Question

Discuss viral vaccines and the advantages and disadvantages of different types. Explain the different strategies used to control viral infections through immunisation. a) Review the different types of viral vaccines and their advantages and disadvantages. Provide examples of the different types. (50%) b) Discuss, using details of named examples, how drugs can be used to treat viral infections (50%)

Answer 1

A vaccine is a biological preparation that provides active acquired immunity to a particular disease. A vaccine typically contains an agent that resembles a disease-causing micro-organism and is often made from weakened or killed forms of the microbe, its toxins are one of its surface proteins. The agent stimulates the body's immune systemto recognize the agent as a threat, destroy it, and keep a record of it, so that the immune system can more easily recognize and destroy any of these micro-organisms that it later encounters. Vaccines can beprophylactic (example: to prevent or ameliorate the effects of a future infection by any natural or "wild" pathogen), or therapeutic(e.g., vaccines against cancer are also being investigated).

There are two classic types of viral vaccines- "live attenuated" and "killed" or "fixed".

>Live, attenuated vaccines are either nonpathogenic relatives of the target virus (like Jenner's cowpox) or are produced by "attenuation" of virulent isolates by passage in animals and/or cell culture.

>Fixed vaccines consist of purified virus which has been activated by UV light or chemical treatment that destroys infectivity without affecting antigenicity.

There are advantages and disadvantages to each type of vaccine:

Live, attenuated vaccines:

>Advantages:

    Because of attenuation, vaccination mimics a subclinical infection resulting in induction of a complete immune response, including IgG and IgA antibodies, CD4+ and CD8+ T lymphocytes and memory cells.

    Dosage administered is small leading to low cost of preparation and limited allergic response

    Theoretically, only one dose is necessary, although often more doses are recommended

>Disadvantages:

    Since the virus is a live agent, virulent replication or reversion to virulence can occur

    Live vaccines are labile

    Interference with natural agents can occur leading to vaccine failure

Fixed vaccines:

>Advantages:

    No side effects are encountered due to viral replication

    It is easier to predict the response to inoculation

    Fixed vaccines are stable

>Disadvantages:

    The immune response induced is similar to that induced against an antigen and thus only IgG is induced, the cell mediated immune response induced is uncertain, and serum immunity wanes and boosters are required for maintenance

    Adjuvent can be required to maximize the immune response

    Fixed vaccines require large amounts of pure antigen

Both live and fixed vaccines are currently in use:

Poliovirus:

    Both fixed (Inactivated Polio Vaccine, IPV, developed by Jonas Salk) and live, attenuated (Oral Polio Vaccine, OPV, developed by Albert Sabin) polio vaccines have been used. Both are trivalent, containing all three poliovirus serotypes. Initially IPV was given, but ~1960 OPV became the standard vaccine and is the vaccine in use in worldwide eradication efforts. In the U.S., three doses of OPV were given to infants because of possible interference due to concurrent inapparent enterovirus infection at the time of vaccination. Unfortunately, the OPV (particularly the Serotype 3 component) reverts to neurovirulence, causing a few (<10) cases of poliomyelitis per year. Thus recently, the vaccine regimen was changed to two doses of IPV followed by a dose of OPV.

Measles, Mumps, Rubella:

    Live, attenuated vaccines for all three viruses were developed in the 1960's. In the U.S., these vaccines are administered by intramuscular injection in trivalent form. The three viruses do not interfere with each other. Initially, only one dose was given at 15-18 months of age, however after the 1989-1991 resurgence of measles, a second dose was added at 5-10 years of age to insure complete coverage.

Antiviral drugs are a class of medication used specifically for treating viral infections. Likeantibiotics and broad-spectrum antibiotics forbacteria, most antivirals are used for specificviral infections, while a broad-spectrum antiviral is effective against a wide range of viruses. Unlike most antibiotics, antiviral drugs do not destroy their target pathogen; instead they inhibit their development.

Antiviral drugs are one class ofantimicrobials, a larger group which also includes antibiotic (also termed antibacterial),antifungal and antiparasitic drugs or antiviral drugs based on monoclonal antibodies. Most antivirals are considered relatively harmless to the host, and therefore can be used to treat infections. They should be distinguished from viricides, which are not medication but deactivate or destroy virus particles, either inside or outside the body. Natural antivirals are produced by some plants such as eucalyptus.