A vaccine is a biological preparation that introduces antigenic components to the immune system so it can recognise and fight the corresponding pathogens without causing the disease. Vaccines confer active immunity by training the immune system to respond rapidly and effectively upon subsequent exposure.
Composition, mechanism and types of vaccines
Most vaccines consist of antigens derived from the target organism combined with stabilizers and often an adjuvant to enhance immunogenicity. When administered, antigen-presenting cells process the vaccine components and present them to T helper cells, which assist B cells in producing specific antibodies. Cytotoxic T cells may also be activated to destroy infected host cells. The immune system then generates memory lymphocytes that persist for years, providing durable protection. Traditional vaccine types include live attenuated vaccines, which use weakened pathogens that replicate briefly in the host; inactivated vaccines, containing killed microorganisms; and subunit vaccines made from purified proteins or polysaccharides. Conjugate vaccines link polysaccharide antigens to carrier proteins to improve immunogenicity in infants. Toxoid vaccines use inactivated toxins to prevent toxin-mediated diseases like tetanus and diphtheria. Newer platforms encompass mRNA vaccines and viral vector vaccines that deliver genetic instructions for antigen production within host cells. Vaccine development requires balancing safety and immunogenicity, and clinical trials and regulatory oversight ensure quality.
Examples of vaccines and global impact
The vaccinia-based smallpox vaccine eradicated smallpox by 1980. Inactivated poliovirus and oral live attenuated poliovirus vaccines have nearly eliminated poliomyelitis worldwide. The combined measles, mumps and rubella vaccine prevents these viral childhood illnesses and reduces complications such as encephalitis. Subunit hepatitis B vaccines protect against chronic liver disease and liver cancer. Human papillomavirus vaccines based on virus-like particles prevent infections with HPV types linked to cervical and other cancers. Seasonal influenza vaccines, updated annually to match circulating strains, reduce hospitalisations and deaths from influenza. During the COVID-19 pandemic, mRNA and adenoviral vector vaccines induced high levels of neutralising antibodies and substantially decreased the incidence of severe disease and death when deployed widely.
Vaccines remain among the most successful interventions in public health. By priming the immune system to respond rapidly to pathogens, they prevent illness, complications and transmission, saving millions of lives each year.
Related Terms: Immunization, Antigen, Adjuvant, Herd immunity, Booster dose