Vaccine Science: Vaccines and the Immune System

How do vaccines work?

The story of Chip and Dale

To understand how vaccines work you need to understand the story of two 5-year-old children, Chip and Dale.


Chip plays with a child in his class who has measles. Ten days later, Chip develops high fever, runny nose, "pink eye” and a rash. The rash consists of red bumps that start on his face and work their way down to the rest of his body. After two more days, Chip starts to have trouble breathing. His breaths are short and rapid. Chip's mother takes him to the doctor where he gets an X-ray of his chest. The X-ray shows that Chip has pneumonia (a common complication of measles infection). Chip is admitted to the hospital where he stays for five days and finally recovers. After having fought off his measles infection, Chip will never get measles again. Or, said another way, Chip has immunity to measles. Chip is immune to measles because he has cells in his body that can make "antibodies" to measles virus. These cells, called "memory B cells,” developed during the infection and will hang around for the rest of Chip's life.


Dale also plays with the child who has measles. However, Dale never develops symptoms of measles. He doesn't get fever, rash or pneumonia. Dale was infected with measles virus, but didn't get any of the symptoms of measles. This is called an "asymptomatic infection.” Because Dale, like Chip, also develops “memory B cells,” he is also immune to measles for the rest of his life.

The difference between Chip and Dale

Whereas Chip had to pay a high price for his immunity, Dale didn't. Dale was lucky. Although some children don't get severe infections when they are exposed to measles, most do. Before a measles vaccine was developed in 1963, measles would infect about 3-4 million children each year in the United States, causing about 48,000 children to be hospitalized, about 1,000 to suffer encephalitis (swelling of the brain), and about  400-500 to die.

Vaccines take the luck out of it

By also causing "asymptomatic infections,” vaccines mimic what happened to Dale. This allows children to benefit from the natural immunity that comes with infection without having to suffer the severe, and occasionally fatal, consequences of natural infection.

Learn how vaccines cause immunity without causing disease.

Vaccines remove the element of luck by controlling:

  • The potential severity of the pathogen
  • The dose of the exposure (smallest amount needed)
  • The timing of exposure (before the period of highest risk)

Watch a video about how the pertussis vaccine works in a community.

What is the difference between “specific” and “nonspecific” immunity?

The immune system is composed of two parts: innate (which provides nonspecific immunity) and adaptive (which provides specific immunity).

The innate immune system is the fast-acting first line of defense and affords nonspecific immunity through both physical and chemical means. Examples of physical components of innate immunity include our skin and nasal hairs. Chemical components include things like acids in the stomach, enzymes in sweat and saliva, and inflammatory responses that cause heat, redness, swelling and pain locally. These defense responses represent “nonspecific” immunity because they occur regardless of the type of threat being posed.

The adaptive immune system takes longer to activate, but responds specifically to an invading pathogen. The immune responses are “specific” for the virus or bacteria posing the threat. Antibodies generated by B cells are examples of the adaptive immune response; they will only work on the pathogen against which they were generated. Immunological memory, in which our bodies remember previous pathogens and activate more rapidly, is also an example of adaptive immunity. Vaccines employ the adaptive immune system to protect us from future encounters with viruses and bacteria.

Is natural infection better than immunization?

It is true that natural infection almost always causes better immunity than vaccines. Whereas immunity from disease often follows a single natural infection, immunity from vaccines usually occurs only after several doses. However, the difference between vaccination and natural infection is the price paid for immunity.

The price paid for immunity after natural infection might be pneumonia from chickenpox (varicella), mental retardation from Haemophilus influenzae type b (Hib), pneumonia from pneumococcus, birth defects from rubella, liver cancer from hepatitis B virus, or death from measles.

Immunization with vaccines, like natural infections, induces long-lived immunity, but unlike natural infection, immunization does not exact such a high price for immunity.

If you could see the world from the perspective of your immune system, you would realize that where the virus or bacteria comes from is irrelevant. Your immune system “sees” something that is foreign, attacks it, disables it and then adds it to the memory bank, so it can react more quickly the next time it encounters it.

The differences between a vaccine and getting the disease naturally are the dose, the known time of exposure, and the potential severity of the pathogen.

  • Dose - When someone is exposed to viruses or bacteria naturally, the dose is often larger, so the immune response that develops will typically be greater (as will the symptoms). However, when scientists are designing vaccines, they determine the smallest amount of virus or bacteria needed to generate a protective immunologic response.
  • Time of exposure – Most of the time, we do not know when we are exposed to viruses and bacteria; however, when we take our children to the doctor’s office for a vaccine, we do know. In essence, we are controlling their exposure to the viruses or bacteria that the vaccines protect against because we know when and where they occur. In contrast, and more typical of the norm, we don’t know what viruses or bacteria they might be exposed to from the door knob to the doctor's office, the books in the waiting room, or the toddler at the restaurant we go to after the office visit.
  • Potential severity of the pathogen – When someone gets sick from exposure to a pathogen in the community, called natural infection, they can experience mild illness or become very sick or even die. On the other hand, vaccination provides immunity without having to suffer the severe, and occasionally fatal, consequences of natural infection.

Of interest, a few vaccines induce a better immune response than natural infection:

  • Human papillomavirus (HPV) vaccine −The high purity of the specific protein in the vaccine leads to a better immune response than natural infection.
  • Tetanus vaccine − The toxin made by tetanus is so potent that the amount that causes disease is actually lower than the amount that induces a long-lasting immune response. This is why people with tetanus disease are still recommended to get the vaccine.
  • Haemophilus influenzae type b (Hib) vaccine − Children younger than 2 years of age do not typically make a good response to the complex sugar coating (polysaccharide) on the surface of Hib that causes disease; however, the vaccine links this polysaccharide to a helper protein that creates a better immune response than would occur naturally. Therefore, children younger than 2 years of age who get Hib disease are still recommended to get the Hib vaccine.
  • Pneumococcal vaccine − This vaccine works the same way as the Hib vaccine to create a better immune response than natural infection.

So, in summary, a vaccine affords us protection with lesser quantities of virus or bacteria, the control of scheduling the exposure, and the knowledge that immunity will be gained without experiencing severe illness.

Watch as Dr. Offit talks about natural infection and vaccination in the short video below, part of the Talking About Vaccines with Dr. Paul Offit series.

What is herd immunity?

One of the contested concepts in vaccines these days is the idea of immunizing one’s children to protect others. People fearful about the safety of vaccines feel they may be risking the health of their children for someone else’s benefit. So, exactly what is herd immunity?

Herd immunity occurs when so many people in a community (the “herd”) are immunized or immune from previous infection that even those who aren’t immune won’t get a disease. Herd immunity is really a numbers game: The more people who are protected from the disease, the less viruses and bacteria are able to spread through the community and infect those who aren’t immune. Vaccination plays an important role in the development of herd immunity because it allows large numbers of community members to become immune without getting ill. In fact, a study done by Susan Van den Hoff and colleagues in The Netherlands, published in 2002, found that unimmunized people in a highly immunized community are less likely to catch measles than immunized people in a community with lower immunization rates. That’s because no vaccine is 100 percent effective, and the more likely you are to be exposed to a virus (by living in a relatively unvaccinated community), the more likely you are to get sick.

The ability to protect community members in this way depends upon a few things:

  • Ability of the disease to spread: Each disease has a certain degree of contagiousness. Some, like measles, are highly contagious. If 10 susceptible people are in an elevator with someone who has measles, nine of them will get measles. In fact, the virus may remain in the air for up to two hours, potentially infecting other susceptible people who might enter the elevator. Other diseases, while contagious, are not as easily spread. Public health officials determine contagiousness by studying susceptible family members or other contacts of the person with the disease. The more contagious a disease, the greater number of people that need to be protected for herd immunity to apply.
  • Effectiveness of the vaccine: Not all vaccines are created equal. Some vaccines protect virtually everyone who gets them after one or two doses, whereas other vaccines require more than two doses. Further, a handful of people who get the vaccine never generate a strong enough immune response to be protected. A less effective vaccine requires more people to have received it to account for those who are not effectively protected.
  • Number of susceptible people in the herd: Some members of the community cannot get a vaccine for medical reasons, such as allergy to a vaccine component; medical conditions, such as cancers or immune deficiencies; or current medical treatments that weaken their immunity, such as steroid treatments. Others may choose not to get a vaccine, also increasing the number of susceptible herd members. The higher the number of susceptible people in the community, the less likely the disease will be stopped.

Because immunization rates in the U.S. have been high for many vaccines such as measles, mumps, rubella, Haemophilus influenzae type b (Hib), and polio, among others, our communities have enjoyed herd immunity. Most of us have not seen these diseases in recent years. In fact, in many of our communities, this still holds true. Unfortunately, however, recent concerns about the safety of vaccines in certain communities have created pockets of susceptible people throughout the country.

Is there a difference between vaccination and immunization?

Although we commonly use the words vaccination and immunization interchangeably, they are not exactly the same.

Vaccination was first coined as a term when Edward Jenner used cowpox to immunize people against smallpox. The word vaccination comes from the Latin word vaccinae meaning “of the cow.”

Immunization means immunity induced by a biological agent. The word immunization comes from the Latin word immunes, referring to “a group of soldiers who once having fought and survived a battle never had to fight again.” In our society, immunity has come to mean freedom from anything burdensome; in the case of vaccines, our children are the soldiers; the vaccines are the battle, and the freedom gained is from disease.

There are two forms of immunization:

  1. Active immunization means administering a vaccine, so that the recipient generates their own immune response. Receipt of hepatitis B vaccine is an example of active immunization.
  2. Passive immunization means administering antibodies or antitoxins from another source to protect the recipient. Antibodies passed from mother to child through breast milk are an example of passive immunization.

Vaccines and antibiotics

Vaccines and antibiotics are two of the most powerful tools against bacterial infections. Vaccines work by preventing infections, whereas antibiotics work by treating them. Antibiotics were first discovered in the early 1900s when a drug called sulfanilamide was found to protect people from fatal bacterial infections such as pneumococcus. Pneumococcus causes pneumonia, bloodstream infections and meningitis.

Perhaps the most well-known antibiotic is penicillin. By the 1940s penicillin could be produced in large quantities and was recognized as an easy way to save people from disease and death caused by pneumococcus. Doctors believed that they could eliminate pneumococcus with these new tools; thus interest in learning more about preventing pneumococcus by vaccine waned.

Dr. Robert Austrian was a physician who continued studying pneumococcal infections, first in New York and later throughout the country. His studies showed that while people treated with penicillin were less likely to die from their infections, pneumococcus was still infecting as many people as it did before penicillin was available. He also found that people who got the most severe infections with pneumococcus died regardless of whether or not they were treated with antibiotics. The only way to protect them would be to prevent their infections in the first place; that is, to immunize them.

While Dr. Austrian was completing his studies, people were continuing to be infected and treated for pneumococcal infections using antibiotics, such as penicillin. Antibiotics resolve infections by stopping the bacteria from reproducing themselves.

However, bacteria respond in a "survival of the fittest" manner. These antibiotic-resistant bacteria continue to reproduce and can be passed on to others.

Antibiotic resistance was discovered shortly after penicillin came into popular use. By 1967, pneumococcal strains resistant to penicillin began to appear. At first, the answer to antibiotic resistance was simple. If the bacteria resisted penicillin, use another antibiotic. However, by the 1980s and 1990s, as pneumococcus and other bacteria became more resistant to antibiotics, doctors were running out of options. Indeed, today there are strains of bacteria for which no existing antibiotic will work. Although scientists continue to research, design and test new antibiotics, the process is slow and expensive.

In the case of pneumococcus, we have had some reprieve — Dr. Austrian's first vaccine became available in the late 1970s and a second version, still used for adults today, was introduced in 1983. Although Dr. Austrian's vaccine worked in adults, it did not work well in young children. In 2000, another version that works better in children also became available. As more people in the community are immunized against a particular type of bacteria, such as pneumococcus, there are fewer opportunities for the bacteria to reproduce and infect others. Thus, vaccines have become one of our most important tools in the fight against antibiotic resistance.

To stem resistance, in addition to getting vaccines, people should:

  • Only use antibiotics when ill with a bacterial infection. Antibiotics are not effective against colds or flu caused by viruses. Overuse of antibiotics can lead to increasingly resistant bacteria that can cause untreatable infections in that person or be passed on to others.
  • Do not use old antibiotics or those prescribed for others, as they may not be as effective at treating your current infection; the drugs may allow the bacteria to continue replicating and spreading to others.

Reviewed by Paul A. Offit, MD on April 22, 2020

Materials in this section are updated as new information and vaccines become available. The Vaccine Education Center staff regularly reviews materials for accuracy.

You should not consider the information in this site to be specific, professional medical advice for your personal health or for your family's personal health. You should not use it to replace any relationship with a physician or other qualified healthcare professional. For medical concerns, including decisions about vaccinations, medications and other treatments, you should always consult your physician or, in serious cases, seek immediate assistance from emergency personnel.