I'm not a molecular biologist. I'm not an immunologist. I'm not a doctor. I'm not a pathologist. I'm not a lot of things relevant to the study of virology and how diseases work.
What I am, though, is inquisitive. I am logical. I reason.
AND I PAID ATTENTION IN SCIENCE CLASS.
The anti-vax movement disapproves of the removal of religious and philosophical exemptions from vaccinations within the state. Some people are against it on the grounds of government overreach but nearly all of them don't really understand how vaccines work nor do they understand their importance.
I encounter these people a lot in my internet meanderings.
Here is a summary of a recent conversation with one of them; I have been told by several people who are fancy-pants biology people who have degrees I can never attain that the below is a reasonable summary of an entire semester of college immunology.
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First off, they claim they are not anti-vax and they launched their counter with the age-old "if vaccines work then why are you so worried about this if you are vaccinated?" (this question comes in many forms). I find this a tiring argument because it shows the level of science ignorance of the person and they are always smug about it, thinking it is the super "gotcha" argument that proves vaccines are worthless.
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Let's have a crash course in this.
Mutations are the root of evolution. They are transcription errors in genetic information being copied during the reproductive process.
Mutations occur generationally, meaning that the change does not happen within a cell, but when a cell is replicating. In single-celled entities (like microbes) each division is a generation of the species, whereas within multi-cellular beings each cellular generation is of no significance to the overall species mutation rate, only gametes matter for multi-cellular beings.
Mutations can be one of three things: inert, beneficial, or detrimental (from the perspective of the organism that possesses the mutation). Inert mutations don't matter, they are just a difference; beneficial mutations help the organism survive; detrimental mutations limit the success of the organism.
Beneficial mutations succeed in reproduction faster than the others in microbes because each subsequent division is a whole generation and a whole opportunity to spread faster and farther.
Each host for a microbe population varies slightly. Each one provides slightly different conditions that favor or hinder the growth of a microbe. This is why different people experience different levels of illness when exposed to the exact same strains of a microbe. This is why treatments work in some people and are completely ineffective in others.
If microbes have no home then they have no opportunity to grow and thrive and... mutate.
When a population is 95% immune it is 95% impossible for a microbe population to even enter the situation and 95% impossible for it to spread.
Which such a spread limitation treatments can be much more focussed and effective because the resources are available to spend on eradication.
This is called herd immunity.
NOW we get into the answer to your question.
Herd immunity minimizes the spread of a disease by minimizing the number of hosts in which it can gain a foothold. It minimizes the number of differing environments that can each exert naturally selective forces on the disease. It limited the number of mutations that can happen to the strain of the microbe, limiting the net volume of variation and mutation that it can acquire.
THAT is why we are so worried about it despite vaccinations.
Because, if we dip too far below the 95% threshold, even as low as 90%, we open a sliver of our population up to becoming incubators and mutation factories.
We open up that sliver of the population to transport the microbes wherever they go and to spread it to the sliver of the population in those locations that are also not vaccinated.
Those differing geographic regions compound the level of environmental diversity created by different individual hosts and create even more diverse naturally selective forces to act upon on microbe population.
Now... let's do some real quick math here based on a bacterium that is well known, even if it doesn't have a ready vaccination for it because we have lots of ways of preventing sickness from it:
An e. coli bacteria has a generation of about 20 minutes. This means that if a single e. coli bacterium finds its way into a human host it doubles in number every 20 minutes. Each individual division is an opportunity for a mutation to occur.
Since the division is exponential this balloons up VERY quickly.
At the end of ONE DAY the number of e. coli cells in the host is an unmanageable number to type (2^720).
If those replications develop a more virulent, faster reproducing, or more lethal strain that is more successful in the host and able to spread to others then we have an outbreak that kills lots of people.
This happens. This is why we have e. coli scares even though simple hand washing and proper cooking techniques prevent the issue.
This same thing happens with EVERY disease. Vaccines work but they only work for existing strains and those similar enough to them for our body to recognize it. If a mutation happens that changes it just enough for our bodies' immune systems to no longer recognize it then EVERYONE, vaccinated or not, gets sick. The more infected hosts the more opportunities for such a mutation to happen; the more opportunities for such an infection to spread. The more opportunities for us to have created a new iteration of measles that we cannot treat with our current medications or which is just lethal enough to make treatments irrelevant altogether.
This, too, is happening. We have cultivated antibiotic-resistant strains of Staphylococcus (known as MRSA) which are now found in about 2% of the population; when it gets into their (or anyone else's) blood it becomes a lethal situation for more than 20% of the people infected.
The easiest and best, way to prevent massive disease outbreaks due to beneficial (to the microbe) mutations is to eliminate their ability to grow and thrive and reproduce and the easiest way to do that is.... TO VACCINATE EVERYONE
It might "just be wikipedia(1)" but it is well documented with primary sources in the bibliography at the bottom.
1. wikipedia has been, repeatedly, tested for accuracy against "real" encyclopedias and found to be more accurate AND have more detailed information (2). https://en.wikipedia.org/wiki/Herd_immunity
2. https://www.wired.com/story/wikipedia-online-encyclopedia-best-place-internet/
