A US study of AstraZeneca’s COVID-19 vaccine showed it was effective and did not increase the risk of rare blood clots reported in Europe, the company announced on Monday.
Although the jab has been authorised in more than 50 countries, it has not yet been given the green light in the US.
The study included more than 30,000 volunteers, of whom two-thirds were given the vaccine, while the rest got placebos.
AstraZeneca said in a statement its vaccine was 79% effective at preventing symptomatic COVID-19 and was 100% effective in stopping severe disease and hospitalisation, though it has not yet published full data.
Investigators said the vaccine worked across all ages, including older people — something experts wanted better data on.
Two doses of the AstraZeneca vaccine were given to people four weeks apart.
“These findings reconfirm previous results observed,” said Ann Falsey, of the University of Rochester School of Medicine, who helped lead the trial. “It’s exciting to see similar efficacy results in people over 65 for the first time.”
The AstraZeneca jab is a pillar of a UN-backed project known as COVAX that aims to get COVID-19 vaccines to poorer countries, and it has also become a key tool in European countries’ efforts to boost their sluggish vaccine rollouts.
The early findings from the US study are just one set of information AstraZeneca must submit to the Food and Drug Administration (FDA) to get its vaccine approved for use in the country.
An FDA advisory committee will publicly debate the evidence behind the shots before the agency decides whether to allow emergency use of the vaccine.
In the past, the time between a company revealing efficacy data and a jab being authorised in the US has been about a month.
Scientists had hoped the US study would clear up some of the confusion about just how well the shots really work, particularly in older people.
Previous research suggested the vaccine was effective in younger populations, but there was no solid data proving its efficacy in those over 65, who are often those most vulnerable to COVID-19.
Britain first authorised the vaccine based on partial results from testing in the United Kingdom, Brazil and South Africa, which suggested the shots were about 70% effective.
But those results were clouded by a manufacturing mistake that led some participants to get just a half dose in their first shot — an error the researchers didn’t immediately acknowledge.
Then came more questions about how well the vaccine protected older adults and how long to wait before the second dose.
Some European countries including Germany, France and Belgium initially withheld the vaccine from older adults and only reversed their decisions after new data suggested it was offering seniors protection.
AstraZeneca’s vaccine development was rocky in the US, too. Last autumn, the FDA suspended the company’s study for six weeks as frustrated regulators sought information about some neurologic complaints reported in Britain. Ultimately, there was no evidence the vaccine was to blame.
Last week, more than a dozen countries, mostly in Europe, temporarily suspended their use of the AstraZeneca vaccine after reports it was linked to rare blood clots — even as international health agencies insisted the benefits of the vaccine outweighed the risks.
On Thursday, the European Medicines Agency concluded after an investigation that the vaccine did not raise the overall risk of blood clots, but could not rule out that it was connected to two very rare types of clots. It recommended adding a warning about these cases to the vaccine's leaflet.
It's not unheard of for such rare problems to crop up as vaccines are rolled out since trials typically look at tens of thousands of people, and some issues are only seen once the jab is used in millions of people.
France, Germany, Italy and other countries subsequently resumed their use of the vaccines on Friday, with senior politicians rolling up their sleeves to show the vaccine was safe.
AstraZeneca said it would continue to analyse the US data before submitting it to the FDA in the coming weeks. It said the data would also soon be published in a peer-reviewed journal.
The AstraZeneca shot is what scientists call a “viral vector” vaccine. The doses are made with a harmless virus, a cold virus that normally infects chimpanzees, which acts as a Trojan horse to carry the coronavirus's spike protein’s genetic material into the body that in turn produces some harmless protein. This primes the immune system to fight if the real virus comes along.
Two other companies, Johnson & Johnson and China’s CanSino Biologics, make COVID-19 vaccines using the same technology but using different cold viruses.