Passive follow-up		Relying on the reporting of cases by health-care providers to identify events of interest in clinical trials and determine outcomes in patients in clinical trials.
Percutaneous		An alternative to the generally preferred intradermal route for tuberculin skin testing or BCG vaccine administration. A device is used to make multiple shallow punctures in the skin and deposit PPD or BCG just below the skin's surface.
Phase I vaccine trial		A clinical trial with a small number (usually 20-30) of healthy volunteers, typically at low risk for infection. Phase I trials provide a first look at vaccine safety and immunogenicity in humans, and are also used to determine a safe dosage range.
Phase II vaccine trial		Controlled clinical study using larger numbers of volunteers to look for safety/risks associated with the test vaccine and to optimize and collect expanded information on its immunogenicity. Advanced Phase II trials can enroll volunteers with characteristics similar to participants in an efficacy (Phase III) trial and look for preliminary indications of efficacy. They enroll up to several hundred participants and generally have two or more arms.
Phase III vaccine trial		Large randomized, controlled study to further evaluate safety and determine the ability of a vaccine to protect against a given infection, disease, or other clinical condition at an optimally selected dose and schedule. Phase III trials seek sufficient information about safety and efficacy to evaluate the overall benefit-risk relationship of the vaccine. Phase III trials can include hundreds and even thousands of volunteers.
Phase IV vaccine trial		Post-marketing studies, commonly lacking control arms, to collect additional information about vaccines, including risks, benefits and optimal use.
Placebo		An inactive substance given to some study participants, while others receive the test substance (e.g. a vaccine). Placebos provide a basis for comparison. (See control).
PPD (purified protein derivative)		A commonly used, FDA-approved tuberculin skin-test preparation. A positive reaction is usually defined as one with a diameter of 5-9, 10-14, or >15 mm of induration, depending on the subject's risk factors for TB. (See Mantoux, induration.)
Pre-exposure vaccine		Vaccine intended to elicit immune response, and therefore provide protective immunity, prior to exposure to the pathogen. BCG is a pre-exposure vaccine, and most new vaccines in development are intended for pre-exposure use as well.
Prevalence		The percentage of a population affected with a particular disease at a given point in time.
Prime		The initial dose(s) of a vaccine. It is intended to induce an immune response and immune memory and may be followed by later vaccine dose(s) (booster).
Prime-boost		A vaccine regimen in which a primary series of vaccine injection(s) is followed later by booster injection(s) with the same or a different (heterologous prime-boost) vaccine preparation. A prime-boost combination may induce stronger or different types of immune responses from those seen with the primary immunization.
Proof of concept		A limited study, involving a relatively small number of participants, to prove that the core ideas of a proposed project or investigational vaccine are workable, feasible and promising enough to go into Phase I or more advanced (Phase III) development.
Protocol		The detailed plan for a clinical trial, outlining a number of parameters. These include rationale, purpose, statistical design and analysis considerations, safety monitoring and laboratory assays. The plan also outlines methodologies, such as vaccine dosages, routes of administration, length of study, eligibility criteria, handling of adverse events and outcome measures.
Post-exposure vaccine		Also called "immunotherapeutic vaccines," post-exposure vaccines are intended for use after the patient has come into contact with the pathogen. Their primary function is to boost the immune response; this may shorten the course of treatment or reduce the severity of disease. Most vaccines are pre-exposure, given prior to pathogen contact, but the current pipeline includes several with potential for use post-exposure.