(CNN) – Drugmaker AstraZeneca announced on Monday that its experimental coronavirus vaccine has shown an average efficacy of 70% in large-scale trials — the latest of several vaccine trials worldwide to post their results this month.
The vaccine, developed with the University of Oxford, showed 90% efficacy in one dosing regimen — when the vaccine was given as a half dose, followed by a full dose at least a month later — and 62% efficacy in a second regimen — when two full doses were given at least a month apart.
That averages to a 70% efficacy, AstraZeneca said.
It is not yet clear why the two dosages produced such different results.
More study participants were given the full dosage regimen, at 62% efficacy, while a smaller number of participants were administered the smaller half dose plus full dose combination at 90% efficacy, according to AstraZeneca’s announcement.
When people were given the smaller dose, the number of asymptomatic infections dropped, indicating a difference in transmission, Professor Andrew Pollard, the trial’s lead investigator at Oxford, said in a call with journalists on Monday.
“What we’ve always tried to do with a vaccine is fool the immune system into thinking that there’s a dangerous infection there that it needs to respond to — but doing it in a very safe way,” Pollard explained. “So we get the immune response and we get the immune memory … waiting and ready if the pathogen itself is then encountered.”
It may be that the best way of “kicking the immune system into action” is to give the body a small amount of the vaccine to begin with — and then follow up with a larger amount, but as the data on that method is preliminary, there is still more work to do, Pollard said.
AstraZeneca is the third drugmaker to unveil promising results in the fight against coronavirus this month, with Moderna announcing earlier in November that its vaccine was 94.5% effective against coronavirus, and Pfizer/BioNTech revealing that its vaccine was 95% effective.
Pollard addressed the efficacy differences between those other trials on Monday, explaining that AstraZeneca had used slightly different protocols to measure the disease that included all aspects of the disease — including mild cases — which are harder to predict.
He said that the results of the trials were “exciting” and demonstrated that they had an “effective vaccine that will save many lives.”
No participants who received the vaccine were hospitalized or reported suffering severe cases of the virus, the company said in a statement on Monday.
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Crucially, the AstraZeneca vaccine can be distributed and administered within existing healthcare systems, as it can be stored, transported and handled in normal refrigerated conditions (from 2-8 degrees Celsius, or 36-46 degrees Fahrenheit) for at least six months, the company said.
It will also be cheaper than rival coronavirus vaccines from makers Pfizer and Moderna. AstraZeneca has pledged to “ensure broad and equitable access to the vaccine at no profit for the duration of the pandemic.”
The company said it was making “rapid progress” in manufacturing, with a capacity of up to 3 billion doses of the vaccine in 2021 on a rolling basis, pending regulatory approval.
AstraZeneca said it would “immediately prepare regulatory submission of the data to authorities around the world that have a framework in place for conditional or early approval,” adding that it will seek emergency-use listing from the World Health Organization to accelerate vaccine availability in low-income countries.
Professor Peter Piot of the London School of Hygiene & Tropical Medicine told the Science Media Centre he was “very pleased” to see the the Oxford-AstraZeneca vaccine’s efficacy rates of 62% to 90%, depending on the doses used.
“The encouraging news is that greater protection at lower dose may mean that more people can be vaccinated with the same amount of vaccines, and that this vaccine can be stored easily at regular fridge temperature,” Piot said, noting that it would also help with equitable access to the vaccine globally.
