At present, the variant is referred to as “SARS-CoV-2 VUI 202012/01” (i.e., the first variant under investigation from 2020, December), or “B.1.1.7.” The press often uses the terms “variant,” “strain,” “lineage,” and “mutant” interchangeably. For the time being in the context of this variant, the first three of these terms are generally being used interchangeably by the scientific community as well.

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Strains

According to Van Regenmortel, a (natural) virus strain is a “variant of a given virus that is recognizable because it possesses some unique phenotypic characteristics that remain stable under natural conditions” [emphasis added by the authors] [[url=https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3535543/#CR30]30]. Such “unique phenotypic characteristics” are biological properties different from the compared reference virus, such as unique antigenic properties, host range or the signs of disease it causes. Importantly, as Van Regenmortel points out, a virus variant with a simple “difference in genome sequence…is not given the status of a separate strain since there is no recognizable distinct viral phenotype” [[url=https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3535543/#CR30]30]. This definition is very similar to that of Fauquet and Stanley, who argued that “strains are viruses that belong to the same species and differ in having stable and heritable biological, serological, and/or molecular characters [sic]” [[url=https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3535543/#CR8]8]. These two definitions are also reflected in the words for “strain” in other languages, such as German (Stamm), which back-translate to “trunk” rather than “branch”, i.e., the word implies something fundamentally different from a reference entity despite it being directly related to it, possibly with little genomic sequence variation. A strain is therefore a genetically stable virus variant that differs from a natural reference virus (type variant) in that it causes a significantly different, observable, phenotype of infection (different kind of disease, infecting a different kind of host, being transmitted by different means etc.). “Genetically stable” means that the genomic changes associated with the phenotypic change are largely preserved over time through natural selection. The extent of genomic sequence variation is irrelevant for the classification of a variant as a strain since a distinct phenotype sometimes arises from few mutations. “Observable phenotype” means, for instance, that within a comparative animal experiment, it would be possible for the researcher to distinguish between the reference control virus-infected animal and the animal infected with the alleged new strain, without knowing which animal received which virus and without having any information about the differences between the two viruses. The designation of a virus variant as a virus strain would be the responsibility of international expert groups. Thus far, despite the abundant indiscriminate use of the word “strain” in the filovirus literature, natural filovirus strains according to this definition have not been reported. All described genetic variants of EBOV, for instance, cause a similar hemorrhagic fever in humans and even experimental animals and are transmitted similarly. None of the known EBOV genetic variants can be distinguished from others on clinical grounds alone. In fact, their variety seems to be limited to subtle differences in growth kinetics and plaque formation in vitro or subtle changes in the duration of disease in experimental animals, and ultimately derives from limited, but often stable, differences in genomic sequence [[url=https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3535543/#CR13]13]. This also holds true for the different genetic variants of MARV, RAVV, BDBV, RESTV, and SUDV (currently, there is only one isolate of TAFV and none of LLOV). We therefore recommend abstaining from using the word “strain” in context of any natural filovirus until either a particular genetic filovirus variant is discovered that causes a difference in disease phenotype and/or until expert groups establish a clear-cut definition of what “phenotype” means and to which extent phenotypes must differ to establish a filovirus as a strain