The COVID-19 vaccine developed by AstraZeneca and Oxford University was 79% effective in preventing symptomatic illness in a large trial in Chile, Peru, and the United States.
And was 100% effective against severe or critical disease and hospitalization.
The study included over 32,000 volunteers across all age groups, and the results could help it get emergency authorization from the U.S. Food and Drug Administration.
Oxford Professor Sarah Gilbert explains:
"This means that AstraZeneca can now go through the process of preparing a submission to the FDA using the data from this trial. That will take some time to achieve, possibly a number of weeks and that will then be reviewed by the FDA who will be able to make their decision on whether to license the vaccine for use in the U.S."
The data - which the company released on Monday (March 22) - should boost faith in the British shot after results from earlier, separate studies raised questions about the robustness of the data.
And allay safety concerns after some European countries briefly halted the use of the AstraZeneca vaccine earlier this month following reports it was linked to blood clots.
Many have since resumed vaccinations after a regional regulator said it was safe, but a survey on Monday showed Europeans remained skeptical.
Leaders in Asia have sought to boost public confidence by taking the AstraZeneca shot themselves.
The AstraZeneca shot was among the first and cheapest of the COVID-19 vaccines to be developed and launched on a large scale.
It is set to be the key vaccination programme in much of the developing world.