World Health Organization (WHO) official Dr. Maria Van Kerkhove said earlier this week that the agency is tracking four omicron sub-variants, including BA.1, BA.1.1, BA.2 and BA.3.

The infectious disease epidemiologist explained in a video clip she tweeted on Thursday that the agency is “watching [the] virus evolve in real-time,” tracking proportions of the sub-variants.

“Now, we already know that omicron has a growth advantage; it’s more transmissible compared to other variants of concern and also has properties of immune escape,” she said. “But, we know some of the sub-lineages, BA.2, has a growth advantage even over BA.1.”