GENEVA — The U.N.-backed program to ship COVID-19 vaccines worldwide has announced supply delays involving a key Indian manufacturer, a major setback for the ambitious rollout aimed at helping low- and middle-income countries vaccinate their populations and fight the pandemic.

Gavi, the Vaccine Alliance, and its partners said Thursday that the Serum Institute of India, a pivotal vaccine maker behind the COVAX program, will face increasing domestic demands as coronavirus infections surge.

"Delays in securing supplies of SII-produced COVID-19 vaccine doses are due to the increased demand for COVID-19 vaccines in India," Gavi said.

The move will affect up to 40 million doses of the Oxford University-AstraZeneca vaccines being manufactured by the Serum Institute that were to be delivered for COVAX this month, as well as 50 million expected next month. 

COVAX, an initiative devised to give countries access to coronavirus vaccines regardless of their wealth, has so far shipped vaccines to some 50 countries and territories. 

The Serum Institute of Indian has been contracted to supply vaccines to 64 countries, and Gavi said the U.N.-backed program has "notified all affected economies of potential delays."

Gavi said the Serum Institute has pledged that "alongside supplying India, it will prioritize the COVAX multilateral solution for equitable distribution."

Gavi, which runs COVAX jointly with the World Health Organization and the Coalition for Epidemic Preparedness Innovations, has distributed 31 million doses of the AstraZeneca vaccine — 28 million from the Serum Institute and another 3 million from a South Korean contractor also producing it.

The program had been aiming to deliver some 237 million AstraZeneca vaccines through the end of May. A Gavi spokesman said the delays were not expected to affect the overall goal of shipping some 2 billion doses worldwide through COVAX by the end of the year.

U.N. officials, governments, advocacy groups and others in recent months have pleaded with manufacturers to do more to speed up and broaden production of COVID-19 vaccines and ensure fair distribution — insisting that the pandemic can only be defeated if everyone is safe from it.

The Serum Institute of India, also known as SII, is the world's largest maker of vaccines. Unlike many other manufacturers, it pledged to prioritize making shots for COVAX. 

India's foreign minister, S. Jaishankar, tweeted a photograph Thursday afternoon of vaccines received by South Sudan, although there have been growing concerns that vaccine exports from India have dwindled in the past week.

India has consistently maintained that it would try to export vaccines to as many countries as possible, but with the caveat that supplies would based on availability and the requirements of India's own immunization program.

India's need for vaccines is set to increase significantly beginning April 1, when it plans to start vaccinating everyone over 45, age groups that account for 88% of all virus-related deaths in India.

The expanded vaccinations coincide with a sharp spike in COVID-19 infections and concerns about more contagious variants circulating in the country. India reported over 50,000 new confirmed cases on Thursday, the highest daily number so far this year,