Hence the variability in the severity of illness was suspected to correlate with genetic differences in the victims. This has been difficult to study in animal models that respond to the virus with hemorrhagic fever in a similar manner as humans, because typical mouse models do not so respond, and the required large numbers of appropriate test subjects are not easily available. In late October 2014, a publication reported a study of the response to a mouse-adapted strain of Zaire ebolavirus presented by a genetically diverse population of mice that was bred to have a range of responses to the virus that includes fatality from hemorrhagic fever. In December 2016, a study found the VSV-EBOV vaccine to be 70–100% effective against the Ebola virus, making it the first vaccine against the disease. VSV-EBOV was approved by the U.S. Food and Drug Administration in December 2019. Ebola virus was first identified as a possible new "strain" of Marburg virus in 1976. The International Committee on Taxonomy of Viruses (ICTV) identifies Ebola virus as species "Zaire ebolavirus", which is part of the genus "Ebolavirus", family "Filoviridae", order "Mononegavirales". The name "Ebola virus" is derived from the Ebola River—a river that was at first thought to be in close proximity to the area in Democratic Republic of Congo, previously called Zaire, where the 1976 Zaire Ebola virus outbreak occurred—and the taxonomic suffix "virus". In 1998, the virus name was changed to "Zaire Ebola virus" and in 2002 to species "Zaire ebolavirus". However, most scientific articles continued to refer to "Ebola virus" or used the terms "Ebola virus" and ""Zaire ebolavirus"" in parallel. Consequently, in 2010, a group of researchers recommended that the name "Ebola virus" be adopted for a subclassification within the species "Zaire ebolavirus", with the corresponding abbreviation EBOV. Previous abbreviations for the virus were EBOV-Z (for "Ebola virus Zaire") and ZEBOV (for "Zaire Ebola virus" or ""Zaire ebolavirus""). In 2011, the ICTV explicitly rejected a proposal (2010.010bV) to recognize this name, as ICTV does not designate names for subtypes, variants, strains, or other subspecies level groupings.