Vaccines are biological preparations, often made from attenuated or killed forms of microorganisms or fractions thereof.
They work by stimulating the immune system to produce antibodies and cells directed against a particular organism, mimicking "natural infection".
Based on their biological and chemical characteristics, vaccines can be categorized in two basic types, "Live-attenuated" (bacterial or viral) vaccines and "inactivated" or "non-live" vaccines.
Examples of live attenuated vaccines include: Measles, Mumps and Rubella, Varicella, Yellow fever, Oral polio (OPV), rotavirus, the "nasal-spray" Live Attenuated Influenza (LAIV) vaccine and BCG.
Attenuation results in micro-organisms that may still infect and multiply in humans, but they usually do not cause severe disease. These vaccines are usually associated to life-long immunity.
Inactivated or non-live vaccines include those against hepatitis A, influenza-virus, Bordetella pertussis, rabies-virus or polysaccharide vaccines directed against encapsulated bacteria (Haemophilus influenzae type b, Streptococcus pneumoniae, Neisseria meningitidis A, C W, Y (not group B)).
Most non-live vaccines induce a less robust immune response compared to live vaccines. They generally require additional doses ("boosters") to maintain long-term protective immunity.
There are many other subcategories of these basic groups, like subunit vaccines, whole cell vaccines, toxoid vaccines, polysaccharide vaccines, recombinant protein vaccines, mucosal vaccines, mRNA or DNA and vector vaccines.