■ The most important prophylactic measures in the face of potential viral infections are active vaccines. Vaccines containing inactivated viruses generally provide shorter-lived and weaker protection than live vaccines. Passive immunization with human immunoglobulin is only used in a small number of cases, usually as postexposure prophylaxis. ■
Value of the different methods. In general, vaccination, i.e., induction of immunity (immune prophylaxis) is the most important factor in prevention of viral infection. Exposure prophylaxis is only relevant to hygienic measures necessitated by an epidemic and is designed to prevent the spread of pathogens in specific situations. Chemoprophylaxis, i.e., administration of chemothera-peutic agents when an infection is expected instead of after it has been diagnosed to block viral metabolism, is now justified in selected cases, e.g., in immunosuppressed patients (see Chemotherapy, p. 404).
There are two basic types of vaccines:
Active immunization. In this method, the antigen (virus) is introduced into the body, either in an inactivated form, or with attenuated pathogenicity but still capable of replication, to enable the body to build up its own immunity.
■ Inactivated vaccines. The immunity that develops after so-called "dead vaccines" are administered is merely humoral and generally does not last long. For this reason, booster vaccinations must be given repeatedly. The most important dead vaccines still in use today are influenza, rabies, some flavivirus, and hepatitis A and B vaccines. Some inactivated vaccines contain the most important immunogenic proteins of the virus. These so-called split vaccines induce more efficient protection and, above all, are better tolerated. Some of them are now produced by genetic engineering methods.
■ Live attenuated vaccines. These vaccines confer effective and long-lasting protection after only a single dose, because the viruses contained in them are capable of replication in the body, inducing not only humoral, but sometimes cellular immunity as well, not to mention local immunity (portal of entry!). Such live vaccines are preferable when available. There are, however, also drawbacks and risks, among them stability, the increased potential for contamination with other viruses, resulting in more stringent testing and the possibility that a back-mutation could produce a pathogenic strain (see Variability and Quasispecies of Viruses, p. 391).
■ Vaccines with recombinant viruses. Since only a small number of (surface) viral proteins are required to induce protective immunization, viral vectors are used in attempts to express them in vaccine recipients (see p. 390). Suitable vectors include the least virulent virus strains among the picorna-viruses, alphaviruses, and poxviruses. There must be no generalized immunity to the vector in the population so that it can replicate in vaccine recipients and the desired protein will at the same time be expressed. Such recombinant vaccines have not yet been approved for use in humans. A rabies vaccine containing the recombinant vaccinia virus for use in animals is the only practical application of this type so far (p. 390).
■ Naked DNA vaccine. Since pure DNA can be inserted into eukaryotic cells (transfection) and the information it carries can be expressed, DNA that codes for the desired (viral) proteins can be used as vaccine material. The advantages of such vaccines, now still in the trial phase, include ease of production and high stability.
Passive immunization. This type of vaccine involves the injection of antibodies using only human immunoglobulins. The protection conferred is of short duration and only effective against viruses that cause viremia. Passive immunization is usually administered as a postexposure prophylactic measure, i.e.,
after an infection or in situations involving a high risk of infection, e.g., to protect against hepatitis B and rabies (locally, bite wound). Table 1.13 (p. 33) and Table 8.7 (rabies, p. 470) list the most important vaccines.
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