Pharmaceutical giant AstraZeneca and the University of Oxford on Monday said that their newly developed Covid-19 vaccine is, on average, 70.4% effective - suggesting that it is not as powerful as two vaccines already announced by other firms.
The interim analysis of phase 3 clinical trials showed the results of two different dosing regimes.
A 90% effectivity rate was achieved when a half dose was given first, followed by a full dose of the vaccine at least a month later. A lower 62% figure came from patients given two full doses, at least a month apart.
The Anglo-Swedish drugmaker said that results showed the vaccine was "highly effective in preventing Covid-19," the disease caused by the new coronavirus, and that there were no hospitalizations or severe cases in anyone who received the experimental drug.
"Today marks an important milestone in our fight against the pandemic," AstraZeneca's chief executive Pascal Soriot said.
The international race to find a vaccine to counter coronavirus infections, and therefore stop the pandemic, has already seen two major competitors.
US pharmaceuticals giant Pfizer and its German partner BioNTech have announced that their drug is 95% effective, and that there were no safety concerns. The companies are already applying for an emergency use authorization from the US Food and Drug Administration (FDA).
Last week the US pharmaceutical firm Moderna announced its Covid-19 vaccine candidate was 94.5% effective.
Oxford University's Andrew Pollard said that "these findings show that we have an effective vaccine that will save many lives."
"Excitingly, we've found that one of our dosing regimens may be around 90 [per cent] effective and if this dosing regime is used, more people could be vaccinated with planned vaccine supply."
Phase 3 trials
The vaccine entered phase 3 trials in September. They were temporarily stopped because of health concerns with a participant in Britain, but have since been resumed.
Unlike the Moderna and Pfizer/BioNTech vaccines, the Astrazeneca drug does not use mRNA technology to fight the virus in the body. It is designed to stimulate the production of antibodies and T-cells, which then attack the virus.
The vaccine "contains the genetic material of the SARS-CoV-2 virus spike protein. After vaccination, the surface spike protein is produced, priming the immune system to attack the SARS-CoV-2 virus if it later infects the body."
Experts cautioned that it may be too early to conclude how effective the newly announced vaccine is, and that more data was required.
This data will be eagerly awaited by several countries that have pre-ordered millions of doses from AstraZeneca and Oxford. The European Union alone has ordered 300 million doses.
The EU, the United States, Britain, Japan and Brazil have all signed initial contracts with AstraZeneca for vaccine deliveries if the medication is approved by regulators.