Prester John said:
.... if you cease vaccination on a population level then you lose then required level of acquired immunity and thus lose herd immunity protection. The disease in question will quickly reestablish itself and run rampant through a susceptable population. Epidemic even.
Absolutely.
Herd immunity only confers protection on the non-immune while they stay in the herd, and while the percentage of immune individuals in that herd remains high enough.
That will only be the case by natural infection shortly after an epidemic, when the vast majority of the survivors will be immune. As time passes, new births (which will become susceptible once their passive maternal antibody wanes, at a few months of age) and new immigrants will reduce the percentage and the disease will have enough susceptible individuals in contact with each other to be able to run through them all again.
So in the natural situation nearly everybody gets the disease, one way or another, often in cycles of epidemic activity.
Gradually, some genetic adaption will occur, as the constant winnowing by disease selects for genetic types who are less susceptible to the worst effects of the disease, but after about 30 generations you've gone about as far as you're going to go. We've had diptheria for a lot longer than 30 generations, and we're not going to adapt much more. And even if we did, that adaptation involves having the disease kill all the most susceptible genetic types before they've had a chance to breed. Nice.
Unnaturally, we've found a way to protect nearly everybody by making them immune without having to catch the disease. We use an inactive or partial form of the bacterium to kick the immune system into making antibodies, while making sure that the organism isn't capable or complete enough to cause disease. We call this "vaccination".
Once that is well enough established, herd immunity can be kept high enough all the time that the few non-immune members (babies too young to be vaccinated, those with immune system diseases and those who have an increased risk of an adverse reaction to the vaccine, mostly due to an allergy) are also protected, as they are in effect surrounded by so many immune individuals that the disease can't find them.
By systematic application of this method, it becomes feasible to eradicate the infectious organism from the earth - make it extinct, in fact. This was achieved with smallpox (and if there hadn't been samples kept in some labs, we'd be at no risk at all now), and there are good hopes of achieving the same for polio and measles. Once that is achieved, it is possible to stop vaccinating, indeed sensible to do so.
The problem we have at the moment is that measles and polio and diptheria have not been eradicated world-wide. However, in the prosperous areas the vaccination strategies have already resulted in very low or non-existent rates of disease. The vaccines aren't 100% perfect, and there is still a small risk of an adverse reaction. Seeing this risk, and seeing the almost non-existent chance of catching the disease in the prosperous population, people like Rouser want to stop vaccinating.
Now on an individual basis, looked at selfishly, there is a point to consider. Not everyone needs to be vaccinated to keep herd immunity up. The vaccinated majority protect the small children who are yet to be vaccinated, and those who can't take the vaccine for some medical reason. Rouser has spotted that if he joins that latter group (one more won't make much difference), he'll share in the protection and yet avoid the slightest risk of experiencing an adverse vaccine reaction.
If he simply stated that he intended to take that attitude, and rely on almost everyone else "risking" the vaccine to keep him safe, he'd be logically right. Morally, I'll leave that for others to judge.
Of course he's only be safe if he stayed in the immune herd, and the herd stayed immune. He couldn't risk, as the poor man in the thread starter post did, travelling to anywhere like Haiti. The herd protection only protects the non-immune so long as they stay in the herd. And he couldn't risk the herd becoming insufficiently immune. The strategy stops working as soon as more than a handful of people adopt it!
However, he seems to keep trying to claim that everyone could and should take the same attitude. That everyone can refuse vaccination and still retain the herd immunity. This is of course impossible.
Rouser, the fact that the chance of getting the disease is as low as the chance of an adverse vaccination reaction relies
absolutely on >90% of the population being vaccinated. If you use that argument to stop the vaccination, the chance of getting the disease will very quickly mushroom so that anyone with any sense will be
begging for vaccination.
Rolfe.