Going against the herd: The importance of mass immunity in society

Why is it that members of the public health community are worried about falling vaccination rates in the US?

Do our personal health decisions for ourselves and our children have an impact on the health of society as a whole?

The answer to this is that yes, our individual decisions do matter to society when it comes to combating the spread of contagious disease.

A large part of this is herd or community immunity; the way in which mass immunity in a population can control the spread of disease among individuals. Herd immunity is a major reason behind why so many deadly diseases have all but disappeared from American society; our vaccination rates protect many of those who are unvaccinated from contagious diseases. However, this is beginning to change in the US and we are beginning to see outbreaks of diseases that have not been of major clinical concern for decades.

Poster from before the 1979 eradication of sma...
Poster from before the 1979 eradication of smallpox, promoting vaccination. (Photo credit: Wikipedia)


Herd immunity works by limiting the number of susceptible individuals that an infected individual can come into contact with and therefore spread disease to.  The most common way doing this in modern society is through vaccination. For many diseases, effective herd immunity can be reached at vaccination levels lower than 100%. For example, herd immunity is protective at levels as low as 80% immunity for mumps, but a minimum of 92% immunity is required for effective pertussis transmission prevention. This is a very important factor, as not all people are good candidates for vaccination such as pregnant women, young infants, or those with compromised immune systems. By having those around them vaccinated, at risk individuals can be protected from disease without being vaccinated themselves (see 3rd panel in image below).

"The top box depicts a community in which no one is immunized and an outbreak occurs. In the middle box, some of the population is immunized but not enough to confer community immunity. In the bottom box, a critical portion of the population is immunized, protecting most community members."
“The top box depicts a community in which no one is immunized and an outbreak occurs. In the middle box, some of the population is immunized but not enough to confer community immunity. In the bottom box, a critical portion of the population is immunized, protecting most community members.” Via NIAID

However, due to the recent trend of not vaccinating our children we are at risk of losing important protective herd immunity to various pathogens with serious consequences (moving from panel 3 in the image to panel 2). In fact, this is already happening in the United States. My home state of Washington has had a significant outbreak of whooping cough in 2012, caused by the bacteria Bordetella pertussis. On April 3rd the State Secretary of Health declared a whooping cough epidemic in the state. By the 16th of June cases numbers had soared above 2500. This was a massive 1,300% increase from the same period in 2011, and the highest number of cases reported in the state since 1942. (From a scientific standpoint there is no controversy as to why this is happening; people are not following the correct vaccination schedules for their children for various reasons. For more information on the modern anti-vaccination movement and how it is a movement of emotions trumping science, see my post on The Panic Virus by Seth Mnoonkin.)

During local whooping cough outbreaks non-vaccinated children sent home from school for fear that they could catch a potentially fatal disease or bring it home to siblings who were too young to be vaccinated. As a result, disease outbreaks such as this not only damage individuals, but can also significantly impact the education of non-infected but susceptible children in the community. The worst part about this outbreak in my home state is that it was entirely preventable if children were properly vaccinated.

The importance of herd immunity in the context of bacterial infections becomes even more clear when we also look at the diminishing reserve of effective antibiotics that we have at our disposal. Some of these pathogens that we vaccinate against can acquire antibiotic resistance, meaning that vaccination is the ONLY line of defense to certain resistant organisms. Without the ability to prevent we are only able to treat, and bacteria are much faster at evolving around our current treatments than we are at developing new ones.

As a global society we are facing issues of overpopulation, overcrowding, antibiotic resistance, and falling vaccination rates in parts of the developed world. These factors coupled with the continual emergence and evolution of human pathogens means that we are entering an unprecedented microbiological era. We have the successes of the last century to bolster us such as vaccination and antibiotics, but these will not be enough if social practices discourage their proper use.

Herd immunity is a major factor in public health and the prevention of outbreaks of disease with serious health, social, and even educational consequences. This is one reason why vaccines and their proper use are such a large part of modern public health efforts and should be utilized properly whenever possible.


Featured Image from Wikimedia Commons, used under Creative Commons License.

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