The company at the center of quality problems that led Johnson & Johnson to discard 15 million doses of its coronavirus vaccine has a string of citations from U.S. health officials for quality control problems. Emergent BioSolutions, a little-known company vital to the vaccine supply chain, was a key to Johnson & Johnson’s plan to deliver 100 million doses of its single-shot vaccine to the United States by the end of May. But the Food and Drug Administration repeatedly has cited Emergent for problems such as poorly trained employees, cracked vials and problems managing mold.