Soriot said it would probably be another "international study, but this one could be faster because we know the efficacy is high so we need a smaller number of patients".
The news comes as AstraZeneca faces questions about its success rate that some experts say could hinder its chances of getting speedy U.S. and EU regulatory approval.
Several scientists have raised doubts about the robustness of results released on Monday showing the experimental vaccine was 90% effective in a sub-group of trial participants who, by error initially, received a half dose followed by a full dose.
Soriot said he did not expect the additional trial to delay UK and European regulatory approvals.
Clearance from the U.S. Food and Drug Administration (FDA) may take longer though because the agency is unlikely to approve the vaccine based on studies carried out elsewhere, especially given the questions over the results, he said.
Authorisation in some countries is still expected before the end of the year, he added.
AstraZeneca research chief Mene Pangalos told Reuters on Monday that researchers had stumbled upon the half-dose regime by accident, saying a sub-group of the trial was given a smaller initial dose by mistake.
Earlier he had said that the firm would start discussions with the FDA to change the design of its experimental COVID-19 vaccine trial to add the more-effective dosage regime.
Business Standard has always strived hard to provide up-to-date information and commentary on developments that are of interest to you and have wider political and economic implications for the country and the world. Your encouragement and constant feedback on how to improve our offering have only made our resolve and commitment to these ideals stronger. Even during these difficult times arising out of Covid-19, we continue to remain committed to keeping you informed and updated with credible news, authoritative views and incisive commentary on topical issues of relevance.
We, however, have a request.
As we battle the economic impact of the pandemic, we need your support even more, so that we can continue to offer you more quality content. Our subscription model has seen an encouraging response from many of you, who have subscribed to our online content. More subscription to our online content can only help us achieve the goals of offering you even better and more relevant content. We believe in free, fair and credible journalism. Your support through more subscriptions can help us practise the journalism to which we are committed.
Support quality journalism and subscribe to Business Standard.