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Oxford/AstraZeneca COVID-19 Vaccine Efficacy Varies by Dose

Focus a vaccine vial
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Key Takeaways

  • Preliminary data from a trial examining a vaccine developed by the University of Oxford and AstraZeneca shows different efficacy based on two different doses.
  • The vaccine is based on a modified chimpanzee cold virus.
  • The vaccine can survive at refrigerated temperatures, unlike other vaccine candidates that require colder, freezing temperatures.

A COVID-19 vaccine created by researchers from the University of Oxford and pharmaceutical company AstraZeneca showed an average of 70% efficacy, according to an assessment of clinical trial data thus far.

Phase 3 trial data was published last week in The Lancet, and AstraZeneca announced findings in a press release on Monday.

The 70% efficacy figure represents an average between two groups that received two different doses of the vaccine. When researchers gave people two full doses of the vaccine in one month, they saw 62% efficacy. In a group that received a smaller dose first and the full dose during their second vaccination, recipients were 90% less likely to develop COVID-19.

“These numbers may change over time, because the vaccine could become less effective later on, but the same is the case for the Pfizer/Moderna vaccines,” Hildegund C.J. Ertl, MD, a vaccine development researcher with The Wistar Institute, tells Verywell.

The efficacy is significantly different between both groups, says Amira Roess, PhD, a professor of global health and epidemiology at George Mason University.

"At this stage we need to examine the data for these two different dosing schemes separately. Combining them in the analysis does not make sense," Roess tells Verywell.


Vaccine efficacy and vaccine effectiveness are two different things. Efficacy is a measurement made during the trial, while effectiveness refers to how the vaccine works once released. A vaccine’s efficacy rate is not necessarily indicative of an effectiveness rate once released.

An independent review confirmed that the assessment met the primary endpoint, in that it demonstrated protection from COVID-19 in 14 days or more after two doses were administered. No serious safety events were reported in relation to the vaccine, AstraZeneca said in its statement.

The vaccine was jointly developed by the University of Oxford along with its company, Vaccitech.

Preliminary data from the Pfizer and BioNTech vaccine saw 95% efficacy, while the Moderna vaccine had a 94.5% efficacy. Pfizer announced on November 20 that they would seek Emergency Use Authorization from the U.S. Food and Drug Administration (FDA), which would expedite the vaccine rollout process.

What This Means For You

Though the AstraZeneca vaccine trial has not yet wrapped up, this easier-to-store vaccine candidate may soon be another COVID-19 prevention method.

How Is the Oxford/AstraZeneca Vaccine Made?

The vaccine draws upon an adenovirus—a cold virus—derived from the stool of chimpanzees. The adenovirus has been genetically modified so it does not replicate in human cells. It directs human cells to generate the spike protein of SARS-CoV-2, which should cause the body to mount an immune response against the virus without actually contracting the virus.

The University of Oxford team says that chimpanzee adenoviral vectors have been well-studied, and make for a safer vaccine option for children, the elderly, and anyone with a pre-existing condition.

“This vaccine uses an entirely different vaccine platform technology and is much cheaper to produce,” Amesh Adalja, MD, a senior scholar at the Johns Hopkins Center for Health Security, tells Verywell, comparing it to vaccine candidates from Pfizer and Moderna. Instead of a weakened virus, both Pfizer's vaccine and Moderna's vaccine use mRNA technology.

The Associated Press (AP) stated that the vaccine does not have to be stored in freezers and can be kept at refrigerated temperatures, which could make it easier to distribute.

A Closer Look at Efficacy

An article in Nature examined theories for the varied response in efficacy between both doses. The higher efficacy was based on 2,741 participants and the lower dose was reported on data from 8,895 volunteers.

“Efficacy statistics from clinical trials often give you a ballpark estimate of where to expect the vaccine to ultimately land when used in normal situations," Adalja says. "It may be a little bit higher and it may be a little bit lower."

Ertl says that according to her knowledge, Oxford has asked regulatory agencies to switch to the more effective low-dose prime regimen, which thus far has comparable efficacy to the Pfizer and Moderna vaccines. She says she is partial to the Oxford/AstraZeneca vaccine, as preclinical experiments have shown that the “vaccine platform induces very long-lasting immune responses.”

According to Ertl, in the Oxford/AstraZeneca vaccine trial, all participants were tested for COVID-19 infection with nasal swabs; in the other trials, patients were only tested if they had symptoms. “So the actual infection rates may be higher in the Pfizer/Moderna trials than reported due to asymptomatic infections," she says.

Erika Schwartz, MD, an internist in New York City, tells Verywell she wants to see more information on the vaccines before recommending any of them. 

“The clarity and transparency of data released at this point is missing enough information [for providers] to make an informed decision for the wellbeing of the patient,” Schwartz says.

"I’m looking forward to seeing a comprehensive analysis of the data as well as the actual dataset," Roess says. "It is certainly promising that we have multiple vaccine candidates with high efficacy. What remains to be seen is whether the high efficacy translates into an equally impressive effectiveness once the vaccines are out in the real-world."

Will We Have a Choice of Vaccines?

Gretchen Chapman, PhD, a professor in the department of social and decision sciences at Carnegie Mellon University, tells Verywell that patients likely won’t have to make a decision as to which vaccine they receive.

“Because of logistics, patients may not have a choice among several vaccines. And if they do, there should be clear recommendations from health authorities about which vaccine is best suited for which type of individual," she says.

“Take any vaccine you can get,” Ertl says. “It will protect you from severe disease and death, and even if you have few side effects, that’s still far better than landing intubated the hospital."

Adalja thinks if there are multiple options available, what you get might depend on where you live.

“It may end up being whichever [vaccine] is allocated to a given state,” he says. “I think the key is to have approved vaccines that are effective, so I’m not really favoring or looking forward to one over another. I just want a vaccine approved."

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