How Vaccines Work

A vaccination works by educating the immune system to identify and fight viruses or bacteria that cause disease. To do this, certain pathogen molecules must be injected into the body in order to stimulate an immune response. All viruses and bacteria include these molecules, which are referred to as antigens. These antigens can be safely injected into the body to help the immune system develop antibodies against them and memorize them for future use. Before the bacterium or virus can spread and infect people, the immune system will promptly recognize the antigens and launch an aggressive fight.

The Herd Immunity Imperative

Vaccines not only protect specific individuals, but also entire populations. Once enough people are immunized, the likelihood of a disease outbreak is so low that even those who aren't immune gain from it. In essence, a bacterium or virus will simply run out of viable hosts and eventually disappear. Herd immunity, often known as "community immunity," is a phenomena that has allowed once-devastating illnesses to be completely eradicated without the need for universal immunization.

Types of Vaccines

The secret to vaccines is introducing the antigens into the body without making the recipient ill. This can be accomplished in a number of ways, and each method results in a distinct kind of vaccination.

Live Attenuated Vaccines: For these types of vaccines, a weaker, asymptomatic form of the virus or bacteria is introduced into the body. Because it is weakened, the pathogen will not spread and cause sickness, but the immune system will still learn to recognize its antigens and know to fight in the future.

• Advantages: Because these vaccines introduce actual live pathogens into the body, it is an excellent simulation for the immune system. So live attenuated vaccines can result in lifelong immunity with just one or two doses.
• 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.

• Advantages: With these vaccines, the chance of an adverse reaction in the patient is much lower, because only a part or the original pathogen is injected into the body instead of the whole thing.
• Disadvantages: Identifying the best antigens in the pathogen for training the immune system and then separating them is not always possible. Only certain vaccines can be produced in this way.
• Specific Vaccines:
• Hepatitis B
• Influenza
• Haemophilus Influenzae Type B (Hib)
• Pertussis (part of DTaP combined immunization)
• Pneumococcal
• Human Papillomavirus (HPV)
• Meningococcal

Toxoid Vaccines: Some bacterial diseases damage the body by secreting harmful chemicals or toxins. For these bacteria, scientists are able to "deactivate" some of the toxins using a mixture of formaldehyde and water. These dead toxins are then safely injected into the body. The immune system learns well enough from the dead toxins to fight off living toxins, should they ever make an appearance.

• Specific Vaccines:
• Diphtheria
• Tetanus

Conjugate Vaccines: Some bacteria, like those of Hib disease, possess an outer coating of sugar molecules that camouflage their antigens and fool young immune systems. To get around this problem, scientists can link an antigen from another recognizable pathogen to the sugary coating of the camouflaged bacteria. As a result, the body's immune system learns to recognize the sugary camouflage itself as harmful and immediately attacks it and its carrier if it enters the body.

• Specific Vaccines:
• Haemophilus Influenzae Type B (Hib)

DNA Vaccines: Still in experimental stages, DNA vaccines would dispense with all unnecessary parts of a bacterium or virus and instead contain just an injection of a few parts of the pathogen's DNA. These DNA strands would instruct the immune system to produce antigens for combating the pathogen all by itself. As a result, these vaccines would be very efficient immune system trainers. They are also cheap and easy to produce.

• Specific Vaccines: DNA vaccines for influenza and herpes are currently in human testing phases.

Recombinant Vector Vaccines: These experimental vaccines are similar to DNA vaccines in that they introduce DNA from a harmful pathogen into the body, triggering the immune system to produce antigens and train itself to identify and combat the disease. The difference is that these vaccines use an attenuated, or weakened, virus or bacterium as a ride, or vector, for the DNA. In essence, scientists are able to take a harmless pathogen, dress it in the DNA of a more dangerous disease, and train the body to recognize and fight both effectively.

• Specific Vaccines: Recombinant vector vaccines for HIV, rabies, and measles are currently being developed.