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Understanding

How Vaccines WorkLast updated July 2011

Diseases that vaccines prevent can be

dangerous, or even deadly. Vaccines greatly

reduce the risk of infection by working with

the bodys natural defenses to safely develop

immunity to disease. This fact sheet explains

how the body fights infection and how

vaccines work to protect people by

producing immunity.

The Immune System

The Bodys Defense Against Infection

To understand how vaccines work, it is helpul to rst look at howthe body ghts illness. When germs, such as bacteria or viruses,invade the body, they attack and multiply. This invasion is calledan inection, and the inection is what causes illness. The immunesystem uses several tools to ght inection. Blood contains redblood cells, or carrying oxygen to tissues and organs, and whiteor immune cells, or ghting inection. These white cells consistprimarily o B-lymphocytes, T-lymphocytes, and macrophages:

Macrophages are white blood cells that swallow up and digestgerms, plus dead or dying cells. The macrophages leave behindparts o the invading germs called antigens. The body identiesantigens as dangerous and stimulates the body to attack them.

Antibodies attack the antigens let behind by the macrophages.Antibodies are produced by deensive white blood cells calledB-lymphocytes.

T-lymphocytes are another type o deensive whiteblood cell. They attack cells in the body that have already beeninected.

The rst time the body encounters a germ, it can take several daysto make and use all the germ-ghting tools needed to get over theinection. Ater the inection, the immune system rememberswhat it learned about how to protect the body against that disease.

The body keeps a ew T-lymphocytes, called memory cells that gointo action quickly i the body encounters the same germ again.When the amiliar antigens are detected, B-lymphocytes produceantibodies to attack them.

How Vaccines WorkVaccines help develop immunity by imitating an inection. Thistype o inection, however, does not cause illness, but it does causethe immune system to produce T-lymphocytes and antibodies.Sometimes, ater getting a vaccine, the imitation inection can causeminor symptoms, such as ever. Such minor symptoms are normaland should be expected as the body builds immunity.

Once the imitation inection goes away, the body is let with asupply o memory T-lymphocytes, as well as B-lymphocytes thatwill remember how to ght that disease in the uture. However, ittypically takes a ew weeks or the body to produce T-lymphocytesand B-lymphocytes ater vaccination. Thereore, it is possible thata person who was inected with a disease just beore or just atervaccination could develop symptoms and get a disease, because thevaccine has not had enough time to provide protection.

Types of Vaccines

Scientists take many approaches to designing vaccines. These

approaches are based on inormation about the germs (viruses

or bacteria) the vaccine will prevent, such as how it inects

cells and how the immune system responds to it. Practical

considerations, such as regions o the world where the vaccine

would be used, are also important because the strain o a

virus and environmental conditions, such as temperature

and risk o exposure, may be diferent in various parts o the

world. The vaccine delivery options available may also difer

geographically. Today there are ve main types o vaccines that

inants and young children commonly receive:

Live, attenuated vaccines ght viruses. These vaccines contain aversion o the living virus that has been weakened so that it doesnot cause serious disease in people with healthy immune systems.Because live, attenuated vaccines are the closest thing to anatural inection, they are good teachers or the immune system.Examples o live, attenuated vaccines include measles, mumps,

For more information on vaccines,

vaccine-preventable diseases, and

vaccine safety:

http://www.cdc.gov/vaccines/conversations

http://www.cdc.gov/vaccines/conversationshttp://www.cdc.gov/vaccines/conversations

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| Types of Vaccines | continued

and rubella vaccine (MMR) and varicella (chickenpox) vaccine.Even though these vaccines are very eective, not everyone canreceive them. Children with weakened immune systemsorexample, those who are undergoing chemotherapycannot getlive vaccines.

Inactivated vaccines also ght viruses. These vaccines are made by

inactivating, or killing, the virus during the process o makingthe vaccine. The inactivated polio vaccine is an example o thistype o vaccine. Inactivated vaccines produce immune responsesin dierent ways than live, attenuated vaccines. Oten, multipledoses are necessary to build up and/or maintain immunity.

Toxoid vaccines prevent diseases caused by bacteria that producetoxins (poisons) in the body. In the process o making thesevaccines, the toxins are weakened so they cannot cause illness.Weakened toxins are called toxoids. When the immune systemreceives a vaccine containing a toxoid, it learns how to ght othe natural toxin. The DTaP vaccine contains diphtheria andtetanus toxoids.

Subunit vaccines include only parts o the virus or bacteria, orsubunits, instead o the entire germ. Because these vaccinescontain only the essential antigens and not all the othermolecules that make up the germ, side eects are less common.The pertussis (whooping cough) component o the DTaP vaccineis an example o a subunit vaccine.

Conjugate vaccines ght a dierent type o bacteria. These bacteriahave antigens with an outer coating o sugar-like substances

called polysaccharides. This type o coating disguises the antigen,making it hard or a young childs immature immune system torecognize it and respond to it. Conjugate vaccines are eectiveor these types o bacteria because they connect (or conjugate) thepolysaccharides to antigens that the immune system respondsto very well. This linkage helps the immature immune systemreact to the coating and develop an immune response. Anexample o this type o vaccine is the Haemophilus infuenzaetype B (Hib) vaccine.

Vaccines Require More Than One DoseThere are our reasons that babiesand even teens or adults or that

matterwho receive a vaccine or the rst time may need morethan one dose:

For some vaccines (primarily inactivated vaccines), the rstdose does not provide as much immunity as possible. So, morethan one dose is needed to build more complete immunity. Thevaccine that protects against the bacteria Hib, which causesmeningitis, is a good example.

In other cases, such as the DTaP vaccine, which protects againstdiphtheria, tetanus, and pertussis, the initial series o our shotsthat children receive as part o their inant immunizations helpsthem build immunity. Ater a while, however, that immunitybegins to wear o. At that point, a booster dose is needed tobring immunity levels back up. This booster dose is needed at 4years through 6 years old or DTaP. Another booster against these

diseases is needed at 11 years or 12 years o age. This booster orolder childrenand teens and adults, toois called Tdap.

For some vaccines (primarily live vaccines), studies have shownthat more than one dose is needed or everyone to develop thebest immune response. For example, ater one dose o the MMRvaccine, some people may not develop enough antibodies toght o inection. The second dose helps make sure that almosteveryone is protected.

Finally, in the case o the fu vaccine, adults and children (olderthan 6 months) need to get a dose every year. Children 6 monthsthrough 8 years old who have never gotten the fu vaccine in thepast or have only gotten one dose in past years need two dosesthe rst year they are vaccinated against fu or best protection.Then, annual fu shots are needed because the disease-causingviruses may be dierent rom year to year. Every year, the fuvaccine is designed to prevent the specic viruses that expertspredict will be circulating.

The Bottom LineSome people believe that naturally acquired immunityimmunityrom having the disease itselis better than the immunity

provided by vaccines. However, natural inections can cause severecomplications and be deadly. This is true even or diseases that mostpeople consider mild, like chickenpox. It is impossible to predictwho will get serious inections that may lead to hospitalization.

Vaccines, like any medication, can cause side eects. The mostcommon side eects are mild. However, many vaccine-preventabledisease symptoms can be serious, or even deadly. Although manyo these diseases are rare in this country, they do circulate aroundthe world and can be brought into the U.S., putting unvaccinatedchildren at risk. Even with advances in health care, the diseases thatvaccines prevent can still be very serious and vaccination is the

best way to prevent them.

.Adapted from the National Institute of Allergy and

Infectious Diseases, Understanding Vaccines

http://www.niaid.nih.gov/topics/vaccines/

For more information on vaccines call 800-CDC-INFO

(800-232-4636) or visit http://www.cdc.gov/vaccines.

http://www3.niaid.nih.gov/topics/vaccines/http://www.cdc.gov/vaccineshttp://www.cdc.gov/vaccineshttp://www3.niaid.nih.gov/topics/vaccines/