Drug company AstraZeneca is to start producing a potential vaccine for coronavirus, its boss has told the BBC.
Trials of the drug are under way but Pascal Soriot said the firm must start making doses now so that it can meet demand if the vaccine proves effective.
"We are starting to manufacture this vaccine right now - and we have to have it ready to be used by the time we have the results," he said.
AstraZeneca says it will be able supply two billion doses of the vaccine.
Speaking to the BBC's Today programme, Mr Soriot said manufacturing was beginning already because, "we want to be as fast as possible".
"Of course, with this decision comes a risk but it's a financial risk and that financial risk is the vaccine doesn't work," he added.
"Then all the materials, all the vaccines, we've manufactured will be wasted."
He said AstraZeneca would not seek to make a profit from producing the drug during the pandemic.
If it works, the company will be able to produce two billion doses after signing two new contracts on Thursday, one of which was with billionaire philanthropist Bill Gates.
AstraZeneca, which is developing the vaccine with scientists at Oxford University, has agreed to supply half of the doses to low and middle-income countries.
One of the new partnerships is with the Serum Institute of India (SII), the world's largest manufacturer of vaccines by volume. The other is a $750m (£595m) deal with two health organisations backed by Bill and Melinda Gates.
The two charities, the Coalition for Epidemic Preparedness Innovations (CEPI) and GAVI vaccines alliance, will help find production facilities to produce and distribute 300 million doses of the vaccine. Delivery is expected to start by the end of the year.
Mr Soriot has said he expects to know by August if the AZD1222 vaccine is effective, while CEPI chief executive Richard Hatchett said there is still a possibility the vaccine may not work.
AstraZeneca's licensing agreement with India's SII is to supply one billion doses for low and middle-income countries, with a commitment to provide 400 million before the end of 2020.
Mr Soirot said the company was building a number of supply chains across the world "to support global access at no profit during the pandemic and has so far secured manufacturing capacity for two billion doses of the vaccine".
"Having a vaccine is one thing but you need to produce it at scale and I can tell you that It is not an easy thing to do," the pharmaceutical boss told Today.
He has described the coronavirus pandemic as "a global tragedy" and "a challenge for all of humanity".
AstraZeneca has already agreed to supply 300 million doses of the potential vaccine to the US and a further 100 million to the UK, with the first deliveries expected in September.
Governments around the world have pledged billions of dollars for a Covid-19 vaccine and a number of pharmaceutical firms are in a race to develop and test potential drugs.
"A vaccine must be seen as a global public good - a people's vaccine, which a growing number of world leaders are calling for," United Nations Secretary General Antonio Guterres said in a video message on Thursday.