The UK has ordered an extra 40 million doses of a coronavirus vaccine from the French pharmaceutical company Valneva, that should become available later in the year and into 2022.
The government says it will give flexibility should people need revaccinating next winter or beyond.
The UK has secured 407 million doses of different coronavirus vaccines - more than enough for the entire population.
Valneva's jab is still being tested in trials.
Although those will take time to satisfy regulators before it can be rolled out, manufacturing at a site in West Lothian, Scotland, has already begun.
The site is already supporting 100 new highly-skilled local jobs for scientists and technicians.
The jab is expected to be given as two doses and contains a dead version of coronavirus that cannot cause disease, but should teach the body's immune system how to fight it.
The government said it signed a deal to almost double its initial order for 60 million doses.
Business Secretary Kwasi Kwarteng said: "This latest deal is yet another weapon in our national arsenal against this terrible disease, and will ensure we have sufficient supplies to protect the British public in 2021 and beyond.
"Backed with major investment from the UK government, Valneva's site in Scotland will be a vaccine production powerhouse, working flat out to ensure we can quickly deploy jabs across the UK if their candidate is approved, while supporting top quality, local jobs."
Vaccines Minister Nadhim Zahawi said any surplus vaccine could be distributed globally.
"If approved, Valneva's vaccine will not only help tackle Covid-19 here in the UK, but aid our mission to ensure there is a fair supply of vaccines across the globe.
"No-one is safe until the whole world is safe."