A suspension of killed or modified live virus or bacterium being injected in to the body to stimulate immunity against the pathogen, is called vaccine. The process of injecting a vaccine in to the body is known as vaccination. Vaccinated body starts to produce antibodies against the antigen (virus or bacterium) present in the vaccine. The antibodies act as a defense system and give protection to the body against the particular pathogen. This type of immunity is called acquired immunity.

Vaccines are used to prevent certain infectious diseases. Eg. Vaccines for hepatitis, herpes, measles, polio, tuberculosis, malaria, leprosy, rheumatic fever etc.

Vaccines are generally divided in to four types: Killed vaccines, Live attenuated vaccines, Toxoids, Vaccines with specific extracted antigen.

Killed bacteria used for vaccination are called killed vaccines. TAB vaccine is a heat killed vaccine. It is available in the trade names Typhoral and Typhium-VI. It is used to prevent typhoid fever. TAB vaccine is a mixture of heat killed Salmonella typhi, S.paratyphi-A and S.paratyphi-B in 4:3:3 ratio. The bacteria are killed by exposing the cultures to 60°C for 30 minutes. The killed bacteria are mixed together and treated with 5% phenol. The resulting suspension is used for vaccination. Some mild pathogenic bacteria or viruses are made less virulent and used for vaccination. Such vaccines are called live attenuated vaccines.

Pathogenic microbes can be attenuated by freeze drying or chemical treatment. Measles vaccine is made by freeze drying the measles virus for 48 hrs before use.

Toxoids are neutralized toxins produced by pathogen and are used as vaccines. The toxin may be an exotoxin and endotoxin. The toxins are isolated from the culture of pathogenic bacteria by using biochemical methods. The isolated toxin is incubated with formaldehyde at pH 7.4-7.6 for 3-4 weeks at 37°C. Formaldehyde neutarlises the toxin and converts it into a toxoid.

The toxoid has no toxic effect but acts as an antigen to form antibodies to fight against the pathogen. The toxoid is mixed with aluminium phosphate thiomersal for storage and vaccination. E.g. Tetanus toxoid.

The cell wall polysaccharide that can alone stimulate immunity, is isolated from deadly pathogens and used for vaccination. Ex. Meningococcal vaccine is used to prevent Cerebrospinal meningitis caused by Meningococci. It is a combination of two polysaccharides isolate from Meningococci A and C. The polysaccharides act as extracted antigens.

Genetically engineered vaccine contains only a part of pathogen that stimulates the immunity. It contains one or a few antigens of the pathogen. It has none of its genes coding for non-antigenic proteins. As it is a subunit of the pathogen, it is known as subunit vaccine. Subunit vaccines are otherwise called biopharmaceutical vaccines. Subunits are extracted antigens. They are more stable than the vaccines prepared from the whole cells. Further, they do not show any side-effect. Eg. Hepatitis-B vaccine and vaccine for foot and mouth disease of cattle. A gene coding for the surface antigen (HBs Ag) of hepatitis-B virus was isolated and cloned in yeasts. HBs Ag was isolated from the recombinant yeasts and mixed with blood serum for storage and vaccination.

The gene for VP1 of Aphthovirus is cloned in E.coli. The recombinant cells produce VP1 antigen. This antigen is isolated, mixed with urea solution and used for vaccination against foot and mouth disease. Disease causing viruses can be made in to harmless ones by deleting some of the genes. Such viruses are used for vaccination. These vaccines are called live virus vaccines. Eg. Vaccines for Newcastle disease (ND), Marek's disease (MD), fowl Cholera(FC), infectious bursal disease (IBD), etc. Vaccinia viruses do not cause severe disease but they infect a wide range of hosts. Genes coding for antigenic proteins are isolated from pathogens and cloned in vaccinia virus. The recombinant vaccinia virus is used as live vaccine for vaccination.

Genes for antigens are isolated from more than one pathogen and cloned in a bacterium. The host cell produces a large polypeptide composed of many small antigenic peptides. The polypeptide is isolated and used for vaccination. These vaccines are called polyvalent vaccines or polyprotein vaccines.

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