CDC Nowcast: Omicron BA.2 Continues Grow But BA.1.1 Still Dominant

 

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Although the BA.2 sublineage of Omicron is gaining ground in the United States it appears to be doing so at a slower rate than we've seen in places like Denmark and the UK.  Revised estimates show BA.2 was at detectable levels 6 weeks ago (.4%), and has been doubling roughly every week. 

This week's estimate is 8.3%, but that number varies from over 19% in New England, to only 3.2% in the midwest (see map above).    

The BA.1.1 subvariant remains unchanged since last week (71.3% share), while the parental strain (B.1.1.529) continues to give up ground having dropped from 38% to 17% over the past 6 weeks.  This week's CDC Nowcast shows the only COVID variant gaining ground to be BA.2. 

Given the limits of surveillance, testing, and genomic sequencing around the country, all of these numbers should be viewed as rough estimates - and as we've seen frequently over the past few weeks - are subject to revision. 

For now, BA.1.1 remains dominant, but if BA.2 performs as it has in other countries, it is expected to eventually over take it.  Assuming something more `biologically fit' doesn't emerge in the meantime. 

We've seen conflicting reports regarding the threat posed by Omicron BA.2 (see here, here, and here), so far we haven't seen any evidence that it is any more severe than BA.1.1 from countries where it is now dominant (see WHO Statement On Omicron BA.2 Sublineage).

Most of our data comes from highly vaccinated countries, which means there could be some regional differences in its impact. BA.2 is reportedly behind Hong Kong's dramatic rise in COVID cases, illustrating how difficult it is to control. 

How all of this plays out in the United States as we move into warmer spring and summer months - and relax our pandemic precautions - is the big question.  We'll be watching the UK carefully, as they just announced that BA.2 passed the 50% mark of new cases last week. 

Stay tuned.