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The term, ‘vaccination’ originated from vacca which means cow. The term was first used by Edward Jenner who discovered (1798) that cow-pox induced immunity to small-pox in persons artificially inoculated with cow-pox. This process of inducing immunity against small pox came to be known as vaccination which saved millions of human lives since then and has eventually led to complete eradication of the disease by 1979.
The agents used in inducing immunity to various contagious diseases caused by viruses, bacteria or other pathogens are generally known as vaccines, although they have nothing to do with cows. Vaccination is a prophylactic measure, i.e. it gives protection against a possible attack by developing active immunity.
Vaccination is the cheapest and the most effective method of prevention of communicable diseases. Through well-organised, planned vaccination programmes, it is possible to reach large sections of world population and to eradicate life-threatening communicable diseases one by one. Such global immunization programmes have greatly reduced incidence of diseases, like diphtheria, pertussis (whooping cough), tetanus, tuberculosis, measles, chicken-pox, poliomyelitis and hepatitis B.
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Although the advent of numerous clinical antibiotics has played a very significant role in the management of bacterial diseases, they are ineffective against viral disease. Till now, vaccination is the most effective means to give protection against diseases caused by viruses.
All types of vaccines contain antigen in some form or other. The antigen may consist of whole cellular organisms or intact viruses, or some of their constituents possessing the property of immunogenicity. The effectively of some antigenic agents has to be sometimes increased by addition of chemicals, called adjuvants.
One of the most common adjuvant is alum (aluminium sulfate). Adjuvants become necessary when antigens used in vaccines consist of comparatively small molecules which by themselves are not strongly immunogenic.
The traditional vaccines generally contain antigens in the form of whole cells or viral particles. Such vaccines are known as the first generation vaccines. The whole-cell vaccines contain either living or killed cells. When living cells are used as antigen, they must be attenuated. Similarly, when whole viruses are used, they also have to be attenuated.
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Attenuation means loss of pathogenicity without the loss of immunogenicity, so that the attenuated strains when artificially introduced into the body can provoke an immune response without causing the actual disease. Thereby, the body gains the ability to resist the real pathogen.
Attenuation can be induced in a pathogenic strain of an organism by growing it through many passages of artificial culture on unfavorable media which results in loss of pathogenicity due to mutations occurring during the multiple passages. Though rare, the non-virulent attenuated strain may occasionally regain pathogenicity by reverse mutation.
However, the advantages of using living attenuated pathogens as antigen in vaccines are more than using killed pathogens, so that the rare occurrence of reversion of pathogenicity can be ignored. One important advantage is that the attenuated antigenic agent continues to grow in the body of the vaccinated person, thereby producing a sustained immune response. On the other hand, killed pathogenic agents are safer, but, at the same time, they are less efficient as immunogens.
Besides, live attenuated and killed pathogens, inactivated viruses or bacterial toxins are also used as antigens in some vaccines. Inactivated polio-virus is the antigen of Salk vaccine. Similarly, inactivated exotoxins (toxoids) of tetanus and diphtheria bacilli are employed as antigens in vaccines. By treatment with formaldehyde, the toxins are made non-toxic toxoids, but they retain their immunogenicity. The antibodies evoked by these antigens are called anti-toxins.
The second-generation vaccines are called the sub-unit vaccines, because the antigen used in them are some component of the pathogenic agent which can evoke an immune response to counteract the whole pathogen. Obviously, the sub-unit vaccines offer a more desirable alternative to live or killed whole-cell vaccines.
The components used in subunit vaccines are generally the surface molecules of the pathogens, like capsular polysaccharides of bacteria and coat proteins of viruses. The immune response evoked by these antigenic molecules yields specific antibodies which combine with the surface structures of actual pathogen making them easy prey to the phagocytic cells of the body.
In case of viruses, the antibodies combine with the binding sites present on the coat or envelope making them incapable of attachment to host cells. The capsular polysaccharides of pneumococci, meningococci, cholera bacilli, typhoid bacilli and Haemophilus have been used as antigens in their respective vaccines.
Recombinant DNA technology has opened new possibilities for developing more advanced types of subunit vaccines. The gene coding some antigenic molecule of a pathogen can now be cloned in a bacterial or yeast cell to produce a transgenic organism, capable of producing the gene-product.
By mass-culturing of such organisms, it is now possible to get the antigenic material in large-scale and to use it in vaccines. The first vaccine developed in this way is the hepatitis B virus vaccine. The gene encoding a surface glycoprotein of the capsid of this virus was cloned in yeast which produced the glycoprotein in culture medium.
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An exciting and revolutionary possibility in the field of immuno-technology is the use of DNA directly to produce the antigenic molecule in the body which would then provoke an immune response. This has given rise to the possibility to develop a DNA-vaccine.
In recent years, it has been shown that plasmids engineered to have a gene coding for a specific antigenic protein can be injected in muscle tissues of experimental animals to induce an immune response. Similarly, it has been found that c-DNA containing the gene encoding haemagglutinin of influenza virus (which is a surface protein of this virus) can induce immune response in muscle tissues of animals producing antibodies effective against the surface antigen. As yet a human DNA vaccine has not been developed, but the possibility certainly exists and in that case these DNA vaccines will certainly be treated as the third generation of vaccines.
The different types of vaccines, antigens used in them and the target disease are enlisted in Table 10.7:
