The challenge of HIV vaccines

Immunization of an animal or human being against an infectious agent provides levels of protection that are relative only. Protection can be overcome by high challenge doses. Even with a low challenge dose, protection can vary from absolute to relative. The invading pathogen may be neutralized and eliminated from the body before it can multiply; in this case, immunization provides protection against infection. More frequently, the infectious agent starts multiplying at the site of entry, which triggers an anamnestic immune response of the host. Rejection of the invader usually takes place before the appearance of overt clinical symptoms; in this case, immunization provides protection against disease. Protection against infection implies a strong pre-existing immune barrier. Protection against disease mostly relies on immune memory and the possibility for the host of mounting a rapid immune response against the infectious agent.