AIDS (ACQUIRED IMMUNE DEFICIENCY SYNDROME)

What is a vaccine?

A vaccine is a medical product designed to stimulate your body's immune system in order to prevent or control an infection. An effective preventive vaccine trains your immune system to fight off a particular microorganism so that it can't establish a serious infection or make you sick.

What is the difference between a preventive HIV vaccine and a therapeutic HIV vaccine?

Therapeutic HIV vaccines are designed to control HIV infection in people who are already HIV positive (see Therapeutic HIV Vaccines Fact Sheet). Preventive HIV vaccines are designed to protect HIV negative people from becoming infected or getting sick. This fact sheet focuses on preventive HIV vaccines. Although there is currently no vaccine to prevent HIV, researchers are developing and testing potential HIV vaccines. The goal is to develop a vaccine that can protect people from HIV infection, or at least lessen the chance of getting HIV or AIDS should a person be exposed to the virus.

How does a preventive vaccine work?

When your body encounters a microorganism, your immune system mounts an attack on the invader. After the microorganism is defeated, your immune system continues to "remember" how to quickly beat the invader should it try to infect you again. A vaccine is designed to resemble a real microorganism. The vaccine trains your immune system to recognize and attack the real microorganism should you ever encounter it. If you've received an effective vaccine, your immune system will "remember" how to quickly attack and defeat a particular microorganism for many years.

What are the different types of vaccine?

There are three main types of vaccines that are being studied for the prevention of HIV infection and AIDS:

• Subunit vaccines, also known as "component" or "protein" vaccines, contain only individual parts of HIV, rather than the whole virus. Instead of collecting these parts from the virus itself, the HIV subunits are made in the laboratory using genetic engineering techniques. These man-made subunits alone—without the rest of the virus—can prompt the body to produce an anti-HIV immune response, although that response may be too weak to actually protect against future HIV infection.

• Recombinant vector vaccines take advantage of non-HIV viruses that either don't cause disease in humans or have been deliberately weakened so that they can't cause disease. These weakened (attenuated) viruses are used as vectors, or carriers, to deliver copies of HIV genes into the cells of the body. Once inside cells, the body uses the instructions carried in the copies of HIV genes to produce HIV proteins. As with subunit vaccines, these HIV proteins can stimulate an anti-HIV immune response. Most of the recombinant vector vaccines for HIV deliver several HIV genes (but not the complete set) and may therefore create a stronger immune response.

Some of the virus vectors being studied for HIV vaccines include ALVAC (a canarypox virus), MVA (a type of cowpox virus), VEE (a virus that normally infects horses), and adenovirus-5 (a human virus that doesn't usually cause serious disease) based vectors.

• DNA vaccines also introduce HIV genes into the body. Unlike recombinant vector vaccines, DNA vaccines do not rely on a virus vector. Instead, "naked" DNA containing HIV genes is injected directly into the body. Cells take up this DNA and use it to produce HIV proteins. As with subunit and recombinant vector vaccines, the HIV proteins trigger the body to produce an immune response against HIV.