How Vaccination Works
Vaccination is a very simple concept that has historically been of enormous benefit all over the world. It is largely considered to be the most effective, and least expensive, method of preventing the occurrence and spread of infectious diseases.
The concept revolves around the fact that the immune system, having been once exposed to a given pathogen, can develop what is called a memory response – the next time the immune system encounters that pathogen, it is able to eliminate it so quickly that the individual may never experience any disease symptoms.
Vaccination takes advantage of this ability to develop memory. The administration of a vaccine derived from a specific pathogen causes the immune system to develop a memory response against it – and when the individual encounters the pathogen itself, they will not develop an infection.
When it comes to debilitating or lethal diseases such as polio and smallpox, vaccination has been of great significance. Polio is all but eradicated (only 1,300 cases were reported worldwide in 2007), and in the case of smallpox, the disease has been eliminated – the last cases of smallpox occurred in 1978. The only remaining samples of the virus exist in just two laboratories worldwide (the Centers for Disease Control and Prevention in the United States and the State Research Center of Virology and Biotechnology in Russia).
Types of Vaccines
There are three general vaccine categories: live, inactivated, and subunit vaccines.
Live vaccines are simply that – a culture of a living bacteria or virus. The pathogen is attenuated—weakened—so that it is unable to cause disease in the vaccine recipient. Despite its weakened state, the organism is able to live long enough to replicate briefly, allowing the immune system time to develop a strong memory response.
Live vaccines are slightly riskier than the other two types, as there is a very small risk that the attenuated organism will revert to its pathogenic state. It must be stressed, however, that this risk is very small – in the case of the live polio virus, for example, the chance of this occurring is around 1 in 750,000. Live vaccines are generally unable to be given to people with compromised immune systems, as even a weakened form of a pathogen can cause disease in someone whose immune system does not function properly.
Inactivated vaccines typically include either killed bacteria or virus particles. These are safer than live vaccines, as there is no living material in the vaccine – however the immune response they elicit often tends to be weaker than those elicited by a live vaccine.
Subunit vaccines contain purified proteins derived from a bacteria or virus culture. These do not contain whole organisms – only whole proteins, or protein segments. Subunit vaccines have one particular weakness, which is that the proteins purified from a pathogen culture may become denatured. If this occurs, the denatured proteins have a different form compared to the same proteins in their proper form in the live organism. The difference in shape can mean that antibodies elicited to the denatured vaccine proteins will not bind to the same protein when it is part of the whole organism.