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So, let's deal now with this concept of herd immunity. What it is, what it was in the pre-vaccine era, natural herd immunity, and what it is in the vaccine era. It's a term that's commonly used in defense of mass vaccinations, and it finds itself at the interface of public health and politics. It's the driving argument for mandatory vaccination legislation and a key public relations slogan used to persuade parents to vaccinate. "We must maintain herd immunity." What does that mean? Let's distinguish, again, natural heard immunity, that which operated in the pre-vaccine era, and artificial herd immunity, which has taken over in the vaccine era. Understanding natural herd immunity. Let's define, initially, what natural herd immunity is at a functional level of population protection. What factors contribute to natural herd immunity? What does it mean against the changing landscape of measles disease over time? How does individual immunity contribute to herd immunity? What is the contribution of the collective individual immunity to a population-level that protects? In other words, how do the individuals who are immune contribute, or not, to the protection of the population and what is it that they are protecting against? Let's define that, first of all.
In the pre-vaccine era, herd immunity was the presence of adequate immunity in a population against a specific infection, like measles, that operated to protect those at high risk of serious infection to avoid that serious infection and consequently reduce serious morbidity and mortality from measles. In other words, historically, natural herd immunity has not operated to prevent measles, but rather to permit it at a time and in a way that it's least harmful, thus leading in the host to improve survival, good health and the benefit of lifelong immunity. So, what are the elements of natural herd immunity? We've looked at their relationship to exposure, to the dose or intensity of exposure and the age of exposure. We've looked also at what Claude Bernard described as the terrain. That is, the human host, the fitness of the human host to deal with the infection, including nutritional status. So, who's at high risk of serious infection? Who do we need to protect? And therefore, who is protected from serious affection and death by natural herd immunity? Measles, as we've learned, is more dangerous at the extremes of age, in the very young and adults, than when the disease is experienced in childhood after the first year of life.
It's more dangerous when experienced at high intensity, that is, a high dose of viral exposure. And it's more severe in those who are immunodeficient, for example, children who are malnourished. So, let us go back to a time when natural herd immunity was low or absent altogether. If we look at the top of this graph, up until 1920, that is what pervaded the situation at the times. Natural herd immunity was low, children were being exposed of high-intensity and early in life, and consequently, the mortality was very, very high. And as circumstances changed, then so the mortality changed. We go to a circumstance where there was no herd immunity whatsoever, a virgin soil population such as the Yanomami Indians in the Orinoco, then they were extremely vulnerable to this infection. Why? Because when it occurred, it would affect people across ages. There was no immunity in the population. The old had not experienced measles as youngsters, and therefore, they were equally susceptible as infants. And so, you see this dramatic exposure at all ages, including those associated with high mortality such as infancy and old age. And consequently. The disease was a major killer in that population.
The terrain. Claude Bernard's terrain. Poor nutritional status. And this is seen with vitamin C deficiency, Australian Aborigines, by Dr. Archie Kalokerinos, who wrote in his book of every second child, how in fact it was during vaccination campaigns among the Aborigines in New South Wales that exposed this extreme sensitivity to the vaccine, very high rate of adverse reaction and death associated with vitamin C deficiency. The World Health Organization recommending that children in third world countries are supplemented with vitamin A in the presence of measles infection. And now, the emerging evidence, even here in North America, of vitamin D deficiency leading to impaired immune function. As I've said, there is one other very important factor, and that is, of course, Olivia Walton, the mother, who provides her babies with passive immunity that protects them at that extremely susceptible time of life. Let us look closer at an example of how natural herd immunity operates to protect those at higher risk of infection. Infants less than one year of age have a limited ability to generate adequate immunity, and more likely to suffer intensive exposure of secondary cases, and therefore, more susceptible to serious measles infection.
In the pre-vaccine era, unlike the post-vaccine era, mothers conferred good passive immunity on their infants by placental transfer and breast milk. This passive immunity protected those infants through a period of vulnerability until they were better able to deal with the infection, until their immune system had developed to the point where they were able to cope with measles and deal with it with life-long immunity. So, what natural herd immunity, what the pattern of exposure has done over time is to constrain the age of exposure to childhood, when the virus is less dangerous, when measles is a milder disease. And, at one end, that is achieved by maternal immunity protecting infants, and at the other end, it is achieved by mass epidemics of measles, making sure, ensuring that everyone has measles during childhood. Therefore, you are not susceptible as an adult and you are not going to go and develop immunity. So, maternal immunity, regular epidemics, ensured that children went on to develop measles once passive immunity had waned. And over time, they were exposed increasingly in the community, for example, at school, at doses that led to a natural, self-limiting measles infection and the induction of life-long immunity.
And the establishment of life-long immunity in children meant that people were not susceptible to measles as adults, a time when the disease could also be more serious, nutritional status improved, and natural herd immunity increasingly predominated. Measles became, with time, a more and more mild disease. So, this is the consequence of natural herd immunity. It's dramatic, it's striking, and it's really what we have to pitch vaccination against, when comparing the ability of the disease or the vaccine to protect the population. And as a consequence of natural herd immunity, in the developed world, measles mortality had fallen by approximately 99.6% before the vaccine was introduced. And a fall in serious morbidity, that is not death, but serious outcome from the disease will also have paralleled this fall in mortality since we can reasonably assume mortality is the more extreme end of morbidity. So, the exercise here is to address the question: What would have been possible consequences of letting nature take its own course? And for this, I want you to consider measles in two different geographical locations, in industrialized countries and in non-industrialized countries, where clearly measles is still recognized to be a much more severe disease.
And to consider what alternatives to mass vaccination, or in addition to mass vaccination, might have improved what Claude Bernard described as the terrain, the ability of the individual infected with the virus to deal effectively with that virus.