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How Vaccines Work

Infections are unpredictable and can have long-term consequences. Even mild or symptom-less infections can be deadly. Vaccines help the body learn how to defend itself from disease without the dangers of a full-blown infection.78

The Body’s Defense Against Infection

To understand how vaccines work, it is helpful 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 is called an infection, and the infection is what causes illness. The immune system uses several tools to fight infection.79

Click on the "components of the immune system" to learn more:

  • are defensive white blood cells within the body that produce antibodies.79

    • The antibodies attack the antigens that were left behind by the macrophages.79
  • are white blood cells that swallow up and digest any germs, dead or dying cells within the body.79

    • During digestion, the macrophages leave behind parts of the invading germs called antigens. The body identifies antigens as dangerous and stimulates the body to attack them.79
  • are another type of defensive white blood cell.79

    • They attack cells in the body that have already been infected.79


The first time the body encounters a germ, it can take several days to make and use all the germ-fighting tools needed, as described above, to get over the infection. After the infection, the immune system remembers what it learned about how to protect the body against that disease.80

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Vaccines Work by Imitating an Infection

Vaccinations work by imitating a bacteria or virus, eliciting the body’s natural defense against infection. This mechanism allows the body to produce antibodies to help remember how to fight that disease in the future.  Vaccines use very small amounts of antigens to help the immune system recognize and learn to fight serious diseases without overloading the immune system.80

Vaccines Strengthen the Body’s Natural Defenses

A person who is immune can resist the bacteria or viruses that cause a disease. The CDC defines being immune as being partially or fully resistant to a specific infectious disease or disease-causing organism.81 Building that immunity can either be passive or activenatural or vaccine induced. Active immunity takes longer to develop but lasts longer than passive immunity, while passive immunity provides protection that is immediate but fades within weeks or months.81


Active immunity comes from being exposed to a disease-causing organism.81

    • Natural immunity results from being infected directly by a disease-causing organism, whether the infection is symptomatic or not.81
    • Vaccine-induced immunity results from being exposed to killed or weakened bacteria or viruses through vaccination.81


Passive immunity is provided by antibodies produced by another human being or animal.81

    • Full-term babies acquire passive immunity from their mother’s antibodies during the final months of pregnancy.81
    • Patients can acquire passive immunity through antibody-containing blood products derived from human or animal sources.81

To read more about the different types of immunity please reference the CDC- Immunity Types webpage.82