Caving under growing public criticism for failing to help India fight a devastating second Covid-19 wave, the United States rolled out a series of belated supportive measures, including sending over raw materials to make vaccines.

A White House readout of a phone call between US President Joe Biden and Indian Prime Minister Narendra Modi on Monday said Washington has begun providing emergency assistance, including oxygen-related supplies, vaccine materials and therapeutics.

The export of these materials, such as single-use bioreactor bags and filters, was severely curtailed after the Biden administration invoked powers under the Defence Production Act (DPA), a Korean War-era law, in January to ensure "a sustainable public health supply chain". It was done to bolster production of vaccines and other items such as testing kits and protective equipment in the US.

This had a crippling impact on Indian vaccine manufacturers, and Mr Adar Poonawalla, CEO of the Serum Institute of India (SII), urged Mr Biden to lift restrictions on production supplies.

"I humbly request you to lift the embargo of raw material exports out of the US so that vaccine production can ramp up," he tweeted on April 16. His firm is producing the AstraZeneca vaccine in India, under the Covishield brand name. It is one of the two vaccines being used in the country.

Besides helping SII ramp up production of Covishield by diverting its pending orders of vaccine supplies such as filters to India, the US also announced on Monday that it will share up to 60 million doses of its AstraZeneca vaccine with other countries as they become available. While the vaccine is being produced in the US, it has not been authorised for use in the country.

Less than 5 per cent of the Indian population has been fully vaccinated and a scarcity of shots at a time when the country has been battered by more than 300,000 cases daily had led to growing calls on the US government to relax its export curbs and even send its vaccines to India.

"The US has something like a vaccine glut. India is in dire straits. Please overturn this export ban ASAP," tweeted writer Salman Rushdie last Saturday.

Until last week, Washington was still defending its export restrictions, with State Department spokesman Ned Price saying "the United States first and foremost is engaged in an ambitious and effective and, so far, successful effort to vaccinate the American people".

The unprecedented rise in demand for Covid-19 vaccines has led to a global shortage of critical input materials. Vaccine makers have ramped up production capacity from zero to 10 billion annually, when the pre-pandemic global vaccine manufacturing capacity was just five billion, said Mr Thomas Cueni, director-general of the International Federation of Pharmaceutical Manufacturers and Associations (Ifpma).

A typical vaccine manufacturing plant uses around 9,000 different materials sourced from about 300 suppliers across some 30 countries, according to an Ifpma estimate. The absence of just one of these elements can have a crippling impact on production.

The SII did not respond to a request for comment.

Mr Rajinder Kumar Suri, CEO of the Developing Countries Vaccine Manufacturers Network, had told an online press conference on April 23 that most of the scarce critical materials come from the US and curbs under the DPA had proved to be a "real bottleneck".

"There is a complete chain. So if any one component is missing, or some of these components are missing, the entire chain comes to a grinding halt," he said. "If a company is manufacturing one billion doses a year… and you do not supply the material for one month, 100 million doses are gone and these 100 million doses can have a tremendous impact on maybe 100 countries," he added.

Meanwhile, as Indian vaccine manufacturers struggled to access materials from the US, Pfizer and Moderna, two American pharmaceutical companies, continued to benefit from a key raw material from India for their m-RNA Covid-19 vaccines.

VAV Lifesciences, a Mumbai-based firm, has been supplying phospholipids since December to a US-based contract development and manufacturing organisation that produces gene-based lipid nanoparticles used in the mRNA vaccines made by Pfizer-BioNTech and Moderna.

It is reportedly one of the world's only four companies producing phospholipids, which are crucial biomolecules used to manufacture the m-RNA-based Covid-19 vaccines. Producers of these vaccines have also been facing a scarcity of raw materials.

"We have been trying to ramp up production and try our best to meet the global demand, and that is such a struggle," Mr Arun Kedia, VAV Lifesciences' managing director, told The Straits Times. The company said it has increased production of its phospholipids by 10 times since June last year and expects it to go up another 10 times by December.