HIV Vaccines 101: How Vaccines Work

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Vaccines have proven to be the most effective means to prevent and even eradicate infectious diseases. Vaccines have safely and effectively prevented millions of illnesses, disabilities and deaths from smallpox, polio, measles, rabies, influenza, pneumonia, human papillomavirus (HPV) and many other diseases.

The development of a safe, effective and inexpensive preventive HIV vaccine is key to ending the global epidemic. Currently there is no preventive vaccine for the human immunodeficiency virus. This is because the virus rapidly mutates and has "unique ways of evading the immune system, and the human body seems incapable of mounting an effective immune response against it," according to the National Institutes of Health.

HIV is called a retrovirus because it has the unique ability to replicate itself as part of a host cell's DNA. Most viruses and bacteria are not nearly that diabolical and have rather predictable life cycles. That's why scientists were able to successfully create effective vaccines to target them.

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What Is an Infection?

An infection is the process in which bacteria, viruses, fungi, microbes and other microorganisms -- known as pathogens -- enter the body and overcome its natural defenses. These organisms will typically overcome your immune system by rapidly multiplying after attaching themselves to cells, crowding out cells, killing cells, releasing toxins and more. Infections have the potential to cause illness but not everyone will become sick.

Disease happens when the cells in your body are damaged and symptoms can be observed. Many of these symptoms of an infection -- such as fever, headache or coughing -- "result from the activities of the immune system trying to eliminate the infection from the body."

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Types of Infections

A localized infection is limited to only one part of the body. Some localized infections -- such as a cut on a finger that is infected with bacteria -- will not cause problems elsewhere. Localized infections that are internal -- such as appendicitis or pneumonia -- can be very serious or even fatal.

The most serious infections occur when microorganisms are spread throughout the body. These are known as systemic infections and are spread by the bloodstream. These include influenza, malaria, tuberculosis, sepsis, measles, hepatitis, HPV, HIV and more. Let's talk more about viruses.

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What Is a Virus?

A virus is the simplest and tiniest microbe. It usually ranges in size from about 20 to 400 nanometers -- one billionth of a meter -- and can only be observed with a microscope. It is an infectious agent that can only multiply in the living cells of plants, animals, humans, bacteria and fungi. Most biologists consider viruses not to be living because they lack a cellular structure and require a living host to replicate.

The human immunodeficiency virus is roughly spherical with a diameter of about 120 nanometers. That's about 60 times smaller than one red blood cell. It only takes one HIV particle to cause an infection.

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How Are Vaccines Developed?

A vaccine is a biological preparation of active, acquired immunity -- or protection -- to a certain disease. Most vaccines include an agent that resembles a disease-causing microorganism and contains little bits of the microbe that are weakened or dead. They are injected into your body to prepare your immune system for the real thing. Some vaccinations are only one injection. Sometimes several injections are needed. Some vaccinations require periodic "booster" shots -- often every five or ten years -- to jack up the immune system.

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Antibodies to the Rescue!

The introduction of the tiny portion of the germ inside your body triggers your immune system for a fight. Every human cell contains a code -- or a uniform -- that identifies it as a natural part of your body. Harmless bacteria will have a similar "uniform." The foreign invader -- in this case, the biologic agent in the vaccine -- will have an entirely different code.

Those foreign molecules are called antigens. Every microbe carries its own unique set of antigens, which are central to creating vaccines. The immune system detects the presence of the antigens and sounds an alarm that produces antibodies. Antibodies are Y-shaped proteins produced by plasma cells. The antibodies bind directly to the surface of the antigens to prevent them from entering or damaging healthy cells. Antibodies also work with other defensive cells to destroy the microbes.

One experimental HIV prevention strategy would be to infuse or inject potent antibodies that broadly neutralize many HIV subtypes.

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The cells that detected the presence of the antigens -- the invaders -- are known as memory B cells. Your body will continue to develop antibodies and memory B cells for several weeks after vaccination. The antibodies will gradually disappear.

The memory B cells will remain dormant for many years. If the microorganism ever returns, the "dormant memory cells will recognize it straight away, and rapidly start multiplying and developing into plasma cells. Because the plasma cells have already been trained to produce antibodies against the organism, they are able to produce a large number of antibodies very quickly (within hours)."

The infection will quickly be cleared. Hooray!

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Live Vaccines

Vaccines are made using several different processes. The vaccine may contain live viruses that have been weakened (attenuated) or altered so as not to cause any harm. These are fairly easy to create for certain viruses. Live viruses are engineered to create vaccines for measles, mumps, rubella (MMR combined vaccine), chickenpox, rotavirus and the nasal spray for influenza. It's more difficult to create live, attenuated vaccines for bacteria.

There are downsides. Live vaccines usually need to be refrigerated to stay potent. Also, "The remote possibility exists that an attenuated microbe in the vaccine could revert to a virulent form and cause disease," notes the Department of Health and Human Services' "[P]eople who have damaged or weakened immune systems -- because they've undergone chemotherapy or have HIV, for example -- cannot be given live vaccines."

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Inactivated Vaccines

One alternative to a live vaccine is an inactivated -- or killed -- vaccine. This is created by using heat, chemicals or radiation to inactivate the pathogen's ability to replicate. The pathogen remains "intact" so that the immune system can recognize it. This is safer and more stable than a live vaccine but stimulates a weaker immune response. Additional vaccinations and booster shots are usually necessary. Vaccines for polio and Hepatitis A are inactivated.

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Subunit Vaccines and Influenza

Subunit vaccines -- as the names suggests -- include only a small part of the target pathogen. One example would be the injection form of the influenza virus. About 36,000 Americans die from the flu every year and about 200,000 are hospitalized. The flu vaccine is recommended for everyone over six months old. It takes about two weeks to work and usually lasts for a year. A new vaccine is engineered for multiple strains each year. It is generally about 50 to 60 percent effective.

People living with HIV are at a higher risk for serious and more prolonged influenza-related complications, according to the Centers for Disease Control and Prevention. People living with HIV are strongly urged to get the inactivated, injection flu vaccine. "Persons with advanced HIV disease may have a poor immune response to vaccination. Therefore, pre-exposure chemoprophylaxis (use of antiviral medications to prevent influenza infection) may be considered for these patients if they are likely to be exposed to people with influenza."

Other subunit vaccines are genetically engineered, such as the Hepatitis B vaccine.

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