Viruses such as SARS-CoV-2, which causes COVID-19, make imperfect copies of themselves as they move from person to person or from an animal to a person. This leads to constant mutations and new variants. The term “variant” describes a version of the virus with a specific set of mutations. Many mutations often don’t provide any advantage to the virus. But sometimes a mutation can make a virus more transmissible or change the severity of the disease the virus causes. When the same mutation emerges in multiple places it could indicate that these mutations provide some advantage for the virus—making it easier to spread or to evade antibodies, for example.