The recent announcement of the Oxford’s and AstraZeneca’s vaccine trial being 70% effective up to 90% effective has raised some pointed questions.
The trial had two treatment arms, one receiving two full doses of the AZD1222 vaccine and a half dose plus a full dose, with the doses being administered 28 days apart. The “half dose then full dose” treatment arm reported the 90% protection. The problem was that the trial was never meant to have such an arm.
It was noticed that some participants were only receiving a half dose because they were experiencing fewer effects than expected such as arm pain and headache. This was subsequently corrected so that they would still receive the full dose on the second administration.
Of particular concern is that the “90% effectiveness” is based on a much smaller subset of the trial participants, with a correspondingly higher statistical uncertainty. So much so that there is statistical overlap with their lower effectiveness of 62% quoted for the two full doses. Furthermore, the participants were from the initial stages of the vaccine trial, where they were aged 18-55 and therefore have little applicability to the results of the main trial which included older age groups as well.
The details of exactly why the half-measure doses came to be administered in the first place have not been revealed by Oxford or AstraZeneca. Meanwhile in the US, a Phase III of the trial is being rolled out with 40 000 participants, and the “half dose then full dose” regimen may be included – however, uncertainty about it and whether it isn’t a statistical fluke will have to be cleared up first.
Source: Ars Technica
