that the pause and Johnson & Johnson’s later authorization — more than two months after Pfizer’s and Moderna’s — deprived it of a “halo effect.” By the time West Virginia had an ample supply of all three vaccines, he said, “people started to get this concept that maybe there’s something better about being immunized with Pfizer and Moderna.”

The Johnson & Johnson shot had also suffered from a “social network effect,” said Andrew C. Anderson, a professor of public health at Tulane University who researches vaccine hesitancy. Most Americans who were inoculated in the early months of the vaccine campaign received Moderna and Pfizer shots, and so their friends and family were less likely to deviate and accept a different brand.

In Louisiana, hospitals in the New Orleans area have started offering the Johnson & Johnson shot to people on their way out of the emergency room; the thinking is that people will be more likely to accept the vaccine when a doctor who has treated them asks them to take it. And in Arkansas, where only a third of the population is fully vaccinated, state officials are offering Johnson & Johnson doses to agriculture, manufacturing, wastewater and poultry workers, with gift certificates for hunting and fishing licenses as a reward.