The concept of herd immunity leading to disease eradication is simple. More vaccines, less disease, right up to the point of everybody gets vaccines, no more disease. But that's simply wrong.
It's not because herd immunity doesn't help decrease disease, but because the idea of herd immunity is based on too-simple math.
We can all get the idea easily. In a homogenous human population, there should exist a level of vaccination at which no more disease can be transmitted. Notice the fatal flaw is not in the concept, it's in the word "homogenous." Human populations will never be like bricks or milk cartons. You can't just line them up and expect everyone to be the same. So the concept of hitting a magic point and having that point mean no more disease is a mathematical graph, not a human reality.
But it's such a nice graph. You can see a version here.
The more you vaccinate, the fewer cases. But if you look at that graph, you see something like measles needs above 95% vaccination for herd immunity.
The fly in that goal's eye is that at 100% vaccination, total vaccination of every human being on the planet, we only get a net benefit of 91% vaccination. Why? Because 9% of recent measles cases in the U.S. were fully vaccinated. In other words, even with the perfect homogenous population and 100% vaccination, we can't eradicate measles. There is no herd immunity solution possible. It doesn't mean we shouldn't vaccinate, we just shouldn't expect measles to disappear from the planet.
The goal of eradication through absolute vaccination has a darker side. Aggressive forced vaccination for polio has been done for decades. We've gotten the numbers down to a miniscule amount in isolated pockets of a few countries. So should we declare victory? No, say the global public health experts. We need to utterly eradicate this disease. But we've already eradicated one part of the disease, wild polio type 2. It's been gone since 1999. Except since 1999 our continued vaccination with polio type 2 vaccine has resulted in 24 outbreaks of polio type 2 -from the vaccine- occurring in 21 countries. Don't believe me, believe the World Health Organization, who I'm quoting and who has every reason to downplay the fiasco. They've moved toward another vaccine without the polio type 2, but clearly we've been causing harm to both children and the credibility of the vaccination effort by using an unnecessary vaccine for over a decade.
Remember smallpox, the only successful vaccine eradication? For decades after the end of smallpox, we continued to vaccinate for smallpox. When we finally decided to stop vaccinating, it was because the World Health Organization recognized that both death and severe neurological damage were side effects from the vaccine. They were one in a million, but when you have 200 million vaccinations, those are real casualties.
Ah, but we succeeded, smallpox is gone, yes? Not entirely. Monkeypox, a milder variant, has been increasing since we stopped smallpox vaccination. "Human monkeypox outbreaks have occurred in Africa throughout the last 25 years, with case-fatality rates of 1.5-10% [34-40]. Monkeypox was transferred to the U.S. through importation of small animals, causing a multi-state outbreak in 2003 [34,35,41]." In other words, we got rid of the nastiest variant of smallpox, but not the disease family. The World Health Organization made a decision to stop vaccination for smallpox because monkeypox eradication wasn't worth it. They declared victory, and moved on.
Eradication is a noble goal, but it's one that meets the potholes of reality and needs to be tempered by caution and human kindness. Herd immunity up to 100% is still unlikely to lead to eradication because none of our existing vaccines are 100% effective.
PS. It's hard in the politically charged atmosphere around vaccines to make any critical, factual point about the complex medical reality of our global vaccination program. Before anyone accuses me or my patients of being "anti-vaxxers" I need to remind you that the politically correct term is "vaccine hesitant." So feel free to comment, but do read my many replies on Quora about this issue as well.
It's not because herd immunity doesn't help decrease disease, but because the idea of herd immunity is based on too-simple math.
We can all get the idea easily. In a homogenous human population, there should exist a level of vaccination at which no more disease can be transmitted. Notice the fatal flaw is not in the concept, it's in the word "homogenous." Human populations will never be like bricks or milk cartons. You can't just line them up and expect everyone to be the same. So the concept of hitting a magic point and having that point mean no more disease is a mathematical graph, not a human reality.
But it's such a nice graph. You can see a version here.
The more you vaccinate, the fewer cases. But if you look at that graph, you see something like measles needs above 95% vaccination for herd immunity.
The fly in that goal's eye is that at 100% vaccination, total vaccination of every human being on the planet, we only get a net benefit of 91% vaccination. Why? Because 9% of recent measles cases in the U.S. were fully vaccinated. In other words, even with the perfect homogenous population and 100% vaccination, we can't eradicate measles. There is no herd immunity solution possible. It doesn't mean we shouldn't vaccinate, we just shouldn't expect measles to disappear from the planet.
The goal of eradication through absolute vaccination has a darker side. Aggressive forced vaccination for polio has been done for decades. We've gotten the numbers down to a miniscule amount in isolated pockets of a few countries. So should we declare victory? No, say the global public health experts. We need to utterly eradicate this disease. But we've already eradicated one part of the disease, wild polio type 2. It's been gone since 1999. Except since 1999 our continued vaccination with polio type 2 vaccine has resulted in 24 outbreaks of polio type 2 -from the vaccine- occurring in 21 countries. Don't believe me, believe the World Health Organization, who I'm quoting and who has every reason to downplay the fiasco. They've moved toward another vaccine without the polio type 2, but clearly we've been causing harm to both children and the credibility of the vaccination effort by using an unnecessary vaccine for over a decade.
Remember smallpox, the only successful vaccine eradication? For decades after the end of smallpox, we continued to vaccinate for smallpox. When we finally decided to stop vaccinating, it was because the World Health Organization recognized that both death and severe neurological damage were side effects from the vaccine. They were one in a million, but when you have 200 million vaccinations, those are real casualties.
Ah, but we succeeded, smallpox is gone, yes? Not entirely. Monkeypox, a milder variant, has been increasing since we stopped smallpox vaccination. "Human monkeypox outbreaks have occurred in Africa throughout the last 25 years, with case-fatality rates of 1.5-10% [34-40]. Monkeypox was transferred to the U.S. through importation of small animals, causing a multi-state outbreak in 2003 [34,35,41]." In other words, we got rid of the nastiest variant of smallpox, but not the disease family. The World Health Organization made a decision to stop vaccination for smallpox because monkeypox eradication wasn't worth it. They declared victory, and moved on.
Eradication is a noble goal, but it's one that meets the potholes of reality and needs to be tempered by caution and human kindness. Herd immunity up to 100% is still unlikely to lead to eradication because none of our existing vaccines are 100% effective.
PS. It's hard in the politically charged atmosphere around vaccines to make any critical, factual point about the complex medical reality of our global vaccination program. Before anyone accuses me or my patients of being "anti-vaxxers" I need to remind you that the politically correct term is "vaccine hesitant." So feel free to comment, but do read my many replies on Quora about this issue as well.
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