is the first virus to be discovered. viral vectors based on tobacco mosaic virus include those of the magnicon and trbo plant expression technologies. hybrids hybrid vectors are vector viruses that are genetically engineered to have qualities of more than one vector. viruses are altered to avoid the shortcomings of typical viral vectors, which may have limited loading capacity, immunogenicity, genotoxicity, and fail to support long-term adequate transgenic expression. through the replacement of undesirable elements with desired abilities, hybrid vectors may in the future outperform standard transfection vectors in terms of safety and therapeutic efficiency. challenges in application the choice of a viral vector to deliver genetic material to cells comes with some logistical problems. there are a limited number of viral vectors available for therapeutic use. any of these few viral vectors can cause the body to develop an immune response if the vector is seen as a foreign invader. once used, the viral vector cannot be effectively used in the patient again because it will be recognized by the body. if the vaccine or gene therapy fails in clinical trials, the virus can't be used again in the patient for a different vaccine or gene therapy in the future. pre-existing immunity against the viral vector could also be present in the patient, rendering the therapy ineffective for that patient. because priming with a naked dna vaccine and boosting with a viral vector results in a robust immune response via yet indefinite mechanism(s), despite pre-existing viral vector immunity, this vaccination strategy can counteract this problem. however, this method may present another expense and obstacle in the vaccine distribution process. pre-existing immunity may also be challenged by increasing vaccine dose or changing the vaccination route. some shortcomings of viral vectors (such as genotoxicity and low transgenic expression) can be overcome through the use of hybrid vectors.