An increasing number of antiviral agents are presently in various stages of development and testing, and an increasing number have recently been licensed for use in humans. These drugs have been used extensively to treat viral infections in immunocompromised individuals, and these studies indicate that for many antiviral agents the response to therapy is highly dependent on the integrity of the underlying host immune response. In particular, the response to zidovudine, acyclovir and ganciclovir in persons with HIV-1 infection is highly dependent upon CD4 number, which can be considered a surrogate marker for the state of host immune function in these subjects. Responses to interferons likewise can be shown to depend on the host immune response, with responses due to both direct antiviral effects of this agent as well as immunomodulatory effects mediated through interferon-induced upregulation of HLA molecule expression.

If vaccines are to reliably prevent disease, they must be developed, produced and quality-controlled according to very strict regulations and procedures. Veterinary viral vaccine registrations are governed by different rules in different countries, but these rules all emphasize that the quality of the raw materials—the cells, eggs, animals or plants that are used in production—need to be carefully controlled. The veterinary vaccine business is also very cost-conscious. Emphasis over the last 5–10 years has therefore been to develop culture systems that minimize labor and sterility problems and thus provide for reliable and cost-effective production. Implementing these often more complex systems in a production environment takes considerable effort, first in scale-up trials and further down the line in convincing production personnel to change their familiar system for something new and possibly untried. To complete scale-up trials successfully, it is absolutely necessary to understand the biochemistry of the cells and the influence of the virus on the cells under scale-up and later production conditions. Once a viral product can be produced on a large scale, it is imperative that the quality of the end-product is controlled in an intelligent way. One needs to know whether the end-product performs in the animal as was intended during its conception in the research and development department. The development of the appropriate tests to demonstrate this plays an important role in the successful development of a vaccine.

A trivalent vector containing genes of Watermelon mosaic virus (WMV) coat protein (CP), and replicase genes of Zucchini yellow mosaic virus (ZYMV) and Cucumber mosaic virus (CMV), was constructed for transformation of watermelon (Citrullus lanatus) plants, mediated by Agrobacterium tumefaciens. The integrated foreign genes were identified in the regenerated progenies by polymerase chain reaction (PCR) and Southern blots. The transformation efficiency was about 1.7‰. Resistance to virus infection was determined by mechanical inoculation in the greenhouse and by field trials. The transgenic watermelon lines showed different phenotypes of susceptible, resistant, immune or recovery from virus infections in the late growth stage. A relatively high level of resistance was shown by T3 plants of the line BH1-7. This result indicates the possibility of creating, by transgenic protocols, new varieties of watermelon resistant to viral infection.