Vaccines chaPtEr 18 TABLE 18-5 Comparison of attenuated (live), inactivated (killed), and DNA vaccines Characteristic Attenuated vaccine Inactivated vaccine DNA vaccine Production Selection for avirulent Virulent pathogen is inactivated by Easily manufactured virulent pathogen chemicals or irradiation with y-rays and purified adverse culture co human pathogen different hosts Booster requirement Generally requires only a single Requires multiple boosters Single injection may suffice Relative stability Less stable More stable Highly stable Type of immunity induced Humoral and cell-mediated Humoral and cell-mediated Reversion tendency May revert to virulent form Cannot revert to virulent form Cannot revert because the three strains of attenuated poliovirus in the vac- cacy of the vaccine. a more convincing argument for vacci- cine interfere with each other's replication in the intestine. nation is the high death rate associated with measles infec With the first immunization, one strain will predominate in tion even in developed countries its growth, inducing immunity to that strain. With the sec Genetic engineering techniques provide a way to attenu ond immunization, the immunity generated by the previous ate a virus irreversibly by selectively removing genes that are immunization will limit the growth of the previously pre- necessary for virulence. This has been done with a her- dominant strain in the vaccine, enabling one of the two re- pesvirus vaccine for pigs, in which the thymidine kinase gene maining strains to predominate and induce immunity. was removed. Because thymidine kinase is required for the Finally, with the third immunization, immunity to all three virus to grow in certain types of cells(e.g, neurons), removal strains is achieved of this gene rendered the virus incapable of causing disease A major disadvantage of attenuated vaccines is the possi- It is possible that similar genetic engineering techniques bility that they will revert to a virulent form. The rate of could eliminate the risk of reversion of the attenuated polio reversion of the Sabin polio vaccine(OPV) leading to subse- vaccine. More recently, a vaccine against rotavirus, a major quent paralytic disease is about one case in 2. 4 million doses cause of infant diarrhea, was developed using genetic engi- of vaccine. This reversion implies that pathogenic forms of neering techniques to modify an animal rotavirus to contain the virus are being passed by a few immunized individuals antigens present on the human viruses and can find their way into the water supply, especially in ar- eas where sanitation standards are not rigorous or where Pathogenic Organisms Are Inactivated waste water must be recycled. This possibility has led to the by Heat or Chemical Treatment (see Table 18-3). The projected eradication of paralytic polio Another common approach in vaccine production is inacti- (Figure 18-4)will be impossible as long as OPV is used any- vation of the pathogen by heat or by chemical means so that where in the world. The alternative inactivated Salk vaccine it is no longer capable of replication in the host. It is critically should be substituted as the number of cases decrease, al- important to maintain the structure of epitopes on surface though there are problems in delivering this vaccine in devel- antigens during inactivation. Heat inactivation is generall oping countries. satisfactory because it causes extensive denaturation of Attenuated vaccines also may be associated with compli- proteins; thus, any epitopes that depend on higher orders of tions similar to those seen in the natural disease. A small protein structure are likely to be altered significantly. Chem percentage of recipients of the measles vaccine, for example, ical inactivation with formaldehyde or various alkylating develop post-vaccine encephalitis or other complications As agents has been successful. The Salk polio vaccine is pro- shown in Table 18-6(page 423), however, the risk of vaccine- duced by formaldehyde inactivation related complications is much lower than risks from infec Attenuated vaccines generally require only one dose to tion. An independent study showed that 75 million doses of induce long-lasting immunity. Killed vaccines, on the measles vaccine were given between 1970 and 1993, with an other hand, often require repeated boosters to maintain incidence of 48 cases of vaccine-related encephalopathy. The the immune status of the host. In addition, killed vaccines low in-cidence of this side effect compared with the rate of induce a predominantly humoral antibody response; they cephalopathy associated with infection argues for the effi- are less effective than attenuated vaccines in inducingVaccines CHAPTER 18 421 because the three strains of attenuated poliovirus in the vac￾cine interfere with each other’s replication in the intestine. With the first immunization, one strain will predominate in its growth, inducing immunity to that strain. With the sec￾ond immunization, the immunity generated by the previous immunization will limit the growth of the previously pre￾dominant strain in the vaccine, enabling one of the two re￾maining strains to predominate and induce immunity. Finally, with the third immunization, immunity to all three strains is achieved. A major disadvantage of attenuated vaccines is the possi￾bility that they will revert to a virulent form. The rate of reversion of the Sabin polio vaccine (OPV) leading to subse￾quent paralytic disease is about one case in 2.4 million doses of vaccine. This reversion implies that pathogenic forms of the virus are being passed by a few immunized individuals and can find their way into the water supply, especially in ar￾eas where sanitation standards are not rigorous or where waste water must be recycled. This possibility has led to the exclusive use of the inactivated polio vaccine in this country (see Table 18-3). The projected eradication of paralytic polio (Figure 18-4) will be impossible as long as OPV is used any￾where in the world. The alternative inactivated Salk vaccine should be substituted as the number of cases decrease, al￾though there are problems in delivering this vaccine in devel￾oping countries. Attenuated vaccines also may be associated with compli￾cations similar to those seen in the natural disease. A small percentage of recipients of the measles vaccine, for example, develop post-vaccine encephalitis or other complications. As shown in Table 18-6 (page 423), however, the risk of vaccine￾related complications is much lower than risks from infec￾tion. An independent study showed that 75 million doses of measles vaccine were given between 1970 and 1993, with an incidence of 48 cases of vaccine-related encephalopathy. The low in-cidence of this side effect compared with the rate of encephalopathy associated with infection argues for the effi￾cacy of the vaccine. A more convincing argument for vacci￾nation is the high death rate associated with measles infec￾tion even in developed countries. Genetic engineering techniques provide a way to attenu￾ate a virus irreversibly by selectively removing genes that are necessary for virulence. This has been done with a her￾pesvirus vaccine for pigs, in which the thymidine kinase gene was removed. Because thymidine kinase is required for the virus to grow in certain types of cells (e.g., neurons), removal of this gene rendered the virus incapable of causing disease. It is possible that similar genetic engineering techniques could eliminate the risk of reversion of the attenuated polio vaccine. More recently, a vaccine against rotavirus, a major cause of infant diarrhea, was developed using genetic engi￾neering techniques to modify an animal rotavirus to contain antigens present on the human viruses. Pathogenic Organisms Are Inactivated by Heat or Chemical Treatment Another common approach in vaccine production is inacti￾vation of the pathogen by heat or by chemical means so that it is no longer capable of replication in the host. It is critically important to maintain the structure of epitopes on surface antigens during inactivation. Heat inactivation is generally unsatisfactory because it causes extensive denaturation of proteins; thus, any epitopes that depend on higher orders of protein structure are likely to be altered significantly. Chem￾ical inactivation with formaldehyde or various alkylating agents has been successful. The Salk polio vaccine is pro￾duced by formaldehyde inactivation. Attenuated vaccines generally require only one dose to induce long-lasting immunity. Killed vaccines, on the other hand, often require repeated boosters to maintain the immune status of the host. In addition, killed vaccines induce a predominantly humoral antibody response; they are less effective than attenuated vaccines in inducing TABLE 18-5 Comparison of attenuated (live), inactivated (killed), and DNA vaccines Characteristic Attenuated vaccine Inactivated vaccine DNA vaccine Production Selection for avirulent organisms: Virulent pathogen is inactivated by Easily manufactured virulent pathogen is grown under chemicals or irradiation with -rays and purified adverse culture conditions or prolonged passage of a virulent human pathogen through different hosts Booster requirement Generally requires only a single Requires multiple boosters Single injection may suffice booster Relative stability Less stable More stable Highly stable Type of immunity induced Humoral and cell-mediated Mainly humoral Humoral and cell-mediated Reversion tendency May revert to virulent form Cannot revert to virulent form Cannot revert