How do vaccines work?

What is a vaccine? A vaccine is a biological preparation that improves immunity to a particular disease. It contains an agent resembling a disease-inducing microorganism– a bacterium, virus or toxin – that activates the body’s immune system. White blood cells – APCs, B cells, and T-cells – recognize, destroy and “remember” this version of the pathogen. That way, the immune system can quickly recognize and destroy this harmful microorganism later on. A vaccine is essentially a pathogen-imposter.

Today, there are five main types of vaccines. Live, attenuated vaccines fight viruses and contain a weakened version of the living virus (e.g., measles-mumps-rubella and varicella vaccine). Inactivated vaccines also fight viruses and contain the killed virus (e.g., polio vaccines). Toxoid vaccines prevent diseases caused by bacteria that produce toxins in the body and contain weakened toxins (e.g., diphtheria and tetanus vaccine). Subunit vaccines include only the essential antigens of the virus or bacteria (e.g. whooping cough vaccine). Conjugate vaccines fight a different type of bacteria which have antigens with an outer coating of sugar-like substances (polysaccharides) that “hide” the antigen from the child’s immature immune system; the vaccine connects (conjugates) the polysaccharides to antigens, so the immune system can react.

A vaccine is essentially a pathogen-imposter.

Once the altered pathogen is introduced into the bloodstream, it is captured by antigen-presenting cell (APC), which float around looking for invaders. When an APC detects the vaccine antigen, it ingests it, breaks it apart, and displays a piece of the antigen on its surface. Then, it travels to areas where immune cells cluster (e.g. lymph nodes) and where so-called naïve T cells specific to the antigen recognize it as foreign and become activated. These T helper cells alert other nearby cells. Naïve B cells recognize the antigens carried by the APCs as well and also become activated.

Some naïve B cells mature into plasma B cells after activation by vaccine antigens and reception of signals from activated helper T cells. They produce antibodies which are “y” shaped proteins that are released at high levels every second. Each antibody tightly attaches to a specific target antigen (like a lock and a key), which can prevent the antigen from entering a cell or mark the antigen for destruction. If the vaccine contains weakened viruses, they enter the cells which are then killed by Killer T cells. What follows is the development of memory (B, T helper and killer T) cells that memorize the vaccine antigen and recognize the real pathogen in the future.

This means that the body’s response will be stronger and faster than if it had never encountered the pathogen before. This is called a secondary response to the pathogen. Furthermore, secondary responses will result in the production of more antibodies to fight the pathogen and more memory cells to identify it promptly. Thus, vaccinations “program” the immune system to remember a specific disease-inducing microorganism by letting it “practice” with a weakened, killed or inactivated version of the pathogen.

Vaccines can prevent outbreaks of contagious diseases through herd immunity (or community immunity). This means that a sufficient portion of the population must be immune to an infectious disease (by vaccination and/or prior illness), so that the disease is less likely to spread from one person to another. As the number of vaccinated people increases, the protective effect of herd immunity increases as well. While the herd immunity threshold may start with 40% of the population vaccinated for some diseases, most diseases require vaccination rates as high as 80% – 95% to prevent outbreaks. Moreover, herd immunity protects those who cannot be vaccinated or for whom the vaccination was not successful, such as people with weak immune systems, chronic illnesses or allergies.

Evidence suggests that the  Chinese used smallpox inoculation as early as 1000 BC

Vaccinations are essentially prophylactic, although there has been an effort in recent years to develop therapeutic vaccinations for infectious diseases like AIDS, tuberculosis, cancer, and various autoimmune diseases. There are also potential vaccinations in development for myasthenia gravis, lupus and diabetes, as well as for cognitive diseases such as Alzheimer’s disease, prion diseases and Huntington’s disease.

Usually, vaccinations are administered in the form of an injection into the skin or a liquid taken orally. However, some vaccinations may also be processed by inhalation through mouth/nose or application onto the skin. The vaccines risks are very low. Most vaccine reactions are usually minor and temporary (i.e. sore arm, fatigue or an elevated temperature). Very serious side effects like severe allergic reactions are extremely rare and are carefully monitored and investigated. The vaccine benefits definitely outweigh the vaccine dangers. In fact, it is far more likely to be seriously harmed by a vaccine-preventable disease than by the vaccine itself.

In recent years, the anti-vaccination movement has been claiming that there is a link between vaccinations and autism. The reason for these claims is a 1998 study, which suggested that the measles-mumps-rubella (MMR) vaccine might cause autism. Its publication started a panic among parents that led to dropping vaccination rates, resulting in subsequent outbreaks of vaccine-preventable diseases. However, this study turned out to be seriously flawed, and the paper was even retracted by the journal that published it. There is absolutely no evidence of a link between vaccines and autism or autistic disorders.

Sources:

• http://www.cdc.gov/vaccines/hcp/patient-ed/conversations/downloads/vacsafe-understand-color-office.pdf
• http://www.historyofvaccines.org/content/how-vaccines-work
• http://www.who.int/topics/vaccines/en/
• http://www.historyofvaccines.org/content/herd-immunity-0
• http://www.immunize.org/timeline/
• http://www.historyofvaccines.org/content/timelines/all