A vaccine is a biological preparation that stimulates an individual’s immune system to recognize and fight specific infectious agents, such as bacteria or viruses, without causing the disease itself. Vaccines typically contain small amounts of the target pathogen or components of the pathogen, which are harmless or inactive but sufficient to induce an immune response.

When a person is vaccinated, their immune system recognizes the vaccine components as foreign and produces specific immune responses, including the production of antibodies and the activation of certain immune cells. This process helps the immune system “remember” the pathogen, so if the person encounters the actual infectious agent in the future, their immune system can respond more quickly and effectively to neutralize or eliminate the pathogen and prevent disease.

There are several types of vaccines, which can be broadly categorized as follows:

  1. Live-attenuated vaccines: These vaccines contain live, weakened forms of the pathogen that are unable to cause disease in healthy individuals. Examples include the measles, mumps, and rubella (MMR) vaccine and the oral poliovirus vaccine (OPV).
  2. Inactivated vaccines: These vaccines contain whole pathogens that have been killed or inactivated, so they cannot cause disease. Examples include the inactivated poliovirus vaccine (IPV) and the whole-cell pertussis vaccine.
  3. Subunit, recombinant, or conjugate vaccines: These vaccines contain purified or recombinant components of the pathogen, such as proteins or polysaccharides, which are sufficient to elicit an immune response. Examples include the hepatitis B vaccine, human papillomavirus (HPV) vaccine, and Haemophilus influenzae type b (Hib) vaccine.
  4. Toxoid vaccines: These vaccines contain inactivated toxins (toxoids) produced by certain bacteria, which stimulate an immune response against the toxin rather than the bacterium itself. Examples include the diphtheria and tetanus vaccines.
  5. mRNA vaccines: These vaccines contain small pieces of the pathogen’s genetic material (mRNA) that encode specific proteins or antigens. Once inside the body, the mRNA is translated by host cells to produce the antigen, which then triggers an immune response. Examples include the Pfizer-BioNTech and Moderna COVID-19 vaccines.
  6. Viral vector vaccines: These vaccines use a harmless, non-replicating virus to deliver a gene encoding an antigen from the target pathogen into host cells. The host cells then produce the antigen, which induces an immune response. Examples include the Oxford-AstraZeneca and Johnson & Johnson COVID-19 vaccines.

Vaccines have been one of the most successful public health interventions in history, leading to the eradication or control of many infectious diseases, such as smallpox, polio, and measles. Vaccination programs not only protect individuals from severe disease but can also contribute to herd immunity, reducing the overall spread of infection within a community and protecting vulnerable individuals who cannot be vaccinated for medical reasons.