NEW DELHI — Adar Poonawalla made big promises. The 40-year-old chief of the world’s largest vaccine maker pledged to take a leading role in the global effort to inoculate the poor against Covid-19. His India-based empire signed deals worth hundreds of millions of dollars to make and export doses to suffering countries.

Those promises have fallen apart. India, engulfed in a coronavirus second wave, is laying claim to his vaccines. Other countries and aid groups are now racing to find scarce doses elsewhere.

At home, politicians and the public have castigated Mr. Poonawalla and his company, the Serum Institute of India, for raising prices mid-pandemic. Serum has suffered production problems that have kept it from expanding output at a time when India needs every dose. He has come under criticism for departing to London amid the crisis, though he said it was only a quick trip. He told a British newspaper he had received threats from politicians and some of India’s “most powerful men,” demanding that he supply them with vaccines. When he returns to India, he will travel with government-assigned armed guards.

In an interview with The New York Times, Mr. Poonawalla defended his company and its ambitions. He had no choice but to hand over vaccines to the government, he said. He cited a lack of raw materials, which he has partially blamed on the United States. Making vaccines, he said, is a painstaking process that requires investment and major risks. He said he would return to India when he had finished his business in London. He shrugged off his earlier comments about threats, saying they were “nothing we can’t handle.”