Mutations are changes in the virus’s nucleotide sequence, the RNA code that determines how it is assembled. A variant is a strain of the virus that contains one or usually several mutations. The most concerning variants are ones with mutations in the shape of the spike protein the virus uses to attach itself to our cells. Vaccines and antibody drugs also target this spike protein.
Dangerous changes in the spike protein include those that make it ‘stickier’ and let it attach to our cells more easily. These kinds of changes can make a variant more easily transmitted, since it can take fewer viral particles (virions) to cause infection, or cause more serious disease since it can enter our cells more easily. Another threatening change in the spike protein can make it harder or impossible for antibodies from vaccines or some drugs to attach to it and thus no longer be effective.