Coronavirus disease is caused by a single species, the SARS-CoV-2 virus, but there are distinct kinds of Coronavirus , and all-cause the same disease.
Confusingly, different types of viruses have been labeled variants, mutants, and strains. Adding to the confusion, those terms are also applied to the disease.
Just to be accurate before we cover more complex terminology: ‘Coronavirus’ is the species or individual virus particle; ‘Covid’ is the disease, and ‘Covid-19’ is the pandemic (although it’s also acceptable to use Covid-19 for the disease).
What do the terms mean?
So what’s the difference between ‘variant’, ‘mutant’, and ‘strain’? And is there a right way to refer to a distinct kind  of Coronavirus, such as B.1.1.7?
Part of the problem is that those three words are technical terms and layman language is looser. In conversation, even scientists use “Coronavirus” and “Covid” interchangeably as it’s often clear from the context within a sentence whether we’re talking about the virus, the disease, or even the pandemic.
Biologists like myself use various words for a group of organisms from the same species that share common traits, including ‘subspecies’ and ‘strain’.
A strain is classified by important genetic characteristics, but precisely which features should be considered important can differ from species to species. For germs that cause disease (pathogens), for instance, a strain might be a microbe with genes that confer resistance to antibiotics that would kill other strains.
But while ‘strain’ is a fuzzy concept, it’s arguably the least ambiguous of the three terms. Before the pandemic, if you had asked someone on the street what they would call a dangerous new virus, they would probably have said “strain.”
The word ‘mutant’ has similar — but not identical — meanings in biology and in popular culture. That double meaning creates ambiguity, which in turn leads to issues when trying to interpret the term.
Scientifically speaking, a mutant is the result of a mutation — a change in the genetic material (DNA or RNA) and produces something new. That mutation may have never appeared before or has reappeared in a population’s gene pool, so it’s a genetic variant that’s either novel or only occurs rarely. A mutant has mutated relative to another type that’s considered common and ‘normal, or ‘wildtype’.
Almost by definition, mutants are either recent or rare. As soon as a new mutant emerges, the clock starts ticking toward the day when they’re no longer news. That means a Coronavirus is only a ‘mutant’ if it mutated recently. Put it this way: if a mutation arose in your ancestor, that doesn’t make you a mutant.
Some scientists purposefully cause mutations that give viruses new abilities in an attempt to better understand the pathogen through gain-of-function mutation researches.
Over the past few months, ‘variant’ has become a widely-used term to describe a distinct type of Coronavirus. The reason why that happened requires speculation.
One possibility is that professionals use different words depending on their training and area of expertise. Scientists who build family trees use the term ‘lineage’ to describe a distinct branch, for instance, whereas those who work in public health call a noteworthy type of disease a ‘Variant Of Interest’ or VOC.
Because a global pandemic is primarily a public health issue, that might explain why ‘variant’ became a widespread term for describing one kind of Coronavirus — as in the ‘UK variant’, which is technically known as the ‘B.1.1.7’ lineage.
Which is the right word?
While professionals might want the general public to use a ‘correct’ term, any linguist will argue that you can’t really control how people use language.
Although there isn’t really one ‘right’ way to describe a distinct type of Coronavirus, there are words you shouldn’t be using with Covid and Covid-19.
Importantly, ‘mutant’ and ‘strain’ can only apply to organisms, not outcomes. Saying “Covid mutant” or “Covid strain” is grammatically incorrect and makes you sound like an idiot. To see why to replace the acronyms ‘COVID’ and ‘COVID-19’ with the words they represent: the phrases ‘disease mutant’ and ‘pandemic strain’ don’t make sense.
Personally, I prefer the biological term ‘strain’ for a distinct virus. When referring to the disease, however, ‘Covid variant’ is the only choice. It can be applied to both the virus and disease without being wrong.
The variant is also technically correct — the best kind of correct.
: For clarity, I deliberately use ‘Coronavirus’ (capital ‘C’) to refer to SARS-CoV-2. That helps distinguish it from ‘coronavirus’ (lowercase ‘c’), which could mean an individual virus particle (what we biologists call a ‘virion’).
: Throughout this article, I use standard words ‘kind’ and ‘type’ because they’re less likely to be confused with more technical terms. The two words are used interchangeably as synonyms, as they’re used in everyday speech.