Immunoprophylaxis against viral illnesses includes the use of vaccines or antibody-containing preparations to provide immune protection against a specific disease.
Active Prophylaxis (Vaccines)
Active immunization involves administering a virus preparation that stimulates the body's immune system to produce its own specific immunity. Viral vaccines now available for use include the following types: (1) attenuated live viruses; (2) killed viruses; (3) recombinant produced antigens. A vaccinee is a person who has been vaccinated.
Immune Response to Vaccines: Vaccination evokes an antibody response and stimulates T lymphocytes. Vaccine effectiveness is assessed in terms of percentage of recipients protected and the duration and degree of protection. Most effective viral vaccines protect more than 90 percent of recipients and produce fairly durable immunity.
Passive immunity is conferred by administering antibodies formed in another host. Human immunoglobulins remain a mainstay of passive prophylaxis (and occasionally therapy) for viral illnesses; they are usually used to protect individuals who have been exposed to a disease and cannot be protected by vaccination.
Many viral diseases are controlled by reducing exposure to the virus by (1) eliminating nonhuman reservoirs, (2) eliminating the vector, and (3) improving sanitation.
There are three types of antiviral agents: (1) virucidal agents, which directly inactivate viruses, (2) antiviral agents, which inhibit viral replication, and (3) immunomodulators, which boost the host immune response.
Virus-infected cells and cells induced with other agents, e.g., double-stranded polynucleotides, can secrete proteins called interferons, which protect normal cells from viral infection. Therapeutic administration of interferon alpha has proven effective for several human viral illnesses.
Cytokines are molecules produced by cells which modify the biological responses of the same or other cells.
Viral diseases range from trivial infections to plagues that alter the course of history. Because of the enormous variations in viruses and in their epidemiology and pathogenesis, there is no single, magic-bullet approach to control. Each virus presents its own set of problems. This chapter covers methods useful to various degrees in controlling selected viral diseases. The most spectacular progress so far has involved vaccines. Vector control and sanitation have contributed greatly. Also, a number of therapeutic antiviral agents are now available, including some for very serious infections such as human immunodeficiency virus type 1 (HIV-1) infection. In addition, interferon alpha is now available for the therapy of several viral diseases.
Immunoprophylaxis against viral illnesses includes the use of vaccines or antibody-containing preparations to provide a susceptible individual with immunologic protection against a specific disease. Immunization against viral illnesses can be either active or passive. With active immunity, protection is achieved by stimulating the body's immune system to produce its own antibodies by immunization with a virus preparation. Passive immunity is conferred by administering antibodies formed in another host. For example, an antibody-containing gamma globulin preparation may protect a susceptible individual exposed to a viral illness.
Active Prophylaxis (Vaccines)
The viral vaccines currently approved for use in the United States are listed in TABLE 51-1. These products are of three types:
(1) Attenuated live viral vaccines: Most live vaccines contain viruses that have been attenuated by laboratory manipulation. These attenuated viruses can infect and replicate in the recipient and produce a protective immune response without causing disease. Live attenuated viral vaccines can often confer lifelong immunity after one immunization series. However, because live viruses can multiply in the body, there is always the possibility that they may revert to a more pathogenic form. Adequate laboratory and animal testing and extensive clinical studies must be performed to assess this possibility. In addition, new recombinant technologies facilitate direct alteration of viral genetic structure, thus permitting scientists to produce attenuated viruses in which the genetic regions likely to lead to pathogenic reversion are modified or deleted.
(2) Killed (inactivated) viral vaccines: Killed viral vaccines contain either whole virus particles, inactivated by chemical or physical means, or some component(s) of the virus. Completely inactivated viral vaccines cannot cause infection. However, they do not generally produce lifelong immunity following one immunization series; additional doses are usually required. In addition, because killed virus does not multiply in the host, the inoculum itself must provide a sufficiently large concentration of viral antigens to induce the desired immune response.
(3) Recombinant-produced antigens: Application of a recombinant DNA strategy to develop new vaccines is performed by identifying the specific component(s) that can elicit the production of protective antibodies, and then cloning and expressing the gene encoding that protein and assembly of a complex in some cases. This approach has made possible a safe and effective recombinant vaccine against hepatitis B virus, which has replaced the vaccine derived from the plasma of hepatitis B virus-infected individuals.