A single-shot vaccine developed by the healthcare giant Johnson & Johnson is up to 85% effective in preventing the most serious coronavirus symptoms, according to data from mass trials.
The company said its latest tests - carried out in several countries - also showed it was 66% effective at preventing moderate to severe illness.
There was some geographic variation, with it being 72% effective against moderate to severe COVID-19 in the US, compared to 57% in South Africa, where it was up against a more transmissible strain.
Unlike other COVID-19 jabs, which require two doses several weeks apart, J&J's is administered in one injection.
As well as being one shot, it also only needs to be kept at fridge temperature, making its storage, distribution and handling much easier.
And costing as little as $10 a dose, it is cheaper than most of the other coronavirus vaccines - coming in at about the same as two shots of the Oxford/AstraZeneca option.
The mostly positive interim data for the J&J jab will be submitted to regulators and trials will continue, too - with information being fed back as part of a rolling review.
Thirty million doses have been ordered by the UK government, pending approval by the Medicines and Healthcare products Regulatory Agency (MHRA).
But there could be another vaccine given the all-clear by the MHRA first - one from Novavax.
Results were released last night from late-stage trials suggesting it was 89% effective in preventing coronavirus.
The prime minister has said the Novavax jab is now going to be assessed by the MHRA.
If approved, the vaccine would start to be rolled out in the second half of 2021. It would follow the Oxford/AstraZeneca, Pfizer/BioNTech and Moderna jabs.
The UK has already ordered 60 million doses, which are going to be manufactured in Stockton-on-Tees.
Meanwhile, the EU regulators are expected to grant restricted approval for the Oxford/AstraZeneca vaccine, which has been the focus of an increasingly bitter dispute in Brussels over supplies.
EU chief Ursula von der Leyen has insisted a contract between Brussels and AstraZeneca contained binding orders.
The contract between the EU and the pharmaceutical firm was published by the bloc on Friday, amid the escalating war of words.