Last week, a COVID-19 variant known as FL.1.5.1 was responsible for an estimated 13% of infections, according to virus tracking data from the Centers for Disease Control and Prevention. But despite a surge in COVID-19 cases this summer that continues to rise as the school year starts, you may not have even heard of this variant. FL.1.5.1 is an alias for XBB.1.9.2.1.5.1, which descends from Omicron and is colloquially called Fornax. If you're confused about what this variant soup of names means and why you should care, you're not alone.
The World Health Organization (WHO) has criteria for determining which variants rise to a high enough threat level to warrant a Greek letter designation like Alpha, Delta or Omicron. Whether a variant rises to the level of a Greek letter designation or not, every variant gets a numeral name in the Pango naming system related to its genetic makeup, like XBB.1.5, which has driven the vast majority of infections in 2023 in the U.S.
