What is Herd Immunity?

This was cited on yesterday's (March 9, 2020) "Peak Prosperity: Coronavirus: New Data Introduces (Some) Hope" video and I thought it would be useful for the COVID discussion even thought the article is five years old...

The term "herd immunity" refers to a means of protecting a whole community from disease by immunizing a critical mass of its populace. Vaccination protects more than just the vaccinated person. By breaking the chain of an infection's transmission, vaccination can also protect people who haven't been immunized. But to work, this protection requires that a certain percentage of people in a community be vaccinated. What factors determine where that critical-mass threshold lies? And once it's in place, how does herd immunity cocoon the most vulnerable among us?

What is "herd immunity?"

There are two ways an individual can become immune to an infectious disease: by becoming infected with the pathogen that causes it or by being vaccinated against it. Because vaccines induce immunity without causing illness, they are a comparatively safe and effective way to fill a community with disease-resistant people. These vaccinated individuals have protected themselves from disease. But, in turn, they are also protecting members of the community who cannot be vaccinated, preventing the chain of disease from reaching them and limiting potential outbreaks. Every vaccinated person adds to the effectiveness of this community-level protection.

What do thresholds have to do with herd immunity?

The microbes that cause disease all have different infectious features. Some, like measles and influenza, pass from person to person more easily than others. Some tend to have more severe consequences in specific demographic groups. For example, the symptoms of pertussis, or whooping cough, are distressing at any age but can be fatal in infants, the age group with the highest death rate from pertussis. Each of these features—such as transmissibility and severity—affects a given disease's threshold, or the minimum percentage of immune individuals a community needs to prevent an outbreak.

To set a threshold, epidemiologists—experts in infectious disease transmission—use a value called "basic reproduction number," often referred to as "R0." This number represents how many people in an unprotected population one infected person could pass the disease along to. For example, R0 for measles is between 12 and 18, while for polio, it is between five and seve. The higher this number is, the higher the immunity threshold must be to protect the community. Because measles is extremely contagious and can spread through the air, for example, the immunity threshold needed to protect a community is high, at 95%. Diseases like polio, which are a little less contagious, have a lower threshold—80% to 85% in the case of polio.

The general concept of an immunity threshold seems simple, but the factors involved in calculating a specific threshold are complex. These factors include how effective the vaccine for a given disease is, how long-lasting immunity is from both vaccination and infection, and which populations form critical links in transmission of the disease. The collective differences in these factors result in different thresholds for different diseases (see below), with a significant factor being R0.

Why is herd immunity important?

Human communities were once relatively small and isolated. Diseases certainly broke out, but their transmission ended wherever geography limited a populace's mobility. But today, our chains of connection traverse the globe—reaching across oceans and over mountain ranges, pervading immense cities and remote villages—linking us all into one vast, interactive human herd. Almost no one anymore lives in isolation from such connections.

These chains of human interaction have resulted in more potent chains of disease transmission. The only thing that can break a chain of transmission is a disease-resistant link. The chicken pox vaccine offers an example of the effectiveness of disease-resistant links. After the chicken pox vaccine debuted in the United States in 1995, deaths rates from chicken pox dropped by as much as 97%. Significantly, even though the vaccine is not administered to infants, no infants died from chicken pox in the United States between 2004 and 2007. These tiniest, most vulnerable links in the chain of human connections avoided exposure thanks to herd immunity.

Let's hope all the companies rushing to develop a vaccine are successful and practical and safe vaccines get to market quickly. There was an interesting charge yesterday showing how vaccine development has sped up the past 20 years with advances in technology.

This subject comes up a lot with the folks who are against vaccinating their children.

Some kids can not be vaccinated due to medical treatments or condition. They are able to “hide in the herd” due to the level of vaccination in the rest.

The kids who are not vaccinated because of their parents are also able to hide in the herd.

The problem becomes when the level of immunity in the herd drops far enough allowing the disease to pass around inside it.

It has come close a few times in CA.

They will have good reason not to take any vaccine developed for this illness as well.

They'll find their reasons not so good when someone they know dies from the disease.

I saw a youtube video featuring a young family that had lost a baby when someone who was not vaccinated against pertussis came to visit and the baby got sick. Following that experience, when they had their second child, they wanted proof of vaccination before they allowed anyone to come visit.

The problem with the anti-vaxxers is that they never lived in a time when infectious disease was the major cause of death, especially among children. Because they have not truly experienced infectious disease, they imagine that our current health status, mostly free of infectious disease, is a natural condition of humanity. It is not.

As you can tell by my tagline I am NOT pro-vaccine.

I think it’s a pretty smelly, corrupt business.

I think most of the vaccines that are required to go to school are there because money changed hands and not for any legitimate health reasons.

I think that if one does not have the right to control what does or does not go into his or her body (or that of our child) then all other rights are meaningless.

For myself I do a basic cost-benefit analysis. The vaccine is only worth it if the deadliness of the disease outweighs the potential harm from the vaccine (and all vaccines will always have some harmful potential).

The happy-happy-joy-joy on that point from vaccine makers is brazen propaganda.

“The problem with the anti-vaxxers is that they never lived in a time when infectious disease was the major cause of death”

Exactly right. A couple of decades without disease and people consider that to be the normal situation. They get lax, let down their guard, say “I don’t need to be vaccinated, there is no more disease.” Reading the history books is almost useless. People need to experience want, deprivation, scarcity, disease and tyranny first-hand to be afraid.

The same is true for:
1. Food - “It’s always plentiful”
2. War - “It’s comparatively scarce and far away. Besides, a very few people are keeping us safe”
3. Liberty and freedom - “We will always be free and communism is good for us, good for equality. The Constitution is an outdated old dead white man document.”

Heard immunity is a complete bull crap argument put out by the vaccine industry

the vaccine industry is pure evil is $100 billion a year and they’re trying to buy off Democrat politicians to force you to inject your self with poison

Doubt me?

Why is there a $5 billion fund to pay off all the families whose children’s lives have been ruined

Adults with fully developed immune system’s are more robust and less likely to be permanently harmed by the injection of vaccines their ad junked and all the crap that’s in them

But the fact remains that vaccines don’t work and that’s really all you need to know

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