Late on Thursday, the FTSE 100-listed firm signed a US$750mln deal with global vaccine organisations CEPI (part of the Bill & Melinda Gates Foundation) and Gavi to deliver 300mln doses at the end of the year.
It also reached a licensing agreement with the Serum Institute of India to supply 1bn doses for low-and-middle-income countries.
The pharma giant has agreed to manufacture and distribute the candidate developed by Oxford University, which is to release results on the second phase of trials by next month.
The first deliveries, announced last month, were scheduled to start in September to distribute jabs to the 30,000 participants of the final phase of clinical trials.
"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," chief executive Pascal Soriot told the BBC on Friday.
"Of course, with this decision comes a risk but it's a financial risk and that financial risk is the vaccine doesn't work… Then all the materials, all the vaccines, we've manufactured will be wasted."
Expenses for the vaccine, which will not be produced for a profit during the pandemic, are expected to be offset by funding by governments.
Shares were flat at 8,509p on Friday at noon.