The Biology of Vaccines, Vaccination, and Immunity

Injection NeedleDespite continued resistance to the medical procedure, vaccines work to save children and adults alike from unnecessary and highly preventable death. The science behind vaccines is both well established and fairly well understood. Vaccination works by priming the immune system to fight viruses, bacteria, and other microbes without exposure to the wild microorganisms. Within the immune system, white blood cells work to remember harmful viruses, bacteria, and other microbes while fighting off the invaders. When the white blood cells first encounter a microorganism, the immune system creates a memory of the best way in which to destroy the invader. If the body ever comes into contact with that specific invader again, the immune system quickly and easily destroys the threat.

The memory created by white blood cells is immunity. Two types of immunity exist: passive immunity and active immunity. Passive immunity occurs when immunity from one person passes to another. The most common form is mother-to-baby passive immunity. Unfortunately, passive immunity is temporary and less effective at fighting off disease. Active immunity occurs when the immune system encounters a virus, bacterium, or other microbe. Unlike passive immunity, active immunity is strong, effective, and often life-long.

In addition to exposure to wild viruses, bacteria, and other microbes, active immunity also forms via vaccination. Natural immunity is active immunity acquired through exposure to disease. Vaccine-acquired immunity is active immunity obtained through vaccination. Natural immunity and vaccine-acquired immunity are largely equal. However, in some cases, natural immunity lasts longer than vaccine-acquired immunity (thus the need for booster shots). In other cases, vaccine-acquired immunity is stronger than natural immunity (tetanus vaccine, Hib vaccine).

Vaccines are categorized into two types: live attenuated and inactivated. Live attenuated vaccines contain live, albeit weaken, microbes. Inactivated vaccines contain dead microbes. Different diseases need different vaccine types to work most effectively. The MMR (measles, mumps, rubella) vaccine is a live attenuated vaccine. The hepatitis B, polio, and pneumococcal vaccines are inactivated vaccines. Live attenuated vaccines tend to provide stronger immunity in smaller amounts and fewer doses than inactivated vaccines, which explains the need for multiple doses of many common vaccines. Other types of vaccines include subunit vaccines such as the influenza vaccines, toxoid vaccines such as the diphtheria vaccine, and conjugate vaccines such as the Hib vaccine.

Vaccines work by creating an immune response without exposure to a wild virus, bacterium, or other microbe. Unlike the diseases prevented by vaccines, vaccines are extremely safe and cause very few side effects. Vaccines undergo years of testing before introduction to the public. The safety of vaccines then undergoes continued safety evaluation as researchers and vaccine producers seek to create better (safer and more effective) vaccines. Vaccines also offer the added benefit of creating an immune response without causing the harmful and potentially deadly side effects of the diseases.

In addition to protecting the individual who received the vaccine, vaccines also work to protect the population by creating herd immunity. Herd immunity occurs when a certain percentage of the population becomes immune to an illness. Even if the disease enters the population, individuals who are not immune are protected by herd immunity because the disease cannot find enough non-immune hosts to infect. Different diseases require different percentages for herd immunity. Measles requires 83% for herd immunity. Pertussis (whooping cough) requires 92%. Herd immunity is especially important for high risk individuals such as the very young and people who cannot receive vaccines for legitimate medicals reasons.

As amazing as vaccines are, vaccination is not a medical miracle that are 100% effective in 100% of the population 100% of the time. Vaccines do not and cannot cause disease. In general, however, vaccinated individuals are less likely to contract a disease prevented by vaccination. Vaccinated individuals also generally contract milder forms of the disease. Vaccines do reduce the risk of many serious illnesses.


Herlihy, Stacy Mintzer & E. Allison Hagood. 2012. Your baby’s best shot: Why vaccines are safe and save lives. Rowman & Littlefield Publishers, Inc.: Lanham.

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