Preexposure

Preexposure refers to actions, measures, or interventions taken before an anticipated exposure to a hazard, risk, or pathogen, with the aim of reducing the likelihood or severity of subsequent harm. The concept appears across medicine, public health, and safety, and it often emphasizes preparedness and proactive risk management.

In public health and medicine, preexposure prophylaxis (PrEP) is a well-known application of preexposure strategy for HIV prevention. PrEP involves taking antiretroviral medications by people at substantial risk of exposure to HIV before potential contact with the virus. When used with high adherence, PrEP can substantially lower the risk of acquiring HIV during sexual exposure or among people who inject drugs. Practitioners typically pair PrEP with regular HIV testing, screening for sexually transmitted infections, kidney function monitoring, and ongoing risk-reduction counseling. Dosing options include daily regimens and event-driven (intermittent) regimens for certain populations, reflecting adherence patterns and exposure risk.

In immunology and vaccinology, preexposure can describe deliberate exposure to an antigen through vaccination to prime the immune system. Vaccines introduce components of a pathogen (or a safe mimic) to stimulate memory B and T cells, enabling a faster and more effective response upon real exposure. This preexposure priming underpins protection against many communicable diseases and is a central goal of immunization programs.

Beyond clinical use, preexposure concepts also inform occupational health and disaster preparedness, where training, protective equipment, and plans are implemented ahead of potential hazards. Limitations of preexposure strategies include imperfect protection, the need for ongoing monitoring, access and equity considerations, and the potential for drug resistance or adverse effects if not implemented with appropriate medical oversight.