A vaccine is a biological preparation that trains the immune system to recognize particular targets and develop protection against them. Most vaccines prevent infectious diseases by presenting material from a pathogen, or instructions for producing that material, without exposing the recipient to the full risks of natural infection. Vaccines are important tools in medicine and public health. Some therapeutic vaccines instead stimulate immune responses against an existing condition, particularly cancer. (fda.gov)
Immunological basis
Vaccines stimulate adaptive immunity through exposure to an antigen: a substance recognized by the immune system. Depending on the vaccine, this may be a whole weakened or inactivated microorganism, a selected component, an inactivated toxin, or genetic instructions that allow the recipient’s cells to produce an antigen. The resulting response prepares the body to react more rapidly during subsequent exposure. (who.int)
B cells can produce antibodies that bind specific targets, while T cells contribute to immune coordination and, in some responses, destruction of infected cells. Immunological memory enables a faster response when the same antigen is encountered again. Vaccination produces active immunity, unlike passive protection supplied by transferred antibodies, including some monoclonal antibody products. Passive protection is generally temporary and does not establish the recipient’s own immune memory. (cdc.gov)
Protection is not necessarily immediate or lifelong. Some vaccines require several initial doses, and later booster doses can reinforce responses that decline over time. Age, maternal antibodies, underlying illness, antigen characteristics, and the route of administration can influence the response. (cdc.gov)
Major vaccine platforms
Vaccine platforms differ in how they deliver antigens and stimulate immunity.
- Live-attenuated vaccines contain weakened organisms capable of replication. Examples include measles, mumps, and rubella vaccines. They often produce strong immune responses, but their replicating organisms can pose risks in people with severe immunodeficiency.
- Inactivated vaccines contain organisms rendered unable to replicate. Examples include inactivated poliovirus and hepatitis A vaccines.
- Subunit vaccines contain selected pathogen components rather than whole organisms. These components can include proteins or polysaccharides; recombinant hepatitis B vaccines are an example.
- Conjugate vaccines link a polysaccharide antigen to a carrier protein, improving immune responses, especially in young children.
- Toxoid vaccines contain inactivated bacterial toxins. Tetanus and diphtheria vaccines induce protection against the toxins responsible for disease. (cdc.gov)
Viral-vector vaccines use a modified virus to deliver genetic instructions for an antigen. Nucleic-acid vaccines deliver DNA or messenger RNA encoding an antigen, which the recipient’s cells then produce. These approaches differ from directly administering the antigen itself. (who.int)
A formulation may also contain an adjuvant to enhance immune responses, stabilizers to preserve the product during storage, or preservatives to prevent contamination. Ingredients vary by product, and not every vaccine contains each of these components. (who.int)
Historical development
Before modern vaccination, variolation deliberately exposed people to smallpox material in an attempt to induce protection. It was practiced in several regions but could cause smallpox and transmit infection. In May 1796, Edward Jenner used material from a cowpox lesion to inoculate James Phipps and subsequently demonstrated protection against smallpox. The word vaccine derives from the Latin vacca, meaning “cow.” (who.int)
Later developments extended vaccination to other diseases and introduced inactivated, purified-component, and genetically engineered products. International vaccination campaigns, combined with surveillance and containment, culminated in the World Health Organization declaring smallpox eradicated in 1980. This was the first eradication of a human infectious disease worldwide. (who.int)
Development and evaluation
Development generally begins with laboratory research and preclinical testing. Human clinical studies then assess safety, dose, immune responses, and protection. Early studies involve relatively small groups; later studies typically enroll larger populations and compare outcomes between vaccine recipients and a control group. Randomized controlled trials are central to evaluating many vaccines. Regulatory review also examines manufacturing processes, quality, and consistency. (fda.gov)
Efficacy measures protection under controlled trial conditions, whereas effectiveness measures performance in real-world use. Both refer to specified outcomes: prevention of infection, symptomatic disease, hospitalization, or death can yield different estimates. An efficacy of 80%, for example, means an 80% lower risk of the measured outcome in the vaccinated group relative to the comparison group—not that 20% of recipients necessarily become ill. (who.int)
Safety and population protection
Vaccines can cause adverse reactions. Common reactions include temporary injection-site pain or fever; serious reactions are uncommon, and their nature and frequency depend on the product. Safety monitoring continues after authorization because rare events may not appear in clinical trials. A health event occurring after vaccination does not by itself establish that vaccination caused it; investigations assess alternative explanations and comparative evidence. (fda.gov)
Where vaccination reduces transmission, high population immunity can provide indirect protection to susceptible people. This effect, termed herd immunity, depends on the pathogen, vaccine performance, and distribution of immunity. Vaccination programmes also require reliable supply and a cold chain: controlled storage and transport conditions that preserve vaccine potency from manufacture to administration. Excessive heat, light, or inappropriate freezing can damage particular products. (who.int)