How Do Vaccines Work?

By Katharine Paljug @kpaljug
September 25, 2017

Getting vaccinated will protect you, your family, and your community. But how do vaccines work to prevent disease without making you sick?

To understand how vaccines protect you from disease, you first need to understand how your immune system works.

How do vaccines work?

When your body is infected by pathogens, such as bacteria or viruses, they attack your cells and organs, causing you to become sick. In response, your immune system fights against germs in three ways:

  • Macrophages, a type of white blood cell, consume and digest germs, leaving behind parts of the germs known as antigens.
  • Antibodies, which are produced by white blood cells, recognize and attack the antigens.
  • T-lymphocytes, a second type of white blood cell, attack infected cells to eradicate disease.


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After the disease is gone, your immune system retains some of the T-lymphocytes it produced. These cells remember the disease and, if it returns, will produce the correct antibodies to fight it.

Vaccines contain pathogens, or parts of pathogens, but these bacteria and viruses are dead or dying, so they are not strong enough to cause illness. However, your immune system still recognizes them as invasive germs and responds by creating T-lymphocytes and antibodies.

If your body is exposed to that disease in the future, your immune system already has the correct memory cells and is prepared to fight it, preventing you from becoming sick.

Vaccines allow your body to develop immunity without risking the dangerous and potentially deadly complications that come from actually catching a disease, such as hearing loss from meningitis or birth defects from rubella.

Are vaccines effective?

Vaccines are one of the most effective tools for reducing severe illness and death due to infection. According to the World Health Organization (WHO), only access to safe drinking water does more to eradicate disease around the world.

Vaccines not only keep individuals healthy but also help remove diseases from entire regions. When enough people in a community are vaccinated against a disease, pathogens are unable to spread easily from person to person and, as a result, the disease disappears from that community entirely. This is called “herd immunity.” Herd immunity is especially important for protecting those who are unable to be vaccinated, such as children with cancer, due to weakened immune systems.

One of the most dramatic examples of the effectiveness of vaccines is the global eradication of smallpox. Smallpox was a highly contagious disease that was fatal in 30 percent of cases. The WHO considered it “one of the world’s most devastating diseases.” After a global vaccination program in the twentieth century, however, smallpox was eradicated worldwide. The last recorded case of smallpox was in 1979.

Polio, which primarily affects children, is another disease that vaccines have nearly eradicated. Polio invades the nervous system and can cause paralysis or death. The WHO reports that vaccines have successfully decreased rates of polio infections 99 percent since 1988, preventing 1.5 million childhood deaths and 16 million cases of paralysis.

Though they have not eradicated all diseases, vaccines are effective at reducing the global burden of illness and preventing millions of deaths. The measles vaccine alone is expected to save 10.3 million lives between 2011 and 2020.

Are vaccines safe?

Vaccines undergo thorough testing and many clinical trials before they are approved to ensure that they are safe.

Because vaccines work by activating your immune system, they can have side effects. These include mild, cold-like symptoms, such as a low fever or runny nose. Some people experience soreness, swelling, or redness at the injection site, and young children may be fussier than normal. These side effects do not usually last for more than a day or two. If you or your child experience symptoms that you feel are more severe, including a fever over 104° F, call your doctor immediately.

Vaccines are safe for infants and, when received according to a proper schedule, cannot overload a baby’s immune system. Babies come into contact with millions of bacteria and germs every day. Even if babies receive multiple vaccines in a single day, the germs those vaccines contain are still only a fraction of the microorganisms their immune systems will encounter and fight off that day.

For most people, the disease-prevention benefits of being vaccinated outweigh any potential side effects. For some people, however, vaccines are not safe. Children and adults with compromised immune systems, such as those undergoing cancer treatments or bone marrow transplants, should not be vaccinated. It is also unsafe to receive a vaccine if you are allergic to one of the ingredients.

Certain vaccines should not be given to pregnant women or people who are currently ill. The United States Department of Health and Human Services provides information on the safety of individual vaccines. You can also ask your doctor if a vaccine is safe for you or your child.

Do vaccines cause autism?

In 1998, a study was published in the medical journal the Lancet alleging a link between the measles-mumps-rubella (MMR) vaccine and autism. However, there were many flaws in the study design, including a sample size of only 12 participants and conflicting financial interests that the authors had failed to disclose. The results were revealed to be highly speculative and fraudulent, and 10 of the 12 original authors issued a retraction, stating that “no causal link was established between MMR vaccine and autism as the data were insufficient.”

Though multiple studies have found that vaccines do not cause autism, the original study caused public panic over the safety of vaccines that decreased immunization rates in developed countries. This contributed to a resurgence of many preventable diseases, including whooping cough and measles epidemics in areas of the United States.

There is no evidence that any vaccines, including the MMR vaccine, cause autistic disorders.


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March 26, 2020

Reviewed By:  

Janet O’Dell, RN