How Vaccines Work

Vaccines provide a safe, cost-effective, and efficient means of preventing illness, disability, and death from infectious diseases. According to statistics, vaccines have saved millions of lives from infectious diseases caused by viruses and bacteria. Vaccines stimulate the immune system to produce immune responses that protect against infection.

Human immune mechanism

To understand how vaccines work, it helps to first look at how the body fights illness. When germs, such as bacteria or viruses, invade the body, they attack and multiply. This invasion, called an infection, is what causes illness. Each virus and bacterium trigger a unique response in the immune system involving specific cells in the blood, in the bone marrow and all over the body, called white cells. These white cells consist mainly of macrophages, B lymphocytes, and T lymphocytes.

Macrophages are a type of white blood cell that are part of your body's defense mechanism. They are made in your bone marrow. When a foreign invader, like bacteria, enters your bloodstream, macrophages secrete certain substances in a battle to help kill the bacteria.

B lymphocytes are defensive white blood cells. Once activated, these white blood cells produce antibodies.

T lymphocytes are another type of defensive white blood cell. These white blood cells kill the contaminated cells.

Principle of vaccine action

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. When a person is given the vaccine, their immune system recognizes the antigen as 'foreign'. This activates the immune system cells so that they kill the disease-causing virus or bacterium and make antibodies against it. 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.

Figure 1. Interaction between antibody and antigen

Herd immunization

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. This is especially important for those people who not only can't be vaccinated but may be more susceptible to the diseases we vaccinate against. No single vaccine provides 100% protection, and herd immunity does not provide full protection to those who cannot safely be vaccinated. But with herd immunity, these people will have substantial protection, thanks to those around them being vaccinated.

Figure 2. The herd immunization

Vaccinating not only protects yourself, but also protects those in the community who are unable to be vaccinated, such as infants and young children, the elderly and those with compromised immune systems. If you understand the importance of herd immunization, protect yourself and your loved ones from preventable diseases by getting vaccinated in a timely manner.

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