The Serum Institute of India (SII) isn't a household name, but it's the world's largest vaccine maker.

The firm churns out 1.5 billion doses every year from the company's vast manufacturing plant in Pune, Western India.

It is currently making Covid vaccines under license for pharmaceutical firms such as AstraZeneca.

"We took a huge calculated risk", by betting on several vaccines in 2020 before regulators had even approved of them, SII's chief executive Adar Poonawalla told the BBC.

"It wasn't a blind risk, because we knew the Oxford scientists from our earlier collaboration with the malaria vaccine."

SII is privately owned, which enabled fast decision-making between Mr Poonawalla and his scientists.

But funding proved a challenge. The firm invested around $260m (£186.7m) and raised the rest from philanthropists, such as Bill Gates, and advances from other countries.

SII managed to secure $800m by May 2020 to make multiple Covid vaccines.

Stashing away doses

How did SII actually scale up production? In April 2020, Mr Poonawalla calculated what they would need, from vials and filters.

"I got 600 million doses worth of glass vials ahead of time and locked it in my warehouse by September," he explained.

"The most important part that enabled us to have so many doses - 70-80 million in January - was because I started manufacturing at risk in August."

"I wish other companies also had taken that risk, because the world would have had many more doses."

Mr Poonawalla criticised the patchwork of global regulatory systems and lack of harmonisation for production delays.

He said the major regulators, including the UK's Medicines and Healthcare products Regulatory Agency (MHRA), the European Medicines Agency (EMA) and the US Food and Drug Administration (FDA), could have united and agreed a quality standard.

He also criticised national governments, claiming that regulators in the countries that are making the vaccines, from India to Europe, could have united to agree a standard international benchmark.

New variants

Mr Poonawalla played down concerns about new variants: "Anyone who has taken that [Oxford AstraZeneca] vaccine so far hasn't had to go to hospital or go on a ventilator and had their life at risk.

"They've also passed that disease on to others. So yes, it's not an ideal situation, but it has protected your life."

In India, SII is also involved the world's largest inoculation programme, to vaccinate 300 million by August. But, according to Bloomberg, only 56% of people eligible to get a shot have actually stepped forward.

"A lot of vaccine hesitancy traditionally has come about when either celebrities or non-experts have said vaccines are not safe," said Mr Poonawalla.

"I always just request celebrities and others who have this tremendous power on the social networks, to just be a bit responsible and read up on the facts before they say anything."