AstraZeneca and the University of Oxford on Sunday began trials to test a modified vaccine against the Beta variant of Covid-19, which was first reported in South Africa.
The booster shot trial will involve about 2,250 people from Britain, South Africa, Brazil and Poland.
They will include people who have been fully vaccinated with two doses of the original Oxford-AstraZeneca vaccine or a messenger-RNA vaccine such as that made by Pfizer and BioNTech.
There will also be people who have not been inoculated against Covid-19.
The new vaccine, known as AZD2816, has been designed using the same base as the main AstraZeneca shot but with minor genetic alterations to the spike protein based on the Beta variant.
"Testing booster doses of existing vaccines and new variant vaccines is important to ensure we are best prepared to stay ahead of the pandemic coronavirus, should their use be needed," said Andrew Pollard, chief investigator and director of the Oxford Vaccine Group at the University of Oxford.
Authorities in the UK have carried out successful vaccination programmes but experts do not know how long protection lasts.
"This study will provide vital evidence on whether further doses, including tweaks against new virus variants, may be needed in the future," said Maheshi Ramasamy, principal investigator at the Oxford Vaccine Group.
Initial data from the trial is expected this year.
Current vaccines are believed to be less effective against the Beta variant, although it is the Delta variant, first recorded in India, which is causing most concern.
Although the main vaccines are believed to be highly effective at preventing serious illness, the highly transmissible Delta variant is causing a sharp increase in cases in countries including Britain.