Vaccine nationalism in countries including the U.S. and India is likely to derail efforts by the World Health Organization to deliver 2 billion doses to poorer and middle-income nations by the end of the year, according to the head of the world’s biggest vaccine maker.

Countries are holding tight to their supplies and restricting access to materials needed to make more, said Adar Poonawalla, chief executive officer of the Serum Institute of India Ltd. The company is responsible for providing more than half of the shots used so far in the WHO-backed COVAX program that aims to provide equitable vaccine access across the world.

Many manufacturers, including Serum, have already missed timelines and commitments, Poonawalla said in an interview with Bloomberg Live. It will take two to three months for shipments to COVAX to really pick up, and reaching the 2021 target of 2 billion doses will be challenging, he said, predicting it will "spill over by a few months.”