Herd Immunity?

[lauradbg]

What is the magic number in vaccine compliance that the CDC looks for in determining herd immunity? Is it 90%, or more, or less?

Have we ever reached that number for any vaccine available diseases? If not, how did the rate of disease infection decline?

Mamakay? Gitti? Deborah? Bueller?

[luminesce]

It depends. Slide 17 has a chart.

http://www.bt.cdc.gov/agent/smallpox...ionhistory.pdf

ETA:
Diphtheria: 85%
Measles: 83 - 94%
Mumps: 75 - 86%
Pertussis: 92 - 94%
Polio: 80 - 86%
Rubella: 80 - 85%
Smallpox: 83 - 85%

[lauradbg]

Thanks for linking that.

Have we ever reached those numbers? For how long? For which diseases?

It's interesting that on that slide they acknowledged that small pox needed other additional efforts to be eradicated--the herd immunity was not enough. They said there could still be outbreaks. Quarantine is what did it for small pox--right?

[shuttlt]

Originally Posted by lauradbg

Thanks for linking that.

Have we ever reached those numbers? For how long? For which diseases?

It's interesting that on that slide they acknowledged that small pox needed other additional efforts to be eradicated--the herd immunity was not enough. They said there could still be outbreaks. Quarantine is what did it for small pox--right?

The claim in the UK is the measles was above herd immunity until Wakefield, I don't have a link right now to prove it though.

[Deborah]

There is a big problem with all of the numbers on herd immunity, because they don't count adults. This is a throw-back to the original situation with childhood illnesses, where the majority of adults were immune (as I am) as a result of having had the actual illnesses. But the first generation to reach adulthood after being vaccinated doesn't have anything like the level of immunity achieved through the circulation of childhood illness through the population. And the next generation will probably have lower immunity.

There are several problems:

1) circulating illness boosts immunity in the entire population, vaxed and naturally immune

2) vaxing CAN provide lifelong immunity, but it doesn't always

3) by counting vaxed children but not paying any attention to the level of immunity in the adult population the public health folks are basically...what? Assuming that outbreaks will occur only among children?

The mumps epidemics in the U.S. and the U.K. show what happens when you assume stuff about herd immunity. Assume wrong stuff...

[mamakay]

Originally Posted by Deborah

There is a big problem with all of the numbers on herd immunity, because they don't count adults. This is a throw-back to the original situation with childhood illnesses, where the majority of adults were immune (as I am) as a result of having had the actual illnesses. But the first generation to reach adulthood after being vaccinated doesn't have anything like the level of immunity achieved through the circulation of childhood illness through the population. And the next generation will probably have lower immunity.

There are several problems:

1) circulating illness boosts immunity in the entire population, vaxed and naturally immune

2) vaxing CAN provide lifelong immunity, but it doesn't always

3) by counting vaxed children but not paying any attention to the level of immunity in the adult population the public health folks are basically...what? Assuming that outbreaks will occur only among children?

The mumps epidemics in the U.S. and the U.K. show what happens when you assume stuff about herd immunity. Assume wrong stuff...

Yeah, it's kind of a paradox. Herd immunity weakens itself once it's achieved.

[Deborah]

Yes, not just with vaccination, either. Isolated communities can achieve herd immunity, halt circulating disease and then have a terrible epidemic when the illness is brought back again by a visitor.

Cities, especially overcrowded pre-modern cities, had the opposite effect. Because there was a continual flow of new people coming in, there were always new bodies to catch whatever was going around. Herd immunity was impossible. Add in filth, overcrowding, malnutrition and ignorance as to causes of disease and the situation became totally impossible. Medical treatment just made things worse.

But vaccines saved us, not a rising standard of living, not increasing knowledge of effective sanitation, not changes in medical treatment--nope, vaccines are the only thing standing between us and devastating epidemics of disease. Whoops, I'm wandering way off-topic, sorry.

[shuttlt]

Originally Posted by Deborah

There is a big problem with all of the numbers on herd immunity, because they don't count adults.

Is this true? I mean, I can see that vaccination levels are easier to get a handle on, so perhaps often when the media says herd immunity, they mean vaccination level, but is this how the CDC calculate it? I've looked and so far failed to find a decent source of figures for herd immunity to work out how it is being calculated.

Originally Posted by Deborah

circulating illness boosts immunity in the entire population, vaxed and naturally immune

I agree completely

Originally Posted by Deborah

vaxing CAN provide lifelong immunity, but it doesn't always

I agree. The same is true of natural immunity though, isn't it? With natural immunity of course you'd be getting your boosters regularly.

Originally Posted by Deborah

Isolated communities can achieve herd immunity, halt circulating disease and then have a terrible epidemic when the illness is brought back again by a visitor.

You are talking about the need for a population of a few hundred thousand before infectious diseases can continuously circulate, irrespective of vaccination?

Originally Posted by Deborah

Cities, especially overcrowded pre-modern cities, had the opposite effect. Because there was a continual flow of new people coming in, there were always new bodies to catch whatever was going around.

I take it you are counting babies as new people?

Originally Posted by mamakay

Yeah, it's kind of a paradox. Herd immunity weakens itself once it's achieved.

Absolutely. That is why IF you are going to vaccinate against something for which herd immunity is relevant, you want to do it as fully and as completely as possible. Doing it without the will, or the means to do it properly can be worse than not doing it at all (even from the most pro-vax standpoint).

[Deborah]

No, I wasn't counting just babies as new people. The nature of cities, especially pre-modern cities, was that they sucked people out of the countryside. Some of these people came from small communities and generally they would have had less exposure to disease just from living in the country. Cities didn't just have the normal childhood illnesses like mumps or measles. They were hotbeds for cholera, typhoid, plague, smallpox, diphtheria and so on. Your healthy young farm worker coming to the city to get rich wasn't likely to have been exposed to all that stuff. A few months of living in an overcrowded tenement, eating crappy food and working 14 hour days in a sweatshop...the death rates were horrendous.

[shuttlt]

Originally Posted by Deborah

No, I wasn't counting just babies as new people. The nature of cities, especially pre-modern cities, was that they sucked people out of the countryside.

Sorry, I thought you might just be counting people coming in. My mistake.

[caned & able]

Regarding herd immunity:

http://www.youtube.com/watch?v=MpZrv...eature=related

[mamakay]

Originally Posted by shuttlt

Sorry, I thought you might just be counting people coming in. My mistake.

Babies should count, though, in terms of disease transmission. It's another person in the population to catch and spread a pathogen. Babies not so much, really, but once they're a toddler, preschooler or child...they're out and about interacting with others and spreading whatever they have.

[Deborah]

Yes, babies and toddlers do count. In early modern cities the infant mortality rates were so high that few babies made it. We tend to assume that all babies were breastfed by their mothers "back then" but many women worked and it was common for babies of the poorest to go out to the country and be cared for by countryfolk. Rich people tended to keep their kids out in the country too, to increase their chances of survival. And even the middle class would put their kids out sometimes. To give a specific example, Jane Austen spent perhaps two or three months with her mother and then went to a wet nurse in the village for a couple of years. Her busy mom visited her once or twice a week! Her mother was running a large household with several children of her own and students being tutored besides. As they weren't wealthy, much of the work of the household rested on her shoulders, although they did have some servants and probably a cook.

Anyway, to get back to the topic, the rare baby that survived being farmed out to the countryside would return to their family aged two or three. Most children in poor families were working by the time they were six.

Anyone who made it to adulthood under those conditions had an immune system like a tank.