One argument against vaccination is so called “Herd Immunity”. Herd Immunity occurs in a population that has very high immunity to a disease. When an infected individual enters the population, the disease cannot spread because most people are already immune. This prevents an outbreak and protects the the population. Those who choose not to vaccinate their children believe that “Herd Immunity” provides sufficient protection from the diseases. “Herd Immunity” is the term used for the idea “Who ever gets measles anymore, anyway?”.
Unfortunately there are serious problems with relying on Herd Immunity to protect someone from disease.
Herd Immunity Protects the Herd, Not Individuals
The biggest problem with Herd Immunity is that it protects the Herd, not individuals. Herd immunity stops a disease from spreading and becoming endemic. A disease is endemic in a population when it is able to exists within a population by spreading from vulnerable individual to vulnerable individual. Once immunity is high enough, the disease cannot continue to spread within the population and it eventually disappears.
However Herd Immunity does nothing to stop the spread from an infected individual to a vulnerable individual. The question is really this, what are the odds of a vulnerable person being exposed to an infected person? Herd Immunity reduces the likelihood of a member of your “herd” from being infected. However people from outside are another question entirely. We live a in global community, and diseases that are rare in developed nations (because of vaccinations) still kill millions of people worldwide. Places like Africa and India are just a flight away. Sick people enter our populations all the time (google can find many news reports). As long as our immunity rates are high, the damage is minimized. Of course, those planes fly both directions, and many of the most beautiful and exciting destinations are also places where infectious diseases are still a problem. Herd Immunity will do nothing to help someone who leaves the herd.
Herd Immunity Requires Very High Immunity Rates
It is estimated that for Herd Immunity to be effective 90 to 95% of the population must be immune. Vaccinations, like all medical procedures, are not 100% effective, so there are a number of individuals who received the vaccinations, but lack immunity. As well, there are some people who cannot be vaccinated, the very young, the immune compromised, etc. Anyone who chooses to not give vaccinations increases the risk that Herd Immunity will fail.
To see how quickly this can happen, consider the false MMR – Autism scare caused by Andrew Wakefield (at best an incompetent and at worse a fraudulent doctor). He produced a study saying that the standard MMR (measles, mumps, and rubella) vaccine was linked to increased rates of Autism. Conveniently he had his own patented MMR vaccine ready to go as a replacement. Instead of causing a switch in the vaccines used, parents stopped having their children vaccinated. Vaccination rates fell from 92% to 80%. In 1998, the year Wakefield’s paper was published, there were 56 cases of measles. In 2006, there were 1348, including 2 deaths. Measles is now considered endemic in Britain.
Summary
Herd Immunity allows diseases to be eradicated from a population without requiring 100% immunisations. This is very useful for public health because 100% immunization is impossible. However it will not protect vulnerable individuals from catching a disease if exposed.