Types of Vaccines

Vaccines are generally divided into two categories: preventive and therapeutic vaccines. Preventive vaccines are mainly used for the prevention of disease, and the recipients are healthy individuals or newborns; therapeutic vaccines are mainly used for sick individuals, and the recipients are patients.

Based on tradition and custom, they can be further divided into live attenuated vaccines, inactivated vaccines, antitoxins, subunit vaccines (including peptide vaccines), vector vaccines, nucleic acid vaccines, and so on.

Live-attenuated vaccine

Viral vaccines in this category are mostly more than 90% effective, and their protection usually lasts for many years. It has the distinct advantage that the pathogen replicates in the host to produce an antigenic stimulus that is similar in number, nature, and location to that of a natural infection, and so is generally highly immunogenic, even without the need for booster immunization. This outstanding advantage is at the same time potentially dangerous: infection can be triggered in some individuals with poor immunity; mutations may restore virulence. The latter can be more rationally attenuated with the understanding of the molecular basis of pathogen virulence, which may make it more definitively attenuated and unable to regain virulence.

Inactivated vaccines

Inactivated vaccines use non-replicating antigens (dead vaccines) compared to live attenuated vaccines, and are therefore safer, but also less immunogenic, and often must be boosted. It should be noted that not all pathogens can become highly effective vaccines after inactivation: some of these vaccines are highly effective, such as the Salk injectable polio vaccine (IPV) or hepatitis A vaccine; others are ineffective, short-duration vaccines, such as inactivated injectable cholera vaccine, which has almost been abandoned; and there are also some partially inactivated vaccines that are low in efficacy, and need to improve their rate of protection and the immune Some other inactivated vaccines have low efficacy and need to improve their protection rate and duration of immunization, such as the traditional inactivated influenza and typhoid vaccines. Most of these ineffective vaccines will be replaced by newer vaccines.

Toxoid vaccines

Toxoid vaccines are of great interest when the pathologic changes in the disease are primarily due to potent exotoxins or enterotoxins, as in the case of vaccines for tetanus and diphtheria. In general, enterotoxigenic toxoids are rarely successful. However the genetically modified detoxified variant of the heat-stable enterotoxin (LT) of enterotoxigenic Escherichia coli holds promise as an effective vaccine against traveler's diarrhea. The corresponding mutation of cholera toxin (CT) may become an even more important vaccine. Variants of both toxins can even induce good mucosal immunity and are also promising adjuvants for mucosal immunity.

Most of the toxoid vaccines currently in use are manufactured using traditional techniques. These vaccines, such as diphtheria and tetanus vaccines, contain many impure components, and the formaldehyde treatment process that changes the toxin into a toxoid also leads to cross-linking with bovine-derived peptides from the culture medium, which ends up creating unwanted antigens. Therefore, the study of a mutated, nontoxic pure molecule as a new vaccine could improve the quality and efficacy of these vaccines, as in the case of the substitution of glutamic acid at position 52 of diphtheria toxin for glycine, which leads to loss of toxicity and can cross-react with diphtheria toxin.

Subunit and peptide vaccines

DNA recombinant technology has made it possible to obtain large quantities of pure antigenic molecules. This has technologically revolutionized vaccines compared to those prepared from pathogens, making them easier to control in terms of quality and more affordable. In terms of efficacy, some subunit vaccines, such as non-acellular pertussis and HBsAg, are highly immunogenic at low doses, while others are less immunogenic and require stronger adjuvants than aluminum salts.

Peptide vaccines are usually manufactured by chemical synthesis techniques. This has the advantage of simpler composition and easier quality control. However, as the molecular weight and structural complexity of the immunogen decrease, the immunogenicity decreases significantly. Therefore, these vaccines generally require special structural designs, special delivery systems or adjuvants.

Vector vaccines

Vector vaccines introduce antigenic genes into the body to induce an immune response through a harmless microorganism, a vector. It is characterized by a combination of the two advantages of the strong immunogenicity of live attenuated vaccines and the accuracy of subunit vaccines. A significant benefit of this live vector vaccine is that it can effectively induce cellular immunity in vivo, which is promising in the context of current methods of inducing cellular immunity that are not yet sufficiently good, and the particular importance of cellular immunity in some diseases. Important vectors used in the experiments are variants of cowpox virus, poliovirus, avian pox virus, adenovirus, herpesvirus, Salmonella, Shigella and others. It is also possible to construct one or more cytokine genes at the same time, which can either enhance the immune response or change the direction of the immune response.

Nucleic acid vaccines

Nucleic acid vaccines are also called DNA vaccines or naked DNA vaccines. The key difference between it and a live vaccine is that the DNA encoding the antigen does not replicate in humans or animals. Nucleic acid vaccines should contain a strong promoter element that is efficiently expressed in mammalian cells, such as the early to mid-stage promoter of human cytomegalovirus; they also need to contain a suitable mRNA transcription termination sequence. After intramuscular injection, the DNA enters the cytoplasm and then reaches the nucleus of the myocyte, but does not integrate into the genome. As the target cells of the gene gun method, both myoblasts and dendritic cells do not divide and proliferate at a high rate, and they do not have a high degree of homology with the plasmid, so homologous recombination is less likely.

Compared with other types of vaccines, nucleic acid vaccines have potential and great superiority: ①DNA vaccines are one of the few methods to induce cytotoxic T-cell response; ②can overcome the problem of protein subunit vaccines are prone to misfolding and incomplete glycosylation; ③good stability, a large number of variations is very unlikely, easy to quality control; ④the production cost is low. ⑤ Theoretically, a multivalent vaccine can be realized by the mixture of multiple plasmids or the construction of complex plasmids. ⑥Theoretically good stability of antigen synthesis will reduce the amount of booster injections, and very small amounts (sometimes on the order of milligrams) of DNA can activate cytotoxic T cells very well.

Theoretically nucleic acid vaccines also have potential problems or side effects. First, while homologous recombination with host DNA is highly unlikely, random insertions are possible. While there is no quantitative data on this issue, whether it induces cancer is still a concern. Second, differences in DNA vaccine potency across antigens or species. The effects of human vaccines in model animals should be properly evaluated. Third, it is possible that the body's immune regulation and effector mechanisms may lead to the destruction of antigen-expressing cells, resulting in the release of intracellular antigens and the activation of autoimmunity. Fourth, stimulation with small doses of antigen over a sustained and prolonged period of time may lead to immune tolerance, which in turn leads to unresponsiveness of the recipient to the antigen. However, these potential side effects have not been identified in practice to date.

Edible vaccines

The carriers of such vaccines are made from the cells of edible plants such as potatoes, bananas, and tomatoes, and the protective immune response is initiated by consumption of their fruits or other components. The plant cells act as a natural biological capsule for effective delivery of antigens to the submucosal lymphatic system. This is one of the few forms of mucosal immunity that can be effectively initiated. Therefore, it has good prospects for mucosal infectious diseases.