Herd Immunity

[Tabitha8]

A simple title for what I think is probably a complex subject.

If we have herd immunity to an illness as a result of vaccinating our children, how is that maintained given that we don't vaccinate ourselves, the grandparents, our neighbours, etc?

Seeker: I know how you feel. There is, however, a logic behind this dispute and it all lies in the definition of three terms: herd immunity, herd immunity effect and herd immunity threshold.

People use herd immunity inappropriately for ease or through misunderstanding. It simply means 'proportion of people vaccinated'

When a particular proportion of people are vaccinated (the threshold value) the disease should die out rather than propogate in a population thus effecting some level of protection to the unimmune. It is this magic number governements/health authorities are always trying to hit.

Bm denies there is a protective effect (herd immunity effect) to the unimmune below threshold values. e.g. if we vaccinate at levels 5% less than threshold the protective effect to unimmune is zero. We think this is because she wants to assert that in diseases where we've not reached threshold, vaccinating does not protect, to any extent, the unimmune.

Thank you. I understand now. I was feeling blinded by anti-science.

Oh? I thought herd immunity referred to the phenomena of what happens to a disease when we reach that number
Not an arbitrary term for levels of protection under that number?
But I guess it's just semantics isn't it?

Math and LeBFG:

""The substantial majority of the cases are explained by this waning immunity," said Dr. William Schaffner, an infectious-disease specialist at Vanderbilt University.

You can argue with Dr Schaffner if you don't believe it.

It's fairly obvious that if immunity from the vaccine wanes then you are left with more susceptibles in the population so the disease will continue to spread. That is the problem they are encountering at the moment and is the reason they are recommending more boosters.

Yes, most people will use the term 'herd immunity' to mean that and many links posted on the thread support that usage although it can also be used to mean the proportion of a population who are immune which is where the confusion has come in. While it is possible to argue that 'a little bit of the population are immune' it is not possible to argue that you can have 'a little bit' of herd immunity in the other usage because, as many links and diagrams have pointed out, a significant proportion of the population need to be immune for that to exist.

Math and LeBFG:

""The substantial majority of the cases are explained by this waning immunity," said Dr. William Schaffner, an infectious-disease specialist at Vanderbilt University.

You can argue with Dr Schaffner if you don't believe it.

It's fairly obvious that if immunity from the vaccine wanes then you are left with more susceptibles in the population so the disease will continue to spread. That is the problem they are encountering at the moment and is the reason they are recommending more boosters.

Yes, most people will use the term 'herd immunity' to mean that and many links posted on the thread support that usage although it can also be used to mean the proportion of a population who are immune which is where the confusion has come in. While it is possible to argue that 'a little bit of the population are immune' it is not possible to argue that you can have 'a little bit' of herd immunity with the other usage because, as many links and diagrams have pointed out, a significant proportion of the population need to be immune for that to exist.

Sorry for double post - phone issue!

I have asked several times on the thread for people to clarify what they are talking about but they still keep jumping between the definitions so I'm not surprised that people are confused.

bm, epidemiological models are not like formulas in geometry or mechanics. They do not produce answers which are right or wrong. They produce estimates which are good or bad. The model you are using to claim my illustrative figure is wrong, is like a road map which only shows the motorway network. I said when I drove from Cambridge to Norwich I saw a red kite. You keep claiming I made a silly mistake because there's no such road. I've tried to persuade you that there is, but you won't have it. So let's get back to the point of the illustraion, the red kite:

Do you now accept that vaccinating any proportion of people, no matter how small, will reduce the incidence of that disease in the unvaccinated proportion of people? Or do you still believe that there is a cliff-edge effect, such that the reproductive rate of a disease doesn't influence the incidence of the disease unless it falls to one or below?

Yes it is semantics minceorotherwise.

I think the confusion arises over herd immunity is many people, scientists included, are talking about threshold effects in the context of vaccination. They'll talk about attaining herd immunity to mean vaccinating to arrive at a level where theoretically the whole herd becomes immune to a disease. In reality, this doesn't happen, threshold just describes the progression of the disease and a population vaccinated to 99% will still have cases of the disease. The unimmune aren't totally excluded from contracting the disease (as pointed out earlier). In this debate we are particularly arguing about protective effects from vaccination below threshold level: bm asserts there are none. The literature says otherwise. In this context, using herd immunity to mean 'proportion of vaccinated/immune people' is the correct usage.

I have no problem with Dr Schaffner's opinion on the subject, bm. I object when you explicitly say 'caused by' thereby implying sole reason, whereas the Dr says 'substantial majority' implying important reason, with other causes. Semantics again.

Think about smallpox in New Zealand and Australian. Smallpox was not in the community only through people arriving in ports and harbours. They vaccinated the people they came in contact with and a people within a certain radius, but did not vaccinate the majority of the population, only a small proportion. Antivaxxers including BM have argued that smallpox could not have been irraditicated by vaccine in these areas because of the low vaccination rates, but that is because they do not understanding how herd immunity works differently for different diseases and communities.

Jo, are you trying to say that, in your example, there is a way that R could have equalled your 2.9? Really? Why can't you just admit you made a silly mistake? I'm also interested to know how you managed to use the HIT formula incorrectly and come to the conclusion that it didn't calculate any of thresholds in your link correctly when it did. Btw "I'm finding it not inconsiderably entertaining to find you demonstrably and unambiguously flailing around"

Why don't you understand that Jo didn't calculate 2.9, she used it for illustrative purposes nothing more.You are arguing to cover your inadequacies, just as you have done on the other threads. You made a huge mistake on this thread with the graph and made out it was nothing. It illustrated how little you really do understand because if you understood the subject you would have realised that the graph could not possible have represented what you were saying it did.
Elaine's "Black Knight " is a perfect description of you.

"They'll talk about attaining herd immunity to mean vaccinating to arrive at a level where theoretically the whole herd becomes immune to a disease."

Not quite BfG, they'll talk about reaching the threshold so that the disease will become stable in the community or exceeding it so that it will die out.

I also haven't said 'there are none'. Why don't you read what I'm actually writing?

Bruffin, a) I'm not anti-vax and b) I wouldn't argue that because what you are talking about is a different method of containment.

R could not have equalled 2.9. I made a silly mistake. I used the HIT formula wrong because I'm congenitally innumerate. There is no road between Norwich and Cambridge.

Will you now answer the question?

Do you now accept that vaccinating any proportion of people, no matter how small, will reduce the incidence of that disease in the unvaccinated proportion of people? Or do you still believe that there is a cliff-edge effect, such that the reproductive rate of a disease doesn't influence the incidence of the disease unless it falls to one or below?

Lol . I've actually been thinking of Jo as the black knight

Thanks Jo :)

I don't think it's a 'cliff edge effect'. If you read my posts and my links you'll understand what I'm saying. Do you think that having 3 people in 1000 (a very small proportion) immune will offer any protection to the non-immune? There is a reason why most of these definitions use terms such as 'a significant proportion' you know.

Well, I am an arts graduate who counts on her fingers. But.

It would seem to me to be logical that any level of immunity within a population must by definition reduce the risk of transmission of an infectious illness a bit- simply because that's one less person who might pass it on. So in a group of 10 people, where 9 are immune, the 1 remaining non immune person is not going to catch disease Z from them. If 5 are immune and 5 aren't, the chances are higher, but still lower than if none of the group is immune. So even 1 immune person in the group is going to reduce the probability of catching Z a bit.

Reducing the risk of catching Z is different from eradicating it. Which would mean,in my example, 9 of them being immune so that Z has nowhere to go after it has infected and either killed or conferred immunity on the one non immune person.

Does this make sense, or have I missed something?

You're explanation is excellent seeker. Bm is still being coy as to whether or not she fully accepts it.

Here is an online simulator you can play with. It shows that increasing the proportion of people who are initially immune, always reduces the proportion of the unimmune people infected in a disease outbreak. The effect of 0.3% immunity is too small too see, but 5% has a visible effect. If you'd like me to talk you through how to use it bm, just ask.

You could try explaining it to me. I can't even count on my fingers.

Well, I understood myself, Tabitha- don't you understand me?

< image of self as cutter through of jargon collapses about ears>

I can follow the 1 person in a group of ten immunity example, but we, in reality, mix with far more than that. They then mix with others.
Now, where am I? I think I'm lost already.

In my head it just works the same with bigger numbers. Imagine a class at school, or a nursery, and imagine 3 out of 30 instead of 1 out of 10. Or 10 out of 100.

BM, 'waning immunity' means no boosters are being administered. Therefore lack of vaccination remains the issue. The Vanderbilt doctor is only partially right, or you have posted a partial quote. Or you have misparaphrased him.

'While it is possible to argue that 'a little bit of the population are immune' it is not possible to argue that you can have 'a little bit' of herd immunity with the other usage because, as many links and diagrams have pointed out, a significant proportion of the population need to be immune for that to exist. '

Depends on the disease and on the part of the population most susceptible. (See Briffin's post about smallpox in NZ and Aus)

BM -- 'I don't think it's a 'cliff edge effect'. If you read my posts and my links you'll understand what I'm saying. Do you think that having 3 people in 1000 (a very small proportion) immune will offer any protection to the non-immune? There is a reason why most of these definitions use terms such as 'a significant proportion' you know.'

Again, you are missing the point about different diseases, different means of spreading them, different susceptibility levels in different parts of the community.

Hospital workers and Hepatitis of various kinds is a case in point.

I think this point about 'significant proportion' needs addressing. I would understand this to mean: a proportion that would have a significant/measurable effect of disease susceptibility in the unimmune. If it were explicitly referring to threshold, they would be emphasising the exact proportion etc.

Bm: explain to us explicitly what your understanding of herd effect is. Do you think that vaccinating 3 in 1000 will have an effect? 400 in 1000, 800 in a 1000?

Tabitha, you ask a good question wrt sources of disease. We brush shoulders with perhaps tens, of people a day (more if you're walking through a shopping mall etc). Still, most of the diseases we vaccinate, although often pretty contagious, are caught laregely from people we have close contact with, very frequently family members. Children have always been very susceptible to disease bacause, amongst other things, they are in close contact with a lot more sources of disease at nursery/school than we are as adults. I reckon my weekly close contacts come to less than ten people (but I live in the country) and even when I was working, you could have added 5 to that number. You may be surprised by how few close contacts people have in a week! Factor in a low vaccination rate - it is quite possible that having one or two of my contacts no longer spreading disease could have a big impact on if I get ill or not.