Vaccines are dead or inactivated organisms or purified products derived from them.
There are several types of vaccines currently in use.1 These represent different strategies used to try to reduce risk of illness, while retaining the ability to induce a beneficial immune response.
Vaccines containing killed microorganisms – these are previously virulent micro-organisms which have been killed with chemicals or heat. Examples are vaccines against flu, cholera, bubonic plague, polio and hepatitis A.
Some vaccines contain live, attenuated virus microorganisms. These are live micro-organisms that have been cultivated under conditions that disable their virulent properties, or which use closely-related but less dangerous organisms to produce a broad immune response. They typically provoke more durable immunological responses and are the preferred type for healthy adults. Examples include yellow fever, measles, rubella, and mumps. The live Mycobacterium tuberculosis vaccine developed by Calmette and Guérin is not made of a contagious strain, but contains a virulently modified strain called “BCG” used to elicit immunogenicity to the vaccine.
Toxoids – these are inactivated toxic compounds in cases where these (rather than the micro-organism itself) cause illness. Examples of toxoid-based vaccines include tetanus and diphtheria. Not all toxoids are for micro-organisms; for example, Crotalus atrox toxoid is used to vaccinate dogs against rattlesnake bites.
Protein subunit – rather than introducing an inactivated or attenuated micro-organism to an immune system (which would constitute a “whole-agent” vaccine), a fragment of it can create an immune response. Characteristic examples include the subunit vaccine against Hepatitis B virus that is composed of only the surface proteins of the virus (produced in yeast) and the virus-like particle (VLP) vaccine against human papillomavirus (HPV) that is composed of the viral major capsid protein.
Conjugate – certain bacteria have polysaccharide outer coats that are poorly immunogenic. By linking these outer coats to proteins (e.g. toxins), the immune system can be led to recognize the polysaccharide as if it were a protein antigen. This approach is used in the Haemophilus influenzae type B vaccine.
A number of innovative vaccines are also in development and in use:
– Recombinant Vector – by combining the physiology of one micro-organism and the DNA of the other, immunity can be created against diseases that have complex infection processes.
– DNA vaccination – in recent years a new type of vaccine called DNA vaccination, created from an infectious agent’s DNA, has been developed. It works by insertion (and expression, triggering immune system recognition) of viral or bacterial DNA into human or animal cells. Some cells of the immune system that recognize the proteins expressed will mount an attack against these proteins and cells expressing them. Because these cells live for a very long time, if the pathogen that normally expresses these proteins is encountered at a later time, they will be attacked instantly by the immune system. One advantage of DNA vaccines is that they are very easy to produce and store. As of 2006, DNA vaccination is still experimental.
– T-cell receptor peptide vaccines are under development for several diseases using models of Valley Fever, stomatitis, and atopic dermatitis. These peptides have been shown to modulate cytokine production and improve cell mediated immunity.
– Targeting of identified bacterial proteins that are involved in complement inhibition would neutralize the key bacterial virulence mechanism 2.
While most vaccines are created using inactivated or attenuated compounds from micro-organisms, synthetic vaccines are composed mainly or wholly of synthetic peptides, carbohydrates or antigens.
Vaccines may be monovalent (also called univalent) or multivalent (also called polyvalent). A monovalent vaccine is designed to immunize against a single antigen or single microorganism.3 A multivalent or polyvalent vaccine is designed to immunize against two or more strains of the same microorganism, or against two or more microorganisms. 4 In certain cases a monovalent vaccine may be preferable for rapidly developing a strong immune response. 5
1.The Main Types of Vaccines, National Network for Immunization Information) Copyright 2000, and the National Immunization Program of the Centers for Disease Control and Prevention (CDC).
2.Meri S, Jördens M, Jarva H. Microbial complement inhibitors as vaccines. Vaccine. 2008 Dec 30;26 Suppl 8:I113-7. Review.PMID: 19388175
3.Monovalent at Dorland’s Medical Dictionary
4.Polyvalent vaccine at Dorland’s Medical Dictionary
5.Questions and answers on monovalent oral polio vaccine type 1 (mOPV1) “Issued jointly by WHO and UNICEF””. http://www.pediatriconcall.com/fordoctor/medical_original_articles/oral_polio_vaccine.asp.
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