Herd Immunity is very important, but there are some growing problems.

Herd immunity involves getting the majority of a population immune to a disease. This has many benefits, including slowing or stopping the spread of the disease, even between non-immune individuals. This saves many lives, but as people forgo vaccines this weakens.

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The Effectiveness of Herd Immunity

Some diseases have been completely eradicated through vaccination, while the rate of infection for others has dropped. The most commonly cited success story is the eradication of smallpox. Smallpox was caused by the virus variola. It was mostly spread through saliva and was very deadly. About 30% of all people infected with it died. Smallpox has existed in humans for at least 3000 years and was officially declared eradicated in 1979. The process that led up to this was painstaking, but due to the hundreds of millions of lives saved, worth it.[1] In the mid 1960s smallpox was still infecting 15 million a year, 150 years after the vaccine was discovered. What it took was a large coordinated effort. Prior efforts had a few problems. They had two few vaccines and too few people to aid in vaccination. Then Bill Foege came up with a new method, surveillance and containment. This method includes finding people who were recently diagnosed with the disease and then to vaccinate everyone who had contact with them. This provided a small area of herd immunity, preventing the spread of the virus completely. [2]

http://www.whale.to/v/obosawin1_files/table4.gif [source](http://www.whale.to/v/obosawin1_files/table4.gif)

Vaccination Today

The method of surveillance and containment only worked on smallpox because a vaccination before the symptoms worked. Other diseases that spread more easily, like the flu, need herd immunity beforehand to prevent an outbreak. For a group of people to become fully immune they must reach a point called the “immunity threshold”. This threshold is dependent on how infectious a disease is. This is measured by the “basic reproduction number”, which is how many people would an average person infect if nobody was immune. For example the basic reproduction number for measles is between 12 and 18, and the rate for smallpox is between 3.6 and 6. The actual method of calculating the immunity threshold is complicated, because each disease and vaccine for it acts differently. This threshold for measles is around 83-94% of the population vaccinated, while smallpox is between 80-85%. [3][4] The vaccination rate for measles is sitting at just under 92% in the United States, and is lower in many other countries. This may not be enough to prevent an outbreak. Another disease commonly vaccinated against is influenza. Unlike measles or smallpox this disease often mutates and comes in many variates. This means a new vaccine for it comes out every year and the threshold immunity changes. Over the five separate outbreaks the number required for herd immunity ranged from 13% to almost 100%, which is insane. We can’t expect out levels of immunity to be safe with those numbers. Only about 75% of elderly and high-risk patients get the vaccine, which is nowhere near enough. [5][6]

Our Herd Immunity is being weakened by people choosing not to take vaccines. This hurts everyone and increases the probability of a pandemic.

### Want to learn more science? Subscribe and Upvote! [[1]](http://www.who.int/csr/disease/smallpox/faq/en/) [[2]](http://www.humanosphere.org/science/2010/08/how-smallpox-got-beat-why-it-matters/) [[3]](http://www.pbs.org/wgbh/nova/body/herd-immunity.html) [[4]](https://academic.oup.com/aje/article/158/2/110/323323/Transmission-Potential-of-Smallpox-Estimates-Based) [[5]](https://www.statista.com/statistics/385577/mmr-vaccination-rate-among-us-children-aged-19-35-months/) [[6]](https://www.ncbi.nlm.nih.gov/pubmed/22414740)