Viruses change over time and when a new mutation occurs, it’s called a variant. During the COVID-19 pandemic, there have been hundreds of different variants found in people infected with the virus.

These changes can affect how the virus spreads, if people get sick or die, and how vaccines work. Some of these variants fade away, while others keep spreading and changing. CDC and other public health partners carefully monitor the COVID-19 variants that are circulating in the community to understand how the virus is spreading, what people are experiencing, and whether the virus is becoming more dangerous for those unvaccinated against it.

The first variant to make a big splash was Alpha (B.1.1.7), which appeared in late 2020 and caused a huge spike in cases. Then it faded and was replaced by the Delta variant. Delta was less contagious and triggered fewer symptoms, like cough and loss of smell, in people than Alpha did. But the strain was still serious enough that it led to dozens of deaths, and researchers feared people would misdiagnose the symptoms for a cold and avoid quarantine, helping it spread even more.

XFG is a mix of two earlier variants, LF.7 and LP.8.1. It has four key mutations in its spike protein that may explain why it’s muscling out other variants. These mutations make it more likely to attach to the ACE2 receptor in cells and cause disease, and they also weaken how well antibodies against the virus neutralize it. XFG is not a Variant of Concern, but it’s still being closely monitored.