vaccination

Vaccination is the administration of a vaccine to stimulate the immune system to develop protection against a specific disease. Vaccines often mimic natural infection by exposing the body to components such as inactivated pathogens, attenuated organisms, toxoids, or genetic material, without causing disease in healthy individuals. This exposure trains the adaptive immune system to recognize and respond more rapidly upon subsequent encounters with the real pathogen.

Vaccines work by presenting antigens to immune cells, prompting the production of antibodies and the activation of T cells. Memory B and T cells then persist, providing faster and stronger responses if the pathogen is encountered again. Some vaccines require booster doses to achieve or maintain protective immunity.

Vaccine types include live-attenuated, inactivated, toxoid, and subunit or conjugate vaccines, as well as newer platforms such as mRNA, DNA, and viral-vector vaccines. Vaccines are designed with considerations of safety, immunogenicity, duration of protection, and practicality of delivery and storage.

Vaccine development proceeds through preclinical testing and phased clinical trials before regulatory approval. After licensure, post-marketing surveillance monitors safety and effectiveness. Regulatory standards cover manufacturing quality and cold-chain requirements. While most vaccines cause only mild side effects, rare serious adverse events can occur and are continuously evaluated.

Immunization programs reduce the burden of vaccine-preventable diseases, save lives, and contribute to herd immunity, which protects those who cannot be vaccinated. Global vaccination efforts face challenges such as access disparities, supply constraints, and misinformation. International organizations coordinate research, funding, and distribution to improve equity and outbreak preparedness.