(I’m straight up guessing, and would be keen for an answer from someone familiar with this kind of study)
This also confused me. Skimming the study, I think they’re calculating efficacy from something like how long it takes people to get malaria after the booster, which makes sense because you can get it more than once. Simplifying a lot (and still guessing), I think this means that if e.g. on average people get malaria once a week, and you reduce it to once every 10 weeks, you could say this has a 90% efficacy, even though if you looked at how many people in each group got it across a year, it would just be ‘everyone’ in both groups.
They’re using 1 minus the hazard ratio, the reduction in the proportion of not yet infected people who are infected at any given time. That is, an 80% efficacy would would that if x% of unvaccinated and as yet uninfected people were infected at some time then (1-0.8)x% of vaccinated and as yet uninfected people would be.
The advantage here is that the proportion of people infected would (unless your vaccine is perfect) eventually go to 100% in both groups, so how long you follow them up for will matter a lot.
(I’m straight up guessing, and would be keen for an answer from someone familiar with this kind of study)
This also confused me. Skimming the study, I think they’re calculating efficacy from something like how long it takes people to get malaria after the booster, which makes sense because you can get it more than once. Simplifying a lot (and still guessing), I think this means that if e.g. on average people get malaria once a week, and you reduce it to once every 10 weeks, you could say this has a 90% efficacy, even though if you looked at how many people in each group got it across a year, it would just be ‘everyone’ in both groups.
This graph seems to back this up:
https://www.thelancet.com/cms/attachment/2eddef00-409b-4ac2-bfea-21344b564686/gr2.jpg
They’re using 1 minus the hazard ratio, the reduction in the proportion of not yet infected people who are infected at any given time. That is, an 80% efficacy would would that if x% of unvaccinated and as yet uninfected people were infected at some time then (1-0.8)x% of vaccinated and as yet uninfected people would be.
The advantage here is that the proportion of people infected would (unless your vaccine is perfect) eventually go to 100% in both groups, so how long you follow them up for will matter a lot.
Nice, that helped clear this up for me!
I think there is a typo here:
Should say:
Right?
(else I’m still confused, heh.)
Yes, thank you for the correction