Cardiff is among 17 areas of the UK where participants are being sought to take part in the final-stage trials of a coronavirus vaccine.
The phase-three trial of the vaccine from pharmaceutical company Janssen, owned by Johnson and Johnson, starts on Monday and will be the first of its two-dose study.
The jab has already undergone phase one and two trials, and interim analysis of the single-dose study suggests the Covid-19 vaccine candidate induces a robust immune response and is generally well-tolerated.
For the two-dose study, researchers are first aiming to recruit around 6,000 UK participants, from a total of 30,000 people globally.
These include Cardiff, Southampton, Bristol, London, Leicester, Sheffield, Manchester, Dundee and Belfast.
Recruitment into the study will complete in March 2021 and the trial will last for 12 months.
The UK has secured 30 million doses of the Janssen vaccine if the trial is successful.
The Janssen vaccine will be the third to be trialled in the UK, along with the Oxford/AstraZeneca candidate, and one from US biotech company Novavax.
It comes a week after the Pfizer vaccine proved to be 90% effective in immunising against the virus, based on results from final "stage three" human trials.
So far around 25,000 people in the UK have participated in vaccine trials, and more than 310,000 have indicated their willingness to take part in clinical studies by signing up to the NHS vaccine research registry.
Professor Saul Faust, principal investigator of the Janssen trial in the UK, said: "We just don't know how each of these vaccines is going to behave and which are going to generate the better short and long-term immunity.
"And we can't be certain that vaccine supply will be efficient and effective and secure from any one manufacturer, wherever it's being made in the world."
Kate Bingham, chairwoman of the UK's Vaccine Taskforce, said: "Because we've got this national citizen registry of volunteers willing to go into clinical trials, it has accelerated our ability to enrol trials.
"And that means that the UK is a very favourable place to come in and run studies and so Novavax has expanded that study, and of course Janssen has come to the UK for the first, I think, of its two-dose study."
Prof Faust said there are 40,000 people on the NHS vaccine registry who are in the postcode areas around the vaccine centres for the Janssen study.
They are expected to be contacted in the second half of this week and invited to take part in the study.
Researchers said recent vaccine trials suggest it generally takes six to nine months to get an outcome, but it could be longer for trials starting out now.
Prof Faust added: "It is going to be some time before the bulk of the population receive a coronavirus vaccine just because of the technical difficulties of manufacturing, achieving the number of vaccines in the UK, and then deployment in that many people."
The researchers also stress that participating in a vaccine trial would not exclude anyone from routine immunisation later.