Predominantly focused in the northeast and Mid-Atlantic where it represents even higher percentages of new cases. Preliminary data suggests the updated booster should provide protection against XBB.1.5 despite its high immune evasiveness.
XBB.1.5 is a recombinant variant resulting from a co-infection of two BA.2 sub-variants, BA.2.10.1 and BA.2.75. Such recombination isn't uncommon with coronaviruses. We see it with flu viruses too. Fortunately unlike influenza, coronaviruses can't undergo reassortment.
There is no indication XBB.1.5 is more severe than other variants. But it is believed to be more transmissible and immune evasive.
OG Omicron was very immune evasive compared to Delta and had a shorter generation time, so it's Rt was high.