(numbers from here and here, numbers inferred are marked with an asterisk)
Some interesting results from the latest vaccine trial. The treatment group was split in two, one of which received 2 full doses of the vaccine, the other received a half dose followed by a full dose (separated by a month in both cases).
In the control group there were 101 COVID infections in ~11,600* participants.
With 2 full doses there were 28* infections in 8,895 participants.
In the half first dose condition there were 2* infections in 2,741 participants.
So does having a low first dose actually improve the immune response?
The best I can figure out the evidence is 8:1 Bayes factor in favour of the “low first dose better” hypothesis vs the “It doesn’t matter either way” hypothesis.
Not sure what a reasonable prior is on this but I don’t think this puts the “low first dose is better” hypothesis as a strong winner on posterior.
On the other hand the evidence is 14:1 in favour of “it doesn’t matter either way” vs “both full doses is better” so at the moment it probably makes sense to give a half dose first and have more doses in total.
I’ll be interested as to what happens as the data matures—the above is apparently based on protection levels 2 weeks after receiving the second dose.
My own wild guess is that perhaps the two-full-dose protocol raised too many antibodies to the adenovirus vector itself, and made the second dose less effective. This has always been a concern with the viral-vector idea.
It sounds like there was a medium-sized prior that the lower first dose would be better—why else would they have tested it?
I was confused as to why they did this too—alternative guesses I had were to increase number of available doses or to decrease side effect severity.
However the site you link to has been updated with a link to Reuters who quote AstraZeneca saying it was an accident—they miscalculated and only noticed when side effects were smaller than predicted.
Oxford / AstraZeneca vaccine effectiveness
(numbers from here and here, numbers inferred are marked with an asterisk)
Some interesting results from the latest vaccine trial. The treatment group was split in two, one of which received 2 full doses of the vaccine, the other received a half dose followed by a full dose (separated by a month in both cases).
In the control group there were 101 COVID infections in ~11,600* participants.
With 2 full doses there were 28* infections in 8,895 participants.
In the half first dose condition there were 2* infections in 2,741 participants.
So does having a low first dose actually improve the immune response?
The best I can figure out the evidence is 8:1 Bayes factor in favour of the “low first dose better” hypothesis vs the “It doesn’t matter either way” hypothesis.
Not sure what a reasonable prior is on this but I don’t think this puts the “low first dose is better” hypothesis as a strong winner on posterior.
On the other hand the evidence is 14:1 in favour of “it doesn’t matter either way” vs “both full doses is better” so at the moment it probably makes sense to give a half dose first and have more doses in total.
I’ll be interested as to what happens as the data matures—the above is apparently based on protection levels 2 weeks after receiving the second dose.
Derek Lowe writes:
It sounds like there was a medium-sized prior that the lower first dose would be better—why else would they have tested it?
I was confused as to why they did this too—alternative guesses I had were to increase number of available doses or to decrease side effect severity.
However the site you link to has been updated with a link to Reuters who quote AstraZeneca saying it was an accident—they miscalculated and only noticed when side effects were smaller than predicted.
I guess the lesson being: Never attribute to n-dimensional chess that which can equally be attributed to stupidity.