Acid-fast bacilli (AFB)		Bacteria that remain stained even after they have been washed in an acid solution. They may be detected under a microscope in a stained smear. (Staining is a procedure whereby cells are treated with a dye to make cell structures visible under a microscope.) Mycobacterium tuberculosis, M. bovis and its attenuated substrain, Bacille Calmette-Guérin (BCG), are examples of AFBs.
Acquired immune deficiency syndrome (AIDS)		The late stage of the illness triggered by infection with the human immunodeficiency virus (HIV). AIDS manifestations include severe deterioration of the immune system and a susceptibility to a range of opportunistic infections, including tuberculosis. The latter is associated with advanced immune deficiency.
Active follow-up		Direct contact with, or re-examination of, a clinical trial participant at set intervals during a trial.
Adenovirus delivery system		Use of an attenuated adenovirus as a vector for vaccine delivery.
Adjuvant		In vaccines, a substance added to a vaccine formulation to enhance or modify the immune response to the vaccine in recipients.
Adverse event or adverse reaction		In a clinical trial, any abnormality in a participant that emerges or worsens relative to the participantŐs pre-vaccination baseline. It may or may not be caused by the study vaccine and can range from mild events such as fever, rash and sore arm, to serious events resulting in hospitalization or death.
Antibody (also called immunoglobulin)		An infection-fighting protein created by the immune system and present in blood or secretory fluids. Serum antibodies are produced by B-cells (humoral immunity) and recognize, coat and mark for immune destruction, or render harmless, foreign matter such as bacteria, viruses and toxins. Antibodies also tag microbe-infected cells, making them vulnerable to attack by the immune system (cell-mediated immunity). Generally, each antibody binds to a single specific chemical sequence (epitope) in an antigen.
Antigen		A component of a microbe or other foreign substance, usually a protein or carbohydrate, which stimulates a response by the immune system. Antigens often contain multiple sub-units (epitopes) that are the targets of different elements of the immune response (specific antibodies or T-cell lymphocytes).
Assay		A laboratory test to determine the amount of a particular constituent of a mixture or the biological or pharmacological potency of a drug or vaccine. It is also a method for measuring immune response to a vaccine.
Attenuated bacterium or virus		A weakened strain of a microbe that can no longer produce disease. Attenuated viruses or bacteria are often used as vaccines because, although they can no longer produce disease, they may still stimulate a strong immune response. Examples include the current TB vaccine, BCG (see BCG), and measles, mumps, rubella, varicella and the Sabin oral vaccine against polio.