The UK has become the first country to approve a dual vaccine which tackles both the original Covid virus and the newer Omicron variant.
Ministers say the vaccine will now form part of the autumn booster campaign.
Moderna thinks 13 million doses of its new vaccine will be available this year, but 26 million people are eligible for some form of booster.
Health officials say people should take whichever booster they are offered as all jabs provide protection.
The original vaccines used in the pandemic were designed to train the body to fight the first form of the virus which emerged in Wuhan, in China, at the end of 2019.
The Covid virus has since mutated substantially, with a stream of new variants emerging that can dodge some of our immune defences. They have caused large surges in cases around the world.
The original vaccines still provide strong protection against becoming severely ill or dying, but companies are tweaking them to match the virus as it evolves.