More than 10,000 human subjects in the UK are being recruited for advanced studies into a potential Covid-19 vaccine.
Companies and academics around the world are racing to find medicines for the coronavirus.
The US has already pledged up to $1.2 billion to AstraZeneca to develop the vaccine it is trialling with Oxford University.
AstraZeneca said it planned to make as many as 30 million doses available in Britain from as early as September.
In the UK trial, adult subjects will be randomly selected to receive one or two doses of either the Oxford drug or an already licensed vaccine against meningitis for comparison.
Volunteers will record their reactions in an e-diary and attend follow-up visits. Some will be given swabs for taking samples at home.
A smaller part of the trial will expand the age range of testing to include children aged from 5 to 12 years old and adults 56 and older. The other, larger group will be studied to test the vaccine's effectiveness in volunteers 18 and older.
The proposed Covid-19 vaccine is made from a weakened version of a common cold virus that has been genetically changed so it is unable grow in humans.
More than 1,000 people have already received the vaccine in an early trial that began in April.
To start rebuilding our economy we need an effective vaccine and to understand how to best treat COVID-19. How soon will this happen? Watch @ITVTonight at 730pm to hear more our leading scientists on "The Race for a Cure"... https://t.co/0QDAYYNi8L
— University of Oxford (@UniofOxford) May 21, 2020
Trials on monkeys also appeared to show the vaccine offered some protection against Covid-19.
The monkeys were found to have less of the virus in their airways and lungs.
Despite positive signs, it is still too early to confirm if the vaccine is effective and safe.
