Author Topic: Oxford Vaccine - What Does “70% Effective” Mean?  (Read 3091 times)

Oxford Vaccine - What Does “70% Effective” Mean?
« on: 03 January, 2021, 02:52:29 pm »
News reports (whether old or new media channels) typically tend to state that the Oxford vaccine is “70% effective” without ever explaining their terms. Does “70% effective” mean that 70% of recipients are rendered 100% immune (and presumably the other 30% of recipients are therefore not rendered immune)? Or that 100% of recipients are rendered 70% immune (compared presumably to some prior / base level of immunity)? Or something else entirely?
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PaulF

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Re: Oxford Vaccine - What Does “70% Effective” Mean?
« Reply #1 on: 03 January, 2021, 02:57:21 pm »
LMGTFY

Quote
Vaccine efficacy is the percentage reduction of disease in a vaccinated group of people compared to an unvaccinated group, using the most favorable conditions.[1] Vaccine efficacy was designed and calculated by Greenwood and Yule in 1915 for the cholera and typhoid vaccines. It is best measured using double-blind, randomized, clinical controlled trials, such that it is studied under “best case scenarios.”[2] Vaccine effectiveness differs from vaccine efficacy in that vaccine effectiveness shows how well a vaccine works when they are always used and in a bigger population whereas vaccine efficacy shows how well a vaccine works in certain, often controlled, conditions.[1] Vaccine efficacy studies are used to measure several possible outcomes such as disease attack rates, hospitalizations, medical visits, and costs.


https://en.m.wikipedia.org/wiki/Vaccine_efficacy

Re: Oxford Vaccine - What Does “70% Effective” Mean?
« Reply #2 on: 03 January, 2021, 03:17:17 pm »
On "How to Vaccinate the World" it was explained like this:

If a vaccine is described as 90% effective in a trial, that means that for every 9 people on the placebo that got the disease, 1 person who had the vaccine also got the disease.

That does not make sense to me. Using that definition, if 10 people get the disease and 5 are in each group it would be described as 50% efficacy, whereas in normal terms the vaccine has no effect so should be describes as 0% effective.

Perhaps they meant to say "for every 10 people".
Quote from: tiermat
that's not science, it's semantics.

Re: Oxford Vaccine - What Does “70% Effective” Mean?
« Reply #3 on: 03 January, 2021, 03:19:03 pm »
It means 70% fewer test subjects got ill compared to the control group.

Whether that's because it only works partially on everybody or it doesn't work at all on some people is a whole other question.

quixoticgeek

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Re: Oxford Vaccine - What Does “70% Effective” Mean?
« Reply #4 on: 03 January, 2021, 03:28:29 pm »
It means 70% fewer test subjects got ill compared to the control group.

Whether that's because it only works partially on everybody or it doesn't work at all on some people is a whole other question.

Does this mean I'm wrong in my understanding that if we vaccinate 100 people, 70 people actually get protection, and 30 nothing happens ?

Then comes the question how many people need to get the vaccine at a specific efficacy rate, for it to make a difference to the population as a whole...

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Re: Oxford Vaccine - What Does “70% Effective” Mean?
« Reply #5 on: 03 January, 2021, 04:03:45 pm »
It means 70% fewer test subjects got ill compared to the control group.

Whether that's because it only works partially on everybody or it doesn't work at all on some people is a whole other question.

Does this mean I'm wrong in my understanding that if we vaccinate 100 people, 70 people actually get protection, and 30 nothing happens ?

It doesn't work like that, it's a random process which is why you need population data. No-one is 100% protected (they can't bathe in virus soup and expect to be safe), and neither is anyone 0% protected (so that a single virus will definitely give them the disease).


Quote
Then comes the question how many people need to get the vaccine at a specific efficacy rate, for it to make a difference to the population as a whole...

J

That depends what you mean by "make a difference". For the million or so people already vaccinated, a number of people in that group would have died over the next year or so without it, so it's already made a difference. For herd immunity it depends on the infection rate for an unvaccinated population as well as the efficacy of the vaccine. Eg, measles is very contagious and needs 90-95% vaccination rate for herd immunity. Or perhaps to eradicate it? Probably more than are willing to have the vaccine, unfortunately.
Quote from: tiermat
that's not science, it's semantics.

Re: Oxford Vaccine - What Does “70% Effective” Mean?
« Reply #6 on: 03 January, 2021, 04:41:32 pm »
One expert today on BBC news explained that those who had the vaccine but still caught the virus, did not have as serious symptoms compared to those who hadn't had the vaccine, so it's not a simple binary result.

Re: Oxford Vaccine - What Does “70% Effective” Mean?
« Reply #7 on: 03 January, 2021, 04:52:20 pm »
Indeed, I believe not a single hospital admission for anyone that had the Oxford vaccine in the trial.

Unfortunately with the increased R for the new variant the % reqd for herd immunity is that bit higher and must be close to 75%. I don’t think that is likely any time soon.

Re: Oxford Vaccine - What Does “70% Effective” Mean?
« Reply #8 on: 03 January, 2021, 05:51:40 pm »
As the folks have explained, its a measure of effectiveness in a large group compared to those not given a placebo instead.

The other very misleading term that gets used is 'immune'. A vaccine doesn't make anyone immune. It primes your body to recognise the particular virus more quickly and to take the appropriate response. So if you have been vaccinated, the chances that the virus is killed off before it can start replicating or replicating at any significant level are greatly reduced. But your natural response is still in a contest, being vaccinated means its much quicker of the line. That contest can still be compromised by other health conditions, age and plain simple chance.

citoyen

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Re: Oxford Vaccine - What Does “70% Effective” Mean?
« Reply #9 on: 03 January, 2021, 11:36:08 pm »
The other very misleading term that gets used is 'immune'.

Working in health journalism, one of my pet hates is when writers refer to “immunity” when they mean “the immune system”. It’s lazy shorthand, and very misleading.
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simonp

Re: Oxford Vaccine - What Does “70% Effective” Mean?
« Reply #10 on: 04 January, 2021, 12:04:15 am »
Indeed, I believe not a single hospital admission for anyone that had the Oxford vaccine in the trial.

Unfortunately with the increased R for the new variant the % reqd for herd immunity is that bit higher and must be close to 75%. I don’t think that is likely any time soon.

If the vaccine is 70% effective, and the R number is 3, you need approx 95% vaccination to give an effective R of 1 with no other measures in place.

(You need 66% of the population to be immune, which is achieved with 94.2% vaccinated at 70% effectiveness).

Given that none of the vaccines are approved for children, 95% is not achievable. Even if it was, you'll be lucky to vaccinate more than 70%.


Jaded

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Re: Oxford Vaccine - What Does “70% Effective” Mean?
« Reply #11 on: 04 January, 2021, 12:08:58 am »
70% is good.

But not as good as Dettol, which kills 99%
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Re: Oxford Vaccine - What Does “70% Effective” Mean?
« Reply #12 on: 04 January, 2021, 12:36:16 am »
Indeed, I believe not a single hospital admission for anyone that had the Oxford vaccine in the trial.

Unfortunately with the increased R for the new variant the % reqd for herd immunity is that bit higher and must be close to 75%. I don’t think that is likely any time soon.

Quite possibly higher than 75%

Re: Oxford Vaccine - What Does “70% Effective” Mean?
« Reply #13 on: 04 January, 2021, 05:45:40 am »
Indeed, I believe not a single hospital admission for anyone that had the Oxford vaccine in the trial.

Unfortunately with the increased R for the new variant the % reqd for herd immunity is that bit higher and must be close to 75%. I don’t think that is likely any time soon.

Quite possibly higher than 75%
75% the corresponds to a basic R number of 4.

Re: Oxford Vaccine - What Does “70% Effective” Mean?
« Reply #14 on: 04 January, 2021, 06:23:53 am »
Indeed, I believe not a single hospital admission for anyone that had the Oxford vaccine in the trial.

Unfortunately with the increased R for the new variant the % reqd for herd immunity is that bit higher and must be close to 75%. I don’t think that is likely any time soon.

If the vaccine is 70% effective, and the R number is 3, you need approx 95% vaccination to give an effective R of 1 with no other measures in place.

(You need 66% of the population to be immune, which is achieved with 94.2% vaccinated at 70% effectiveness).

Given that none of the vaccines are approved for children, 95% is not achievable. Even if it was, you'll be lucky to vaccinate more than 70%.
Your figure is based on 0% of the population being excluded from infection by other means for example previous infection. I was just saying that if the basic R is for the new variant is 4 rather than 3 then 75% rather than 66% would need to be not susceptible whether through vaccination or other means. This is all very simplistic and no statistics on the effect of the vaccines on transmissibility is yet available. On a separate point the level of detail in epidemic modelling now is incredible. The imperial college model (and others I am sure) use census and other data to model down to individual household level.

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Re: Oxford Vaccine - What Does “70% Effective” Mean?
« Reply #15 on: 04 January, 2021, 07:56:05 am »
70% is good.

But not as good as Dettol, which kills 99%

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Oxford Vaccine - What Does “70% Effective” Mean?
« Reply #16 on: 04 January, 2021, 07:59:13 am »
In answer to the original question about 70% ....

There were 23,848 people involved in the trial with half getting a placebo.

131 got covid with symptoms

These 131 were split 101 unvaccinated control versus 30 vaccinated, so 70% less people got covid with symptoms than would be expected if it did nothing.

However there was effectively two trials because some got the wrong dose.

So overall it was 101 vs 30 - 70%
But wrong dose it was 30 vs 3 - 90%
Correct dose it was 71 vs 27 - 62%

The numbers of people taking part in the trial 23,848 is large and so the vaccine is very safe. The numbers getting infected were very small. They are large enough to see it does something very positive but I would not read too much into the efficacy rates. Just looking at the 30 vs 3, that could so easily been vs 2 or 4, based on how much someone coughed.

Edit: the other aspect is that within the control group there were the expected number of hospitalisations and fatalites. Within the vaccinated group nobody was severe enough to require hospital.

Re: Oxford Vaccine - What Does “70% Effective” Mean?
« Reply #17 on: 04 January, 2021, 09:37:39 am »
Interesting, that the number of viral particles in the vaccine full doses was approx twice the number of white blood cells we all have.

Re: Oxford Vaccine - What Does “70% Effective” Mean?
« Reply #18 on: 04 January, 2021, 10:14:07 am »
Indeed, I believe not a single hospital admission for anyone that had the Oxford vaccine in the trial.

Unfortunately with the increased R for the new variant the % reqd for herd immunity is that bit higher and must be close to 75%. I don’t think that is likely any time soon.

Quite possibly higher than 75%
75% the corresponds to a basic R number of 4.

I’m afraid so.

Re: Oxford Vaccine - What Does “70% Effective” Mean?
« Reply #19 on: 04 January, 2021, 10:34:23 am »
Indeed, I believe not a single hospital admission for anyone that had the Oxford vaccine in the trial.

Unfortunately with the increased R for the new variant the % reqd for herd immunity is that bit higher and must be close to 75%. I don’t think that is likely any time soon.

Quite possibly higher than 75%
75% the corresponds to a basic R number of 4.

I’m afraid so.
I had thought they were saying basic R had gone up by 0.4 - 0.7. It was the effective R had gone up by that amount, so it is much worse than I thought. Even more reason to get vaccinated.

Re: Oxford Vaccine - What Does “70% Effective” Mean?
« Reply #20 on: 04 January, 2021, 10:42:08 am »
Indeed, I believe not a single hospital admission for anyone that had the Oxford vaccine in the trial.

Unfortunately with the increased R for the new variant the % reqd for herd immunity is that bit higher and must be close to 75%. I don’t think that is likely any time soon.

Quite possibly higher than 75%
75% the corresponds to a basic R number of 4.

I’m afraid so.
I had thought they were saying basic R had gone up by 0.4 - 0.7. It was the effective R had gone up by that amount, so it is much worse than I thought. Even more reason to get vaccinated.

When a figure of 3 is bandied about, that’s the R0 number not the R number.  R0 is what you get with this virus in a population with little or no immunity.  But you only get that if you remove all the restrictions.  No intervention to social norms and way of doing things. So if life was continuing on as it had before the pandemic hit, you’d see R settle around 3.  There are some estimates R0 is as high as 6. Maybe it is for the new variant? So if you want to remove all restrictions you need that level of vaccination / immunity to keep it under control / but not fully eliminated.

For comparison the R0 number of measles is somewhere between 12-18.

Re: Oxford Vaccine - What Does “70% Effective” Mean?
« Reply #21 on: 04 January, 2021, 10:49:55 am »
I had thought they were saying basic R had gone up by 0.4 - 0.7. It was the effective R had gone up by that amount, so it is much worse than I thought. Even more reason to get vaccinated.

I don't really know how to interpret an increase in R specified as an absolute value. There's a big difference between, e.g., going from 1.0 to 1.4 compared to going from 3.0 to 3.4. Surely it should be a percentage?

Oxford Vaccine - What Does “70% Effective” Mean?
« Reply #22 on: 04 January, 2021, 10:51:33 am »
Indeed, I believe not a single hospital admission for anyone that had the Oxford vaccine in the trial.

Unfortunately with the increased R for the new variant the % reqd for herd immunity is that bit higher and must be close to 75%. I don’t think that is likely any time soon.

Quite possibly higher than 75%
75% the corresponds to a basic R number of 4.

I’m afraid so.
I had thought they were saying basic R had gone up by 0.4 - 0.7. It was the effective R had gone up by that amount, so it is much worse than I thought. Even more reason to get vaccinated.

When a figure of 3 is bandied about, that’s the R0 number not the R number.  R0 is what you get with this virus in a population with little or no immunity.  But you only get that if you remove all the restrictions.  No intervention to social norms and way of doing things. So if life was continuing on as it had before the pandemic hit, you’d see R settle around 3.  There are some estimates R0 is as high as 6. Maybe it is for the new variant? So if you want to remove all restrictions you need that level of vaccination / immunity to keep it under control / but not fully eliminated.

For comparison the R0 number of measles is somewhere between 12-18.
The basic R number,  R0 for the new variant in an environment as it was before covid would be expected to be somewhere between 4.4 and 5.8 based on the effective R increasing somewhere between 0.4-0.7 which is a factor of 140-180%. I had misread and thought the 0.4-0.7 was the change in basic reproduction number.

simonp

Re: Oxford Vaccine - What Does “70% Effective” Mean?
« Reply #23 on: 04 January, 2021, 10:52:33 am »
The transmissibility of the virus has gone up by around 50%, according to contact tracing data. Around 10% of contacts became infected with the old strain, and around 15% became infected if it's the new strain.

That would scale R0=2 to R0=3. Or R0 = 2.6 (a figure from the BMJ earlier in the pandemic) to R0 = 3.9.

If the Re was around 1 then at 50% would be consistent with a quoted 0.4 to 0.7 increase in Re, given a 50% increase in transmissibility.



Oxford Vaccine - What Does “70% Effective” Mean?
« Reply #24 on: 04 January, 2021, 11:06:31 am »
Sorry I was not saying an increase from 0.4 to 0.7. I was saying an increase in the range [0.4-0.7] Or if you prefer 0.55 give or take absolute increase to the current effective R

This was an increase of between 40% and 80%

If the basic R0 went up from 3.0 by a similar % it would be in range [4.2-5.4]