AstraZeneca and the University of Oxford have said their jointly developed vaccine against the coronavirus has shown "an average efficacy of 70%" in late-stage trials.
However, efficacy rose to 90% in a group that first received half of the standard dose, and then a full dose, according to the biopharmaceutical firm's data.
"This vaccine's efficacy and safety confirm that it will be highly effective against COVID-19 and will have an immediate impact on this public health emergency," AstraZeneca's chief executive Pascal Soriot said in a statement on Monday.
Read more: Can we trust recent COVID vaccine successes?
Later, Soriot told a briefing that the lower first dose of its experimental vaccine was "a really big plus" as it meant "being able to vaccinate more people, faster."
The vaccine was most effective when it was administered as a half dose followed by a full dose at least one month apart. Surprisingly, the group that received two full doses showed only 62% efficacy.
British Prime Minister Boris Johnson greeted the news with enthusiasm, tweeting: "Incredibly exciting news the Oxford vaccine has proved so effective in trials. There are still further safety checks ahead, but these are fantastic results."
AstraZeneca's proposed vaccine produced less effective results when compared with others produced by rivals BioNTech/Pfizer and Moderna, which were shown to be over 90% effective.
Nevertheless, the preliminary trial results still represent a fresh breakthrough in the fight against a virus that has killed nearly 1.4 million people, brought global travel to a standstill and devastated the world's economy.
"The announcement today takes us another step closer to the time when we can use vaccines to bring an end to the devastation caused by SARS-CoV-2," said Sarah Gilbert, professor of vaccinology at the University of Oxford.
"We will continue to work to provide the detailed information to regulators. It has been a privilege to be part of this multinational effort, which will reap benefits for the whole world," she added.
jsi/rt (Reuters, dpa, AFP, AP)