In the third year of the COVID-19 pandemic, there is no shortage of new coronavirus variants to track.

On April 11, Japan reported its first case of the omicron XE variant, in a woman who arrived at Narita Airport from the U.S. That was followed by news on May 2 that a man in his 50s who flew to Haneda Airport from the U.S. was also infected with XE. Also, on April 28, a previously unknown omicron variant was reported in a patient in the city of Sendai.

So what are these variants? Why do new variants keep emerging? And how alarmed should we be? Here’s what you need to know about the current situation regarding coronavirus variants.