Volunteers from around the UK are being recruited to take part in a new study for an Omicron booster jab.
Led by vaccine giants Moderna, the study will include around 3,000 participants.
The jab, which is one of the world's first to specifically tackle the Omicron variant, will be trialed at a number of locations throughout the UK.
Research - led by a team based at St George's, University of London - will be conducted through a partnership between Moderna and the National Institute for Health Research (NIHR).
The study will see half of the volunteers receive the new Omicron jab, with the other half injected with the regular Moderna booster.
Levels of Covid immunity drop several months after a booster dose is administered.
This means that clinical trials are needed to determine whether people may require a fourth dose.
The new trial will also seek to recruit people who have had just two doses of a vaccine and have not yet had a booster.
A separate COV-Boost sub-study will also run to compare using Moderna's Omicron variant vaccine as a fourth dose, with a standard dose of the Pfizer/BioNTech jab.
Health and Social Care Secretary, Sajid Javid, said: "The UK is a world leader when it comes to the research and development of vaccines and medicines, bolstered by our renowned life sciences industry.
"It's fantastic to see these capabilities being put to good use, with almost 3,000 people expected to take part in this important clinical trial.
"I want this country to be the best place in the world to launch clinical trials.
"I urge anyone eligible to take part in this vital research and play their part in protecting the country for years to come as we learn to live with Covid-19."
Stephane Bancel, chief executive officer of Moderna, said: "The UK and NIHR have been pioneering in their work to study vaccines and therapeutics throughout the global pandemic and have built up world-class clinical research capabilities.
"This is the first Moderna-sponsored Phase 3 study to be conducted outside of the US with our Omicron-specific booster candidate and we appreciate the collaboration with the NIHR.
"We thank the clinical trial teams and the participants in the study for helping to advance our understanding of this booster candidate."
Volunteers will need to have not tested positive for Covid-19 since the beginning of November 2021, and to have had their last vaccine at least three months prior to joining the study, which is recruiting for the next four weeks.
The study will take place at up to 29 research sites across Scotland, England and Wales, with the trial lasting up to 13 months.
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