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EFFICACY: (Of a drug or treatment). The maximum ability of a drug or treatment to produce a result regardless of dosage. A drug passes efficacy trials if it is effective at the dose tested and against the illness for which it is prescribed. In the procedure mandated by the Food and Drug Administration, phase II clinical trials gauge efficacy, phase III trials confirm it.

ELISA: (Enzyme-Linked Immunosorbent Assay). A laboratory test to determine the presence of antibodies to HIV in the blood. A positive ELISA test generally is confirmed by the Western Blot test. See also Antibodies; Western Blot.

END-STAGE DISEASE: Final period or phase in the course of a disease leading to a person's death.

ENVELOPE: In virology, a protein covering that packages the virus's genetic information. The outer coat, or envelope, of HIV is composed of two layers of fat-like molecules called lipids taken from the membranes of human cells. Embedded in the envelope are numerous cellular protein, as well as mushroom-shaped HIV proteins that protrude from the surface. Each mushroom is thought to consist of a cap made of four glycoprotein molecules called gp120 and a stem consisting of four gp41 molecules embedded in the envelope. The virus uses these proteins to attach to and infect cells. See also Glycoprotein; gp41; gp120; Lipid.

ENZYME: A protein that accelerates a specific chemical reaction without altering itself (i.e., a catalyst).

EPIDEMIC: A disease that spreads rapidly through a demographic segment of the human population, such as everyone in a given geographic area, a military base, or similar population unit, or everyone of a certain age or sex, such as the children or women of a region. Epidemic diseases can be spread from person to person or from a contaminated source such as food or water.

EPIDEMIOLOGY: The branch of medical science that deals with the incidence, distribution and control of a disease in a population.

EXCLUSION/INCLUSION CRITERIA: The medical or social standards determining whether a person may or may not be allowed to enter a clinical trial. For example, some trials may not include people with chronic liver disease, or may exclude people with certain drug allergies; others may exclude men or women or only include people with a lowered T-cell count.

EXPANDED ACCESS: A general term for methods of distributing experimental drugs to patients who are unable to participate in ongoing clinical trials and have no other treatment options. Specific types of expanded access mechanisms include parallel track, Treatment IND, and compassionate use. See also Investigational New Drug.

EXPRESSION SYSTEM: In HIV vaccine production, cells into which an HIV gene has been inserted to produce desired HIV proteins.