Stanford University

Viruses and Viral Vectors

These substances have become staples of the molecular biology community. As such, it is important for users to understand their origins and potential implications. Working with Viral Vectors contains  virus-specific details about virology, clinical features, epidemiology, treatment, laboratory hazards, personal protective equipment (PPE), disinfection, and use with animals. Refer to Recombinant Viral Vector Biosafety Levels for recombinant vectors and their biosafety levels.

  1. Adenoviruses are infectious human viruses which often cause mild respiratory illness, pink eye or gastroenteritis. Rare cases of severe disease can occur, and its use as a genetic vector therefore requires the use of adequate containment equipment and practices. Biosafety Level 2 (BSL-2) is appropriate for many constructs. Particular care should be given to vectors containing genes that make products similar to those of the deleted adenovirus genes. See the Adenovirus Fact Sheet for more information.

Adeno-Associated Virus
  1. These are infectious human viruses with no known disease association. Some AAV types are common in the general population, and these viruses have the ability to integrate into the host chromosome. The NIH Guidelines state that “adeno-associated virus (AAV) types 1 through 4, and recombinant AAV constructs, in which the transgene does not encode either a potentially tumorigenic gene product or a toxin molecule and are produced in the absence of a helper virus” can in most cases be handled at biosafety level 1 (BSL-1). This level of containment is modified by other considerations (see above). See the Adeno-Associated Virus Fact Sheet for more information.

Epstein-Barr Virus
  1. Epstein-Barr virus, frequently referred to as EBV, is a member of the herpesvirus family and one of the most common human viruses. The virus is found worldwide, and most people become infected with EBV sometime during their lives. In the United States, as many as 95% of adults between 35 and 40 years of age have been infected. Infants become susceptible to EBV as soon as maternal antibody protection (present at birth) disappears. Many children become infected with EBV, and these infections usually cause no symptoms or are indistinguishable from the other mild, brief illnesses of childhood. In the United States and in other developed countries, many persons are not infected with EBV in their childhood years. When infection with EBV occurs during adolescence or young adulthood, it causes infectious mononucleosis 35% to 50% of the time. EBV also establishes a lifelong dormant infection in some cells of the body’s immune system. A late event in a very few carriers of this virus is the emergence of Burkitt’s lymphoma and nasopharyngeal carcinoma. EBV is a transforming virus and is often used to produce immortalized cell lines. BSL-2 is appropriate for most experiments. See the Epstein-Barr Fact Sheet for more information.

  1. Herpesviruses include infectious human viruses such as herpes simplex virus type-1 (HSV-1), which is the most commonly used vector system. HSV-1 is common in the general population, but can cause encephalitis in rare cases; its utility as a vector system stems from its broad host cell range, ability to transduce neurons, and its large insert capacity. Biosafety Level 2 (BSL-2) is appropriate for many constructs. See the Herpesvirus Fact Sheet for more information.

  1. These are infectious viruses which can integrate into transduced cells with high frequency, and which may have oncogenic potential in their natural hosts. Retrovirus vector systems are typically based on murine viruses – most commonly, these systems include ecotropic viruses (which can infect only murine cells), amphotropic viruses (which can infect human cells) or pseudotyped viruses, when vector particles express glycoproteins (GPs) derived from other enveloped viruses (which can also infect human cells). The most common GP currently used is VSV-g, however there are newer pseudotypes being derived from viruses such as measles (Rubeola), Ebola and Marburg. Pseudotyping vectors often results in a higher Biosafety level. Containment for vectors with the ability to infect human cells (amphotropic) will usually be recommended at BSL-2/2+, whereas for ecotropic vectors with no ability to infect human cells, BSL-1 containment may be appropriate.


    The host range of recombinant MMLV vectors is dependent on the specificity of the viral envelope. The ecotropic env gene produces particles that infect only rodent cells. Amphotropic env gene allows infection of murine and nonmurine cells, including human cells. VSV-G envelope allows infection in a wide range of mammalian and non-mammalian cells. Biosafety Level 2 (BSL-2) is appropriate for many constructs, while higher levels may be required depending upon the construct. See the MMLV Fact Sheet for more information.


    Lentiviruses are a subset of retroviruses, with the ability to integrate into host chromosomes, and to infect non-dividing cells. These viruses can cause severe immunologic and neurologic disease in their natural hosts. Lentivirus vector systems can include viruses of non-human/non-primate origin (feline immunodeficiency virus, equine infectious anemia virus) as well as simian viruses (simian immunodeficiency virus) and human viruses (HIV). The more recent generation vectors have been designed to significantly diminish the possibility for recombination to occur resulting in a wild type- potentially infectious virus. Typical lentivirus vectors are packaged using pseudotyped enveloped proteins. The most common envelope protein used for this purpose is from vesicular stomatitis virus (VSV). It is usually recommended that work with nonhuman lentiviruses that are incapable of establishing productive infections in humans be conducted at BSL-2. Work with simian or human lentiviruses (SIV, HIV) is typically conducted at a higher containment level. See the Lentivirus Fact Sheet for more information.

  1. Poxvirus vectors include avian viruses (avipox vectors) such as NYVAC and ALVAC, which cannot establish productive infections in humans, as well as mammalian poxviruses, which can productively infect humans -such as vaccinia virus and modified vaccinia viruses (MVA). Poxviruses are highly stable, and vaccinia virus can cause severe infections in immunocompromised persons, persons with certain underlying skin conditions, or pregnant women. Such individuals should not work with vaccinia virus. The use of BSL-2 is appropriate for many poxviruses and constructs. See the Poxvirus Fact Sheet for more information.

  1. Non-mammalian virus vectors that infect insects, these are very stable and may remain dormant in the environment for years before infecting insects. Work is mostly done at the BSL-1 level.

Rabies Virus
  1. Rabies virus is a member of the Rhabdoviridae family and is a common zoonotic infection from bats and other wild mammals. Infection results in encephalitis or paralysis, and is often fatal. Due to its neuronal tropism, pseudotyped rabies virus vectors can be used to study neuronal trafficking or express endogenous genes efficiently in neurons. Biosafety Level 2 (BSL-2) is appropriate for many constructs. See the Rabies Virus Fact Sheet for more information.

Sendai Virus
  1. Sendai virus (SeV) causes respiratory disease in rodents and sometimes swine. There is limited evidence of zoonotic transmission to humans, but the virus is capable of infecting human cell lines, and is similar to human parainfluenza virus type 1. For these reasons, SeV work is usually classified as BSL-2. See the Sendai Virus Fact Sheet for more information.

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