Virus/Virus Interactions

Definition of Virus-Virus Interaction (VVI):

We define a VVI as a measurable difference in the course of infection of one virus as a result of a concurrent or prior infection by a different species or strain of virus. (For a complete description of our classification scheme, see this reprint which was published as "A systematic approach to virus-virus interactions", Virus Res. 2010 Apr;149(1):1-9.) A concurrent infection may include infection of the same cell by two or more virus species, or two viruses may infect different cell types within one organism and produce measurable VVIs. Measurable differences include changes in tissue permissiveness or tropism, viral replication, patterns of progeny production and release, latency, pathology including immunopathology, and immunological responses.

The simple case of a prior viral infection conferring protective immunity against future infections with an immunologically identical virus is well known in vertebrates. But this interaction follows directly from the role of the adaptive immune system, and is therefore a rather uninteresting sort of interaction and not included in this definition. Virus-induced generalized immunosuppression, as seen with human immunodeficiency virus (HIV)/AIDS is also not included here, since generalized immunosuppression, regardless of the cause, results in increased replication and pathology by some viruses and other pathogens that have a benign disease course in immunocompetent persons. Therefore, the inclusion of VVI involving virus-induced immunosuppression will be limited here to those in which there is evidence of other viruses impacting the course of infection of the immunosuppressive virus, or evidence of direct interaction of proteins or nucleic acids of the immunosuppressive virus with proteins or nucleic acids of a coinfecting virus. These predictable examples aside, reports of VVIs involving unexpected direct, indirect environmental, and immunological interactions continue to accumulate. We welcome the help of the scientific community in bringing our attention to published evidence of virus-virus interactions that are new or that we have missed.

Background and purpose:

By design, viral infections in the laboratory almost always occur in the absence of other viruses. While this may be a logical starting point for virus research, it is now clear that viral infections in nature rarely occur in isolation. Polymerase chain reaction (PCR) based techniques have revealed that persistent viral infections are present in all domains of life, and virus coinfections would seem to be the rule rather than the exception in nature. Yet, they have rarely been systematically studied for possible effects of one virus on the other, with most documented interactions having been discovered accidently. It is certainly true that coinfection may not result in interactions for all virus species. However, because many viruses induce profound changes in their host, it seems likely that virus-virus interactions are common and may be critical to understanding viral pathogenesis and evolution. Therefore, the purpose of this webpage is to provide a forum for the continued identification and categorization of virus-virus interactions (VVIs) with the expectation that a central repository of such information will encourage more research into these interesting biological phenomena.

About the database:

This database was started by Professor Laura Kasman as part of a graduate course in Emerging Infectious Disease with three graduate students (Telma De Palma, Bently Doonan, and Nicole Trager) in the Microbiology and Immunology Graduate Program at the Medical University of South Carolina in Charleston.


We welcome the help of the scientific community in bringing our attention to published evidence of virus-virus interactions that are new or that we have missed. We also welcome feedback from the scientific community regarding the accuracy of this database. Please email with comments or published reports of VVI.

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Virus/Virus Interaction Homepage / Laura Kasman / Medical University of South Carolina

Website: Alex Kasman, College of Charleston