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Explanation:
  A vaccine works by training the immune system to recognize and combat pathogens, either viruses or bacteria. To do this, certain molecules from the pathogen must be introduced into the body to trigger an immune response.
These molecules are called antigens, and they are present on all viruses and bacteria. By injecting these antigens into the body, the immune system can safely learn to recognize them as hostile invaders, produce antibodies, and remember them for the future. If the bacteria or virus reappears, the immune system will recognize the antigens immediately and attack aggressively well before the pathogen can spread and cause sickness.
Vaccines don't just work on an individual level, they protect entire populations. Once enough people are immunized, opportunities for an outbreak of disease become so low even people who aren't immunized benefit. Essentially, a bacteria or virus simply won't have enough eligible hosts to establish a foothold and will eventually die out entirely. This phenomenon is called "herd immunity" or "community immunity," and it has allowed once-devastating diseases to be eliminated entirely, without needing to vaccinate every individual.
This is critical because there will always be a percentage of the population that cannot be vaccinated, including infants, young children, the elderly, people with severe allergies, pregnant women, or people with compromised immune systems. Thanks to herd immunity, these people are kept safe because diseases are never given a chance to spread through a population.
Public health officials and scientists continue to study herd immunity and identify key thresholds, but one telling example is the country of Gambia, where a vaccination rate of just 70% of the population was enough to eliminate Hib disease entirely.
For example, in 1997, prominent medical journal The Lancet published research claiming to have found a link between the measles vaccine and autism. As a result, in following years the parents of over a million British children decided not to vaccinate their kids. The research has since been thoroughly debunked, but the number of measles cases has skyrocketed, from just several dozen a year in 1997 to over 2,000 cases in 2011. Similar outbreaks have occurred throughout the United States, involving both measles and whooping cough, with doctors and officials blaming low rates of vaccination.
Types of Vaccines
The key to vaccines is injecting the antigens into the body without causing the person to get sick at the same time. Scientists have developed several ways of doing this, and each approach makes for a different type of vaccine.
Disadvantages: Because they contain living pathogens, live attenuated vaccines are not given to people with weakened immune systems, such as people undergoing chemotherapy or HIV treatment, as there is a risk the pathogen could get stronger and cause sickness. Additionally, these vaccines must be refrigerated at all times so the weakened pathogen doesn't die.
Specific Vaccines:
Measles
Mumps
Rubella (MMR combined vaccine)
Varicella (chickenpox)
Influenza (nasal spray)
Rotavirus
Inactivated Vaccines: For these vaccines, the specific virus or bacteria is killed with heat or chemicals, and its dead cells are introduced into the body. Even though the pathogen is dead, the immune system can still learn from its antigens how to fight live versions of it in the future.
Advantages: These vaccines can be freeze dried and easily stored because there is no risk of killing the pathogen as there is with live attenuated vaccines. They are also safer, without the risk of the virus or bacteria mutating back into its disease-causing form.
Disadvantages: Because the virus or bacteria is dead, it's not as accurate a simulation of the real thing as a live attenuated virus. Therefore, it often takes several doses and "booster shots" to train the body to defend itself.
Specific Vaccines:
Polio (IPV)
Hepatitis A
Rabies
Subunit/conjugate Vaccines: For some diseases, scientists are able to isolate a specific protein or carbohydrate from the pathogen that, when injected into the body, can train the immune system to react without provoking sickness.
