Because few drugs are useful against viral infections, prevention of infection by the use of vaccines is very important. Prevention of viral diseases can be achieved by the use of vaccines that induce active immunity or by the administration of preformed antibody that provides passive immunity.
There are two types of vaccines that induce active immunity: those that contain live virus whose pathogenicity has been attenuated and those that contain killed virus. An attenuated virus is one that is unable to cause disease, but retains its antigenicity and can induce protection.
Some vaccines, such as the hepatitis B vaccine, contain purified viral proteins and are often called subunit vaccines. The features of subunit vaccines resemble those of killed vaccines because no viral replication occurs in these vaccines. The attributes of live and killed vaccines are listed in Table 36–1.
TABLE 36–1Characteristics of Live and Killed Viral Vaccines |Favorite Table|Download (.pdf) TABLE 36–1 Characteristics of Live and Killed Viral Vaccines
|Characteristic ||Live Vaccine ||Killed Vaccine |
|Duration of immunity ||Longer ||Shorter |
|Effectiveness of protection ||Greater ||Lower |
|Immunoglobulins (Ig) produced ||IgA1 and IgG ||IgG |
|Cell-mediated immunity produced ||Yes ||Weakly or none |
|Interruption of transmission of virulent virus ||More effective ||Less effective |
|Reversion to virulence ||Possible ||No |
|Stability at room temperature ||Low ||High |
|Excretion of vaccine virus and transmission to nonimmune contacts ||Possible ||No |
In general, live vaccines are preferred to vaccines containing killed virus because their protection is greater and longer-lasting. With live vaccines, the virus multiplies in the host, producing a prolonged antigenic stimulus, and IgA and IgG are elicited when the vaccine is administered by the natural route of infection (e.g., when polio vaccine is given orally). Killed vaccines, which are usually given intramuscularly, do not stimulate a major IgA response. Killed vaccines typically do not stimulate a cytotoxic T-cell response, because the virus in the vaccine does not replicate. In the absence of replication, no viral epitopes are presented in association with class I MHC proteins, and the cytotoxic T-cell response is not activated (see Chapter 58). Although live vaccines stimulate a long-lasting response, booster doses are now recommended with measles and polio vaccines.
One unique form of a live, attenuated viral vaccine is the influenza vaccine that contains a temperature-sensitive mutant of the virus as the immunogen. The temperature-sensitive mutant will replicate in the cooler air passages of the nose, where it induces IgA-based immunity, whereas it will not replicate in the warmer lung tissue and therefore will not cause disease.
There are three concerns about the use of live vaccines:
They are composed of attenuated viral mutants, which can revert to virulence either during vaccine production or in the immunized ...