Viruses multiply by copying themselves over and over, but these copies can alter slightly as they go along – becoming a new variant. Most of the time, these variants don’t alter much about the way the virus works. However, sometimes they can alter factors such as how quickly the virus spreads, whether it can evade the body’s immune response more effectively, or even the severity of the disease it causes. When this happens, a variant will go from one that’s simply being monitored (also known as a variant under investigation) to one of concern – and that requires action to reduce its spread.