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Assessing individual risk v risk to public health?

The home for all non-political Coronavirus (Covid-19) discussions on The Lemon Fool
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This is the home for all non-political Coronavirus (Covid-19) discussions on The Lemon Fool
Clariman
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Assessing individual risk v risk to public health?

#424102

Postby Clariman » July 2nd, 2021, 4:59 am

Government policy and scientific advice on Covid quite correctly focuses on Public Health (balanced with economic/social impact etc.) rather than on individual health. In other words, it is about statistics and percentages. It is clear from a public health perspective, the vaccines have been very effective. However, how do we each assess our own individual risk as case numbers increase exponentially again?

As a starting point, when it is said that a vaccine has been shown to be n% effective, what does that actually mean?

  • n% of the population cannot catch Covid-19, but 100-n% can
  • n% of the population might catch it but will not be symptomatic, whereas 100-n% will be symptomatic
  • n% of the population will not be hospitalised, whereas 100-n% could be
  • n% of the population might catch it but will not die, whereas 100-n% could die
  • each individual is n% protected from Covid-19 so probably won't be too unwell if they do catch it
  • Something else - what?

With the positive news on vaccines, a Public Health plan to open up further makes sense, despite increasing case numbers. However if one has, for example, a 15% chance of still catching Covid and being very unwell and potentially dying, then high case number might lead one to make a personal decision to remain very cautious.

So, how does one assess individual risk? I don't see much clarity on that.

C

servodude
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Re: Assessing individual risk v risk to public health?

#424104

Postby servodude » July 2nd, 2021, 5:42 am

Clariman wrote:As a starting point, when it is said that a vaccine has been shown to be n% effective, what does that actually mean?


The clinical study efficacy reported is the ratio of symptomatic cases in the vaccinated and un-vaccinated cohorts in the test participants at the point where they have decided enough people have caught the bug

For equal cohorts it would be:
(unvax cases - vax cases) / total cases (expressed as a percentage normally)

if the cohorts aren't equal you adjust to get an "attack rate" but it is the same principle

An example would be:
- find 30000 people and vaccinate half of them
- wait until 200 symptomatic cases have been seen
if N is the number that are in the vaccinated group the efficacy is

( (200 - N) - N ) / 200

So if it were 95% effective it means you would be (ceterus paribus) 19 times more likely to catch it if you were unvaccinated
- where it is symptomatic disease

The figures/ratios thrown about other than the 3rd stage trial efficacy are inferred from the ongoing data monitoring by extracting out the vaxed vs not figures for various statistics (hospitalisations, serious disease metrics, deaths etc)
- and can normally be treated the same way; basically as an odds of that "condition" given your vax status
- with the caveat that oftentimes the denominators are more opaque than the strict methods used in the clinical testing (or at least need a bit of consideration)

- sd

Dod101
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Re: Assessing individual risk v risk to public health?

#424108

Postby Dod101 » July 2nd, 2021, 6:44 am

It is 6.30 am or so and my brain is maybe not as sharp at this hour than it might be so I hope that servotude's answer has answered Clariman's query. personally I need something in plain English for me to understand what is being said.

Question: How does one assess individual risk? As Clariman has said, I don't see much clarity on that.

Dod

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Re: Assessing individual risk v risk to public health?

#424140

Postby Gersemi » July 2nd, 2021, 9:26 am

I think the answer depends on the question, ie what is your risk of what? There are different percentages quoted depending on exactly what question you ask. As Servodude has said the main percentage quoted as around your risk of developing symptomatic illness compared to an unvaccinated person. If you want to know what your risk of developing Covid and ending up in hospital that is a different question and of course it also depends on the amount of Covid circulating, which is changing all the time.

The Zoe Covid symptom study published stats on 17 June whch showed at that point:

Current risk of new daily COVID infection

in the unvaccinated: 1 in 2,093
after 1 vaccine dose: 1 in 5,508
after 2 vaccine doses : 1 in 16,101

Based on people using the app to log symptoms and subsequently getting a PCR test when prompted to do so by the app.

https://covid.joinzoe.com/post/vaccines ... ching-peak

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Re: Assessing individual risk v risk to public health?

#424153

Postby murraypaul » July 2nd, 2021, 9:56 am

Dod101 wrote:Question: How does one assess individual risk? As Clariman has said, I don't see much clarity on that.


Well really only the individual can do that.
It is a balance of 'what is the risk', which others can help to put numbers on, vs 'what am I losing out on by continuing to be cautious', which is obviously very personal.
I'm still acting the way I was immediately at the start of lockdown, because it really isn't costing me very much, and is decreasing my personal risk, but also because of inertia.
I'd say that unless things get much worse in the next month or so, I'm going to start relaxing that and, for example, start going back to shops again.

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Re: Assessing individual risk v risk to public health?

#424159

Postby Dod101 » July 2nd, 2021, 10:10 am

murraypaul wrote:
Dod101 wrote:Question: How does one assess individual risk? As Clariman has said, I don't see much clarity on that.


Well really only the individual can do that.
It is a balance of 'what is the risk', which others can help to put numbers on, vs 'what am I losing out on by continuing to be cautious', which is obviously very personal.
I'm still acting the way I was immediately at the start of lockdown, because it really isn't costing me very much, and is decreasing my personal risk, but also because of inertia.
I'd say that unless things get much worse in the next month or so, I'm going to start relaxing that and, for example, start going back to shops again.


Personally I have never stopped going to the shops. In the last few weeks, I have been to several restaurants, stayed in a hotel twice, travelled to Northern Ireland once, stayed a couple of nights with an old friend (not in the style that Matt Hancock might do) and generally throughout have been cautious and careful with precautions, but I need to live, have had both vaccines and so far have avoided any symptoms. I appreciate that that does not mean I have been in the clear. On the basis of my experience I would say the risks of catching Covid are not that great and of ending up in hospital probably lower again.

Dod

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Re: Assessing individual risk v risk to public health?

#424199

Postby Julian » July 2nd, 2021, 11:50 am

I'm not sure you can assess individual risk at all accurately.

Servodude explained the numbers for the trials but the problem there is that different trials recruit different numbers of patients in different countries so have different profiles in terms of the mixture of variants around at the times the trials were done and there's no guarantee that the efficacy numbers against whatever severity you care about (e.g. if you only care about getting a severe case or above) are valid in the environment that you live in since the profile of the variants currently active in your country might be so different to the variant profile for the trial participants such that the efficacy numbers against severe disease don't reflect what will be seen in your environment. Yes, you might get some idea, but there is likely to be a very big margin for error if using the original phase 3 trial results to estimate your personal risk.

Because of the above, and because of the much higher sample sizes vs phase 3 trials, I now tend to look more at observational studies from PHE (https://www.gov.uk/government/publicati ... nce-report) to see efficacy against hospitalisation. I'd also like to see data on severe disease but that's much harder to collect for observational studies since for the data to be valid a precise definition of when the severe threshold is crossed and the level of monitoring isn't available in mass observations studies to make that determination so the mass observation reports only look at hospitalisations since that is easily collected data.

- Julian

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Re: Assessing individual risk v risk to public health?

#424223

Postby jfgw » July 2nd, 2021, 12:46 pm

Clariman wrote:As a starting point, when it is said that a vaccine has been shown to be n% effective, what does that actually mean?


It depends. Effective at what? The source of the data should tell you this.

If we assume that we are talking about a vaccine's effectiveness at preventing infection,

If a vaccine is 50% effective, 50% of the people who would have become infected don't.
If a vaccine is 85% effective, 85% of the people who would have become infected don't.
Etc.

From an individual perspective, all you can say is that, on average, you are x% less likely to become infected (which is not really an individual perspective).

This figure will not tell you whether:
The viral load needed to cause an infection is increased equally in everyone;
The vaccine is 100% effective in some people and 0% effective in everyone-else;
or something in-between.

It will be something in-between but a simple percentage will not tell you the details.


Julian F. G. W.

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Re: Assessing individual risk v risk to public health?

#424335

Postby 9873210 » July 3rd, 2021, 12:17 am

Clariman wrote:As a starting point, when it is said that a vaccine has been shown to be n% effective, what does that actually mean?

  • n% of the population cannot catch Covid-19, but 100-n% can
  • n% of the population might catch it but will not be symptomatic, whereas 100-n% will be symptomatic
  • n% of the population will not be hospitalised, whereas 100-n% could be
  • n% of the population might catch it but will not die, whereas 100-n% could die
  • each individual is n% protected from Covid-19 so probably won't be too unwell if they do catch it
  • Something else - what?


Any of the above and probably a few more*, depending on context. You have to read the footnotes. If there are no footnotes discount the report.

For the early vaccine trials the headline number mostly compared the symptomatic infection rate in the placebo cohort to symptomatic infection rate in the vaccinated cohort. If the infection rate in the vaccinated is one quarter the infection rate in the placebo group the vaccine is said to be 75% effective.

This was chosen mostly because it would generate numbers quickly and cheaply, thus speeding authorization to use the vaccine. Later vaccine trials might compare non-symptomatic rates, since tests are now more readily available. Many trials also reported secondary numbers for severe infections, hospitalizations or deaths. Although the small numbers of these outcomes usually give low statistical significance.

* a very important number is the fraction of exposed people who become infectious. Unfortunately this is hard to measure, so other numbers are used as a proxy.


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