Back in September, Orac wrote a post,
A sad, premature death cynically exploited by antivaccinationists, on the unexpected death of Jasmine Renata, an 18-year-old New Zealand resident. Her mother is now claiming that Jasmine was killed by the Gardasil vaccine.
In the comments, the conversation turned to the concept of herd immunity. A New Zealand anti-vaccinationist,
Ron Law, made some astonishing claims. Several commenters responded:
ChrisP:
The risk of an uninfected person becoming infected is reduced with high levels of herd immunity.
Ron Law:
One thing that puzzles me is that pre vaccine days we are told that 95% of people got infected… so if herd immunity was real then the disease would have burnt itself out, wouldn’t it?
Science Mom:
What a rookie mistake. Basic epidemiology Ron; new susceptibles enter the herd via birth and/or waned immunity. I would imagine that such an enlightened risk analyst such as yourself with so many years of “studying the medical literature” would have devoted at least a modicum of time to understanding some statistics and epidemiology. The numbers of VPD pre and post vaccine are rather self-explanatory.
Ron Law
Herd immunity is a theory… predicated on most people in the herd having been infected naturally…
Krebiozen
[Ron Law wrote] Herd immunity is a theory… predicated on most people in the herd having been infected naturally…
That’s not actually true. The theory was formulated to explain observations made of the natural cycle of contagious diseases and natural immunity, but is not predicated upon that. In non-vaccinated populations an epidemic occurs when each person infected infects more than one other person, causing an exponential growth in the number of infected people, and continues until there are not enough non-immune people left to continue that rate of infection. The disease then becomes endemic, where each infected person only infects (on average) one other person.
As more and more children are born, the proportion of non-immune people rises until there are enough to sustain an epidemic and sooner or later this occurs and the cycle repeats. The periodicity of an epidemic cycle is largely related to how contagious the disease is – the more contagious the disease, the higher R0 is, the higher the herd immunity threshold and the shorter the period of the cycle.
So herd immunity is really a mathematical model that successfully explains those observations, and observations of vaccinated populations, and allows us to make accurate predictions about the effects of various public health policies. Of course in the real world and on an individual level things are more complicated, as a lot of the assumptions made are not true in every case (random distribution of immune people in the population, the number of people a contagious person comes into contact with etc.). As has been discussed here before, it doesn’t actually matter how immunity is achieved, whether naturally, by vaccinations or by a large enough proportion of the population wearing biohazard suits (not practical, obviously, but it suffices to make the point). The only important factor is that they cannot become vectors for spread of the infection; the rest of the model emerges from that.
So there you have it. Herd immunity is a mathematical model.
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