UKH

We hear that a single dose of one (can’t remember which) of the vaccines gives 70% protection. I’m not really clear what this means. If I understand the science correctly (a big IF I grant you) vaccines work by stimulating the body’s immune system, creating an immune response. Also, I believe, every person has their own degree of response, depending on how well-functioning their immune system is. So does this statistic mean that everyone who has the jab has a 70% chance of eluding covid, or does it mean some who have a high functioning immune system might have a higher chance, perhaps even a 100% chance of doing so, while those with an impaired (or just average) immune system would have a less than 70% chance, and that the 70% figure is just an average chance?

I’m not really sure whether this makes sense to anyone else, or if I’m just displaying a failure to understand a simple concept.

In a large group, 70% fewer detected cases of Covid. An average from a large scale trial. Not studies of individuals.

For the individual impossible to know? I guess there must be many or most cases where several people get the same exposure but only some are infected and nobody really knows why.

while im not 100% certain on the specifics, generally when talking about vaccine effectiveness, 70% effective means that in 100 people who were given the vaccine, 30 still got the disease. it has nothing to do with how much more protected any given indivdual is, as that would be almost impossible to quantify... 

i had this argument with someone online who thought that their friend had a 90% chance of dying if they got covid because they were in an at risk group, despite the fact that they were otherwise young fit and healthy (aside from the underlying a risk factor). 90% of poeple who were admitted to hospital with covid and the underlying health factor were dying, but that in no way meant that every individual with the health issue had a 90% chance of death! (i have made that figure up, i cant remember the exact one).

I asked a similar question a couple of weeks ago, you might find it if you look back though the threads (probably off belay?). I got a number of useful replies, but probably don't have enough understanding to explain it without making mistakes! Lol

If you want you can read the full report here

https://www.thelancet.com/journals/lancet/article/PIIS0140-6736(20)32661-1/...

but to summarise when 131 of the participants had tested positive it was found that 33 had had the vaccine and 86 were in a control group.

Also of those 33 no one was seriously ill. Of the 86 who didn't have the vaccine (off the top of my head so might be slightly out) 1 died, 3 need ventilators and 10 were hospitalised.

I think this addresses you other point

We have reported immunogenicity data showing similar immune responses following vaccination with two doses of ChAdOx1 nCov-19 in older adults, including those older than 70 years of age, when compared with those younger than 55 years.

 As older age groups were recruited later than younger age groups, there has been less time for cases to accrue and as a result, efficacy data in these cohorts are currently limited by the small number of cases, but additional data will be available in future analyses.

the extra factor not included in your question and which is also very important is the effect of having people out there who do not transmit - as I understand it the efficacy of this is not fully established but as I do not believe it to be close to zero it will affect the reinfection rate exponent.  If it is reasonably high then once significant numbers of people have a vaccine then transmission should begin to fall at an increasing rate.  This will act a s aprotector of everyone from probability of infection

"70% effective means that in 100 people who were given the vaccine, 30 still got the disease"

This is wrong.

It's just 30% are still susceptible, can still get the disease, not got it. It's compared to the placebo group that matters. 

Short answer: Not as much as you think it means

Longer answer: Read up on efficacy vs effectiveness. Different things.

The numbers you hear are derived from the clinical trials. You take the number of people who caught covid in the vaccine arm and the number who caught covid in the control arm in the carefully controlled clinical trial setting, do some stats that make it less than straightforward to follow, and then come up with a number. That number will change when it's used at a population level. And there's some doubt (is doubt the right word? Lack of clarity maybe) over whether people who caught covid in the first few days after being vaccinated are counted. Maybe that'll come out in the publications but information is scant and rumour isn't.

Even shorter answer: It means they're plenty good enough

In reply to geode:

Yep, sorry should check stuff rather than write stuff off the top of my head. The 86 and 33 were from different parts of the trials (they do seemed to have created confusion by the way the trials were set up). 

I should have used 101 and 30.

https://www.linkedin.com/pulse/weighting-waiting-stephen-senn

The calculations used are in this, and it shows efficacy.

Re effectiveness v efficacy. It's basically idealized trial (efficacy) world v real world (effectiveness). That's my understanding anyway.

Thanks. I suppose we will find out fairly soon, how effective it is.