Monday, September 7, 2015

We need to rename Herd Immunity

One morning last week, I found myself thinking about vaccines and immunizations as I drove to work. Perhaps NPR had something on the radio, perhaps not.

It occurred to me that the phrase "Herd Immunity" is flawed and I realized that I wished doctors and researchers could arrive at a better one.

It defined "noun: herd immunity"
  1. general immunity to a pathogen in a population based on the acquired immunity to it by a high proportion of members over time. 
My difficulty with this phrase and its definition is that it's not a definition of immunity. It's a probability statement.

Immunity is the capability of the body to resist harmful microorganisms or viruses from entering it, acting as a barrier, the capability to act as an eliminator of a wide range of pathogens irrespective of antigenic specificity, and the capability to adapt to each new disease encountered and generate pathogen-specific immunity.

If you are immune, you can't get that disease. Either your body blocks it from entering (skin or other barrier), it's not compatible with humans in the first place (not zoonotic), or you have antibodies in general that can destroy it, or you have gotten the disease before and developed specific antigens for it.

"Herd immunity," on the other hand, is not a thing you have or a feature of being human. You can get the disease just as easily as anyone, but the probability is low that you'll come into contact with a carrier ....  except in schools, hospitals, churches and any other place where people congregate.

"Community Immunity" (NIH)
Look at the way herd "immunity" works (source):
Top: If no one is vaccinated and a disease carrier enters the group, the group catches the disease. A random few have a natural immunity, or did not attend church that day, or live far enough away from the carriers and did not contract the disease.

Middle: A few are vaccinated, a carrier mingles with the group and again, many people contract the disease.

Bottom: Many people are vaccinated and the carriers do not inflect as many people.

But there's problems with that. This image shows a nice statistical spread, a random positioning, that allows the unvaccinated to avoid being infected.  Schools, churches, hospitals, and other gathering places, are all scenarios in which this nice statistical spread is not in place.

School children are grouped together all day. If there is a red person and a blue person anywhere in that building, they will come in contact at some time during the day. The statement "If you have a high enough percentage in the group who are vaccinated" now runs afoul of the reality that schools are not random distributions with unvaccinated children able to stay away from any potential carriers.  They will come into contact with the un-vaccinated and they will be infected.

Why does this matter?

I think that the term is incorrectly giving people the impression that they are safe if they don't vaccinate when the truth is that they are in danger of infection when they no longer are spread out into their respective suburban cul-de-sacs and are commingled in schools and other gatherings.

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