The Omicron variant (Pango Lineage B.1.1.529) has been classified into the following sublineages: BA.1, BA.1.1, BA.2, BA.3, BA.4, and BA.5. Each sublineage is further classified into several distinct sublineages due to accumulation of additional mutations. 

Of note, several sublineages of BA.2, BA.4, and BA.5 have acquired similar mutations that are associated with immune evasion and resistance to treatments. One specific mutation at position 346 in the receptor binding domain has appeared in many variants such as BA.2.75.2 and XBB (BA.2 sublineages), BA.4.6 (BA.4 sublineage), BF.7 and BQ.1.1 (BA.5 sublineages). Mutations at other locations on the receptor binding domain are also common and have been shown to be antigenically significant in prior variants of concern.

Current Sublineage Proportions

As of February 15, 2023, for the month of January, BQ.1.1 (36.8%), BQ.1 (24.5%), XBB.1.5 (16.4%), BA.5 (7.5%), BA.2.75 (5.5%), and BF.7 (1.9%) make up the confirmed Omicron cases sequenced in California. Since there is a known delay in the availability of sequencing results, CDPH models provide estimates of sublineage proportions for the most recent weeks. Models are updated weekly on Thursdays. Based on CDPH model estimates, variants with highest proportions in California are XBB.1.5 (59.4% (44.0-80.7%)), BQ.1.1 (25.2% (13.5-40.9)), BQ.1 (7.5% (4.4-15.3%)), XBB (5.4% (1.8-11.2%), and BA.5 (2.6% (1.2-7.1%)).​

Sublineage Proportions and Clinical Decision-Making

Per National Institutes of Health (NIH) guidance, the prevalence of certain Omicron subvariants (i.e., BA.4.6, BA.2.75.2, BA.5.2.6, BF.7, BQ.1, BQ.1.1, and XBB.1.5) that are resistant to tixagevimab plus cilgavimab (Evusheld) is rapidly increasing and the decision to administer Evusheld to a given patient should be based on the regional prevalence of the resistant subvariants, the individual patient's risks, the available resources, and logistics. Based on the CDPH model estimates, >90% of circulating sublineages in most California regions are likely to be resistant to Evusheld.

Please also note that bebtelovimab is not currently authorized for the treatment of COVID-19 in any region of the United States due to increasing prevalence of sublineages that have been associated with resistance to treatment with bebtelovimab. Providers should prioritize use of nirmatrelvir/ritonavir (Paxlovid) and remdesivir for treatment of mild to moderate COVID-19 in outpatients at risk for disease progression as these drugs continue to be effective against all Omicron sublineages. If neither of these are clinically appropriate, please see the NIH COVID-19 Treatment Guidelines for additional, effective options.