The Oxford-AstraZeneca Covid-19 vaccine showed 66.7% efficacy against symptomatic disease starting two weeks after the second shot, according to a preprint posted Tuesday by researchers at the University of Oxford.
The new analysis adds new trial sites and a month of new data to the mix, building upon earlier results announced by AstraZeneca that its vaccine had showed an estimated 70.4% efficacy.
However, the latest analysis also suggests the vaccine may offer substantial protection after a single shot.
The study estimates 76% efficacy up to three months following one dose. This is based on a subset of 88 symptomatic infections, split unevenly between the vaccine and placebo groups between 22 and 90 days after vaccination. The study also found relatively stable levels of antibodies during this time frame, “with minimal waning by day 90.”
Furthermore, the authors suggest there could be higher efficacy with more spaced-out doses. Among adults 18 to 55, vaccine efficacy appeared to rise when the time between shots was spaced out from less than six weeks to more than 12. However, more information is needed to know how statistically different that finding is.
Taken together, the findings may bolster the UK’s decision to recommend spacing out doses up to 12 weeks apart, according to a statement from the chief investigator of the Oxford Vaccine Trial, Andrew Pollard.
Some context: AstraZeneca announced last month it had completed enrollment in its Phase 3 trial in the United States, which will serve as “the primary basis” for the company’s eventual application to the US Food and Drug Administration.
The vaccine has already been authorized in a number countries such as the UK and India, but authorization may not come in the US until late March at the earliest, according to Operation Warp Speed’s Moncef Slaoui.