Why is Oxford’s half-dose coronavirus vaccine more effective than the full dose?

In reporting that its coronavirus vaccine reached up to 90% efficacy in clinical trials, AstraZeneca and Oxford University also noted that the jab was more effective when given in a half-dose regimen as opposed to a full. 

AstraZeneca elaborated on the findings in a press release posted Monday.

“One dosing regimen showed vaccine efficacy of 90% when AZD1222 [the vaccine] was given as a half dose, followed by a full dose at least one month apart, and another dosing regimen showed 62% efficacy when given as two full doses at least one month apart,” the press release stated.

“The combined analysis from both dosing regimens resulted in an average efficacy of 70%,” it said.


When pressed on the implications of these findings, an AstraZeneca spokesperson told Fox News that the company is still investigating. 

“We are excited to see 90% efficacy from the half dose: full-dose regimen,” Brendan McEvoy, AstraZeneca spokesperson, said. “We see a lot of merit in this regimen and we will now start discussions with regulators into incorporating this dose combination for further clinical investigation. We will continue to follow the science to better understand these data.”

Oxford University did not immediately return Fox News’ request for comment.

Findings from the phase 2 trial, published in The Lancet on November 19, included that the vaccine was safe, boosted immune response, and was better tolerated among older adults. The trial was designed to administer different doses to participants.

Interestingly, Pfizer and Moderna’s coronavirus vaccine candidates (which both use a new messenger RNA, or mRNA, platform) work in a dose-dependent manner, or in other words, neutralizing antibodies are ramped up after the second dose, “showing clear benefit of a 2-dose regimen,” per an earlier statement from Pfizer.

“Dose-dependent increases in immunogenicity were seen across the three dose levels, and between prime and boost within the 25 µg and 100 µg dose levels,” per Moderna’s phase 1 data.


The Oxford-AstraZeneca vaccine does not use mRNA technology and instead involves an inactivated common cold virus isolated from chimpanzees, altered with genes to express the spike protein of the SARS-CoV-2 virus.

While the Oxford-AstraZeneca team works to better understand the data, the Monday announcement disclosed that full results from the interim analysis were submitted for publication in a peer-reviewed journal.

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