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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 too is immune to measles for the rest of his life.
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 4 million children each year, cause 48,000 children to be hospitalized, and kill 500.
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:
Watch a video about how the pertussis vaccine works in a community.
The immune system is composed of two parts: innate and adaptive.
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.
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, 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, does not extract 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 and the known time of exposure.
Of interest, a few vaccines induce a better immune response than natural infection:
So, in summary, a vaccine affords us protection with lesser quantities of virus or bacteria and the control of scheduling the exposure.
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.
View this video with a transcript
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 are immunized that even those who aren’t immunized 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 vaccinated. 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 (the “herd”) in this way depends upon a few things:
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.
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:
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:
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.