30,000 healthy adults are being recruited there.
The latest phase of the trial is looking at the safety and efficacy of the Oxford Vaccine in preventing coronavirus.
The US leg is being led by AstraZeneca and will use healthy volunteers who have stable underlying medical conditions, including HIV.
As well as in the UK, Oxford vaccine trials are already happening in Brazil and South Africa, and are due to start soon in Peru, Chile, Japan and Russia.
Early results in July showed the ChAdOx1 nCoV-19 vaccine generated robust immune responses against the virus.
“Oxford and AstraZeneca are collaborating with clinical partners around the world as part of a global clinical programme to trial the Oxford vaccine.
This latest trial in the US, led by AstraZeneca, will expand our safety database further and will help to provide the possibility of an earlier understanding of whether the vaccine can prevent infection.” said Professor Andrew Pollard, Director of the Oxford Vaccine Group.
The US trial is funded by the Biomedical Advanced Development Authority (BARDA).
It's recruiting 30,000 adults aged 18 years or over from diverse ethnic and geographic groups.
Participants are being randomised to receive two doses of either ChAdOx1 nCoV-19 or a saline control, four weeks apart, with twice as many participants receiving the potential vaccine than the saline control.
The trial is assessing efficacy and safety of the vaccine in all participants, and local and systemic reactions and immune responses will be assessed in 3,000 participants.
The Phase III clinical trial will enrol up to 50,000 participants globally.
Results from the late-stage trials are anticipated later this year, depending on the rate of infection within the clinical trial communities.