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Vaccine Currently, there are no HIV vaccines are approved for use outside of clinical trials. Vaccine development involves using a disease-causing agent (e.g., virus, bacterium) to stimulate the immune system to recognize the agent and launch a protective response. Once the immune system recognizes the virus or bacterium and develops antibodies, vaccination (injecting a person with a vaccine) establishes immunity to the disease.

A vaccine that protects people from infection (i.e., people who are HIV-negative) is called a preventative vaccine and a vaccine that prevents disease in infected people (i.e., people who are HIV-positive) is called a therapeutic vaccine. In some cases, people who have been repeatedly exposed to HIV remain HIV-negative or never develop AIDS. These people may have developed natural immunity to the human immunodeficiency virus (i.e., their immune system recognized HIV and their immune response successfully eliminated the virus). The fact that a person can be exposed to HIV and never develop HIV/AIDS indicates that the immune system is capable of successfully responding to HIV infection.

Some studies have shown that patients who are infected with HIV-2 may be less likely to become infected with HIV-1. In these cases, HIV-2, which is weaker than HIV-1, may be acting like a vaccine and instructing the immune system to recognize and mount an immune response to the more potent HIV-1.

Vaccine Types

There are three types of vaccine:

• Subunit vaccines are made up of parts of the virus. This is the safest type of vaccine because it does not contain infectious material.
• Killed whole-virus vaccines are whole viruses that have been inactivated so that they are no longer infectious. These vaccines often cause side effects associated with infection.
• Live vaccines are made of viruses that have been weakened so that they are much less infectious. These vaccines are the least safe because they may cause rather than prevent disease, especially in people with compromised immune systems.

Experimental Vaccines
Once vaccines have been developed in the laboratory and tested in animal models, they are tested in humans in a series of clinic trials. It takes approximately 15 years to complete all 3 phases of testing and receive approval from the Food and Drug Administration (FDA).

Clinical trials are scientific studies that test the safety and effectiveness of certain treatments (e.g., medications). Vaccines are subjected to the following clinical trials:

• Phase I: Studies involve fewer than 20–80 healthy volunteers. Safety, dosage, the immune response, and immunization schedule are determined. This phase may take 1–2 years to complete.
• Phase II: Studies involve 100–300 volunteers, composed of low-risk and high-risk people in the population where the Phase III trial is expected to occur. Safety, efficacy, dosage, and the immunization schedule are evaluated. This phase takes at least 2 years to complete.
• Phase III: Studies involve 1000–3000 high-risk volunteers in areas where HIV is spreading. Safety and efficacy are tested. This phase takes 3 or more years to complete.

In most cases, it takes another 1–2 years for the FDA to review and approve (or not approve) the vaccine.

The first HIV vaccine clinical trial began in 1987, and since then, development of a safe, effective HIV vaccine has become a worldwide priority. In July of 2005, the National Institute of Allergy and Infectious Diseases (NIAID), a component of the National Institutes of Health (NIH), announced funding to establish the Center for HIV/AIDS Vaccine Immunology (CHAVI) to help accelerate vaccine development.

Developing an HIV/AIDS Vaccine

Scientists face the following challenges when designing and developing a safe, effective HIV/AIDS vaccine:

• The immune system response to HIV is not well understood.
• There is no ideal animal model to help scientists study the immune system response.
• The many modes of transmission and forms of HIV must be understood and targeted to develop a safe, effective vaccine.
• HIV is constantly mutating.
• HIV slowly works into immune system cells, where it may reside for years before affecting health.

Despite these challenges, the history of vaccine development provides encouragement:

• Several vaccines have been developed in the past without a complete understanding of the immune response (e.g., whooping cough).
• Several vaccines have been developed without the use of an ideal animal model (e.g., measles, mumps, rubella). The less than ideal SIV (simian immunodeficiency virus) monkey model has provided important, relevant information about HIV.
• Vaccines can be combined (e.g., measles, mumps, rubella). The complex immune response to HIV could be targeted by more than one vaccine.
• Scientists developed an effective vaccine for hepatitis B, which is also transmitted in many different ways.
• Scientists have developed vaccines for other slow-acting viruses (e.g., feline leukemia virus, equine infectious anemia virus).
• Vaccines can control viruses even after a person has been infected for years (e.g., measles).

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