The trial sub-group that gave the 90% efficacy read-out comprised 2,741 volunteers, a fraction of the tens of thousands in trials that resulted in the above-90% efficacy data released earlier this month for Pfizer-BioNTech's and Moderna's vaccines.
"Sub-group analyses in randomised controlled trials are always fraught with difficulties," said Paul Hunter, a professor of medicine at Britain's University of East Anglia.
"In order to have faith in the results," Hunter added, any sub-group analysis have large numbers of volunteers.
Shares in AstraZeneca were down 0.9% at around 1310 GMT. They have fallen about 7% since it reported the vaccine data on Monday.
Vaccine optimism helped buoy the stock to record peaks this year, making the drugmaker the most valuable listed British company, but its shares have lost over 17% since late July and it has fallen behind Unilever and Shell.
Moderna shares have rallied 22% since it released its trial data on Nov. 16, while Pfizer and BioNTech are up 6% and 14% respectively since announcing data on Nov. 9.
A peer-reviewed analysis of data from the AstraZeneca-Oxford trial will be published in The Lancet in coming weeks.
The U.S. Food and Drug Administration (FDA), has not commented on the trial results. The European Medicines Agency said on Thursday it would "assess data on the efficacy and safety of the vaccine in the coming weeks".
Moncef Slaoui, scientific adviser for the U.S. government's vaccine programme Operation Warp Speed, has noted that no-one in the sub-group that got the initial half dose was older than 55 - suggesting that regimen's efficacy in older people was unproven.
"There are a number of variables that we need to understand, and what has been the role of each one of them in achieving the difference in efficacy," Slaoui told a briefing on Tuesday.
