**2.4. DNA Vaccine**

**2.2. Attenuated**

**2.3. Subunit**

Some vaccines contain live, attenuated microorganisms. Many of these are live viruses that have been cultivated under conditions that disable their virulent properties, or which use closely related but less dangerous organisms to produce a broad immune response [1]. They typically provoke more durable immunological responses and are the preferred type for healthy adults. Examples include the viral diseases yellow fever, measles, rubella, and mumps. Attenuated vaccines have some advantages and disadvantages. They have the ca‐ pacity of transient growth so they give prolonged protection, and no booster dose is re‐

Protein subunit- rather than introducing an inactivated or attenuated micro-organism to an immune system (which would constitute a "whole-agent" vaccine), a fragment of it can cre‐ ate an immune response. Examples include the subunit vaccine against Hepatitis B virus that is composed of only the surface proteins of the virus (previously extracted from the blood stream of chronically infected patients, but now produced by recombination of the vi‐ ral genes into yeast), the virus like particle (VLP) vaccine against human papillomavirus (HPV) that is composed of the viral major capsidprotein, and the hemagglutinin and neura‐ minidase subunits of the influenza virus. One method of production involves isolation of a specific protein from a virus and administering this by itself. A weakness of this technique is that isolated proteins can be denatured and will then bind to different antibodies than the proteins in the virus. A second method of subunit vaccine is the recombinant vaccine, which involves putting a protein gene from the targeted virus into another virus. The second virus will express the protein, but will not present a risk to the injector. This is the type of vaccine currently in use for hepatitis, and it is experimentally popular, being used to try to develop

quired. But they may get reverted to the virulent form and cause the disease.

240 Current Issues in Molecular Virology - Viral Genetics and Biotechnological Applications

**Figure 1.** H1N1 flu nasal spray as an example of attenuated vaccine

new vaccines for difficult to vaccinate viruses such as Ebola and HIV.

In the past decade and a half, the DNA vaccine concept has been tested and applied against various pathogens and tumor antigens [2]. The optimized gene sequence of interest is deliv‐ ered to the skin, subcutaneum or muscle by one of several delivery methods [3]. The expres‐ sion of plasmid-encoded genes will produce foreign antigens and elicits immunological response. Until now, four DNA vaccine products have been approved, all in the area of vet‐ erinary medicine [4].

Vaccines are very effective on stable viruses, but are of limited use in treating a patient who has already been infected. It is also difficult to successfully deploy them against rapidly mu‐ tating viruses, such as influenza (the vaccine for which is updated every year) and HIV [5]. Antiviral drugs are particularly useful in these cases.


**Table 1.** Current licensed DNA therapies (Adapted from Kutzler MA & Weiner et. al)
