is jenner's use of cowpox to protect against smallpox. a current example is the use of bcg vaccine made from mycobacterium bovis to protect against tuberculosis. genetic vaccine the subgroup of genetic vaccines encompass viral vector vaccines, rna vaccines and dna vaccines. viral vector main article: viral vector vaccine viral vector vaccines use a safe virus to insert pathogen genes in the body to produce specific antigens, such as surface proteins, to stimulate an immune response. rna main article: rna vaccine an mrna vaccine (or rna vaccine) is a novel type of vaccine which is composed of the nucleic acid rna, packaged within a vector such as lipid nanoparticles. among the covid-19 vaccines are a number of rna vaccines to combat the covid-19 pandemic and some have been approved or have received emergency use authorization in some countries. for example, the pfizer-biontech vaccine and moderna mrna vaccine are approved for use in adults and children in the us. dna main article: dna vaccine dna vaccination – the proposed mechanism is the insertion and expression of viral or bacterial dna in human or animal cells (enhanced by the use of electroporation), triggering immune system recognition. 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 potential advantage of dna vaccines is that they are very easy to produce and store. in august 2021, indian authorities gave emergency approval to zycov-d. developed by cadila healthcare, it is the first dna vaccine approved for humans. experimental electroporation system for experimental "dna vaccine" delivery many innovative vaccines are also in development and use. dendritic cell vaccines combine dendritic cells with antigens to present the antigens to the body's white blood cells, thus stimulating an immune reaction. these vaccines have shown some positive preliminary results for treating brain tumors and are also tested in malignant melanoma. recombinant vector – by combining the physiology of one micro-organism and the dna of another, immunity can be created against diseases that have complex infection processes. an example is the rvsv-zebov vaccine licensed to merck that is being used in 2018 to combat ebola in congo. 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. the use of plasmids has been validated in preclinical studies as a protective vaccine strategy for cancer and infectious diseases. however, in human studies, this approach has failed to provide clinically relevant benefit. the overall efficacy of plasmid dna immunization depends on increasing the plasmid's immunogenicity while also correcting for factors involved in the specific activation of immune effector cells. bacterial vector – similar in principle to viral vector vaccines, but using bacteria instead. antigen-presenting cell 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. valence 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. a multivalent or polyvalent vaccine is designed to immunize against two or more strains of the same microorganism, or against two or more microorganisms. the valency of a multivalent vaccine may be denoted with a greek or latin prefix (e.g., bivalent, trivalent, or tetravalent/quadrivalent). in certain cases, a monovalent vaccine may be preferable for rapidly developing a strong immune response. when two or more vaccines are mixed in the same formulation, the two vaccines can interfere. this most frequently occurs with live attenuated vaccines, where one of the vaccine components is more robust than the others and suppresses the growth and immune response to the other components. this phenomenon was first noted in the trivalent sabin polio vaccine, where the amount of serotype 2 virus in the vaccine had to be reduced to stop it from interfering with the "take" of the serotype 1 and 3 viruses in the vaccine. this phenomenon has also been found to be a problem with the dengue vaccines currently being researched, where the den-3 serotype was found to predominate and suppress the response to den-1, -2 and -4 serotypes. other contents this section is an excerpt from vaccine ingredients. graphic from the world health organization describing the main ingredients typically in vaccines a vaccine dose contains many ingredients, very little of which is the active ingredient, the immunogen. a single dose may have merely nanograms of virus particles, or micrograms of bacterial polysaccharides. a vaccine injection, oral drops or nasal spray is mostly water. other ingredients are added to boost the immune response, to ensure safety or help with storage, and a tiny amount of material is left-over from the manufacturing process. very rarely, these materials can cause an allergic reaction in people who are very sensitive to them. adjuvants main article: immunologic adjuvant vaccines typically contain one or more adjuvants, used to boost the immune response. tetanus toxoid, for instance, is usually adsorbed onto alum. this presents the antigen in such a way as to produce a greater action than the simple aqueous tetanus toxoid. people who have an adverse reaction to adsorbed tetanus toxoid may be given the simple vaccine when the time comes for a booster. in the preparation for the 1990 persian gulf campaign, the whole cell pertussis vaccine was used as an adjuvant for anthrax vaccine. this produces a more rapid immune response than giving only the anthrax vaccine, which is of some benefit if exposure might be imminent. preservatives vaccines may also contain preservatives to prevent contamination with bacteria or fungi. until recent years, the preservative thiomersal (a.k.a. thimerosal in the us and japan) was used in many vaccines that did not contain live viruses. as of 2005, the only childhood vaccine in the u.s. that contains thiomersal in greater than trace amounts is the influenza vaccine, which is currently recommended only for children with certain risk factors. single-dose influenza vaccines supplied in the uk do not list thiomersal in the ingredients. preservatives may be used at various stages of the production of vaccines, and the most sophisticated methods of measurement might detect traces of them in the finished product, as they may in the environment and population as a whole. many vaccines need preservatives to prevent serious adverse effects such as staphylococcus infection, which in one 1928 incident killed 12 of 21 children inoculated with a diphtheria vaccine that lacked a preservative. several preservatives are available, including thiomersal, phenoxyethanol, and formaldehyde. thiomersal is more effective against bacteria, has a better shelf-life, and improves vaccine stability, potency, and safety; but, in the u.s., the european union, and a few other affluent countries, it is no longer used as a preservative in childhood vaccines, as a precautionary measure due to its mercury content. although controversial claims have been made that thiomersal contributes to autism, no convincing scientific evidence supports these claims. furthermore, a 10–11-year study of 657,461 children found that the mmr vaccine does not cause autism and actually reduced the risk of autism by seven percent. excipients beside the active vaccine itself, the following excipients and residual manufacturing compounds are present or may be present in vaccine preparations: aluminum salts or gels are added as adjuvants. adjuvants are added to promote an earlier, more potent response, and more persistent immune response to the vaccine; they allow for a lower vaccine dosage. antibiotics are added to some vaccines to prevent the growth of bacteria during production and storage of the vaccine. egg protein is present in the influenza vaccine and yellow fever vaccine as they are prepared using chicken eggs. other proteins may be present. formaldehyde is used to inactivate bacterial products for toxoid vaccines. formaldehyde is also used to inactivate unwanted viruses and kill bacteria that might contaminate the vaccine during production. monosodium glutamate (msg) and 2-phenoxyethanol are used as stabilizers in a few vaccines to help the vaccine remain unchanged when the vaccine is exposed to heat, light, acidity, or humidity. thiomersal is a mercury-containing antimicrobial that is added to vials of vaccines that contain more than one dose to prevent contamination and growth of potentially harmful bacteria. due to the controversy surrounding thiomersal, it has been removed from most vaccines except multi-use influenza, where it was reduced to levels so that a single dose contained less than a microgram of mercury, a level similar to eating ten grams of canned tuna. nomenclature various fairly standardized abbreviations for vaccine names have developed, although the standardization is by no means centralized or global. for example, the vaccine names used in the united states have well-established abbreviations that are also widely known and used elsewhere. an extensive list of them provided in a sortable table and freely accessible is available at a us centers for disease control and prevention web page. the page explains that "the abbreviations this table (column 3) were standardized jointly by staff of the centers for disease control and prevention, acip work groups, the editor of the morbidity and mortality weekly report (mmwr), the editor of epidemiology and prevention of vaccine-preventable diseases (the pink book), acip members, and liaison organizations to the acip." some examples are "dtap" for diphtheria and tetanus toxoids and acellular pertussis vaccine, "dt" for diphtheria and tetanus toxoids, and "td" for tetanus and diphtheria toxoids. at its page on tetanus vaccination, the cdc further explains that "upper-case letters in these abbreviations denote full-strength doses of diphtheria (d) and tetanus (t) toxoids and pertussis (p) vaccine. lower-case "d" and "p" denote reduced doses of diphtheria and pertussis used in the adolescent/adult-formulations. the 'a' in dtap and tdap stands for 'acellular', meaning that the pertussis component contains only a part of the pertussis organism." another list of established vaccine abbreviations is at the cdc's page called "vaccine acronyms and abbreviations", with abbreviations used on u.s. immunization records. the united states adopted name system has some conventions for the word order of vaccine names, placing head nouns first and adjectives postpositively. this is why the usan for "opv" is "poliovirus vaccine live oral" rather than "oral poliovirus vaccine". licensing a vaccine licensure occurs after the successful conclusion of the development cycle and further the clinical trials and other programs involved through phases i–iii demonstrating safety, immunoactivity, immunogenetic safety at a given specific dose, proven effectiveness in preventing infection for target populations, and enduring preventive effect (time endurance or need for revaccination must be estimated). because preventive vaccines are predominantly evaluated in healthy population cohorts and distributed among the general population, a high standard of safety is required. as part of a multinational licensing of a vaccine, the world health organization expert committee on biological standardization developed guidelines of international standards for manufacturing and quality control of vaccines, a process intended as a platform for national regulatory agencies to apply for their own licensing process. vaccine manufacturers do not receive licensing until a complete clinical cycle of development and trials proves the vaccine is safe and has long-term effectiveness, following scientific review by a multinational or national regulatory organization, such as the european medicines agency (ema) or the us food and drug administration (fda). upon developing countries adopting who guidelines for vaccine development and licensure, each country has its own responsibility to issue a national licensure, and to manage, deploy, and monitor the vaccine throughout its use in each nation. building trust and acceptance of a licensed vaccine among the public is a task of communication by governments and healthcare personnel to ensure a vaccination campaign proceeds smoothly, saves lives, and enables economic recovery. when a vaccine is licensed, it will initially be in limited supply due to variable manufacturing, distribution, and logistical factors, requiring an allocation plan for the limited supply and which population segments should be prioritized to first receive the vaccine. world health organization vaccines developed for multinational distribution via the united nations children's fund (unicef) require pre-qualification by the