Credit CDC Nowcast
#15,591
Two weeks ago, in CDC Nowcast: Omicron BA.2 Beginning To Show Up, we saw the CDC's first attempt to estimate the prevalence (3.6%) of the Omicron BA.2 subvariant across the country.
At the time, I warned "It takes a few weeks for new variants to begin showing up reliably in these Nowcasts" and we'd likely see `adjustments' made to current and past estimates over the next several weeks.
Last week, CDC Nowcast: Now Tracking 3 Omicron Variants (B.1.1.529, BA.1.1 & BA.2), we saw the additional breakout of the BA.1.1 subvariant (73%), and a rollback of the previous week's BA.2 estimate to just 1.6%, with last week's prevalence estimated at 3.9%.
Yesterday the CDC released their 3rd week of BA.2 estimates, and once again they have `adjusted' the first two weeks downward (1% and 2%), and now estimate this week's prevalence as 3.8% (range 3.0-4.8). BA.1.1 is estimated to be 75.6%.
Although BA.2 is pretty much at the same level the CDC first estimated 2 weeks ago, when you factor in the previous week's `adjusted' estimates, it is still doubling every week (1% to 2% to 3.8%). It's prevalence is significantly higher in New England and the West Coast than in the deep south and midwest.
As we discussed yesterday (see WHO Statement On Omicron BA.2 Sublineage), while we've seen some studies suggesting that BA.2 may pose a greater public health threat than BA.1, countries currently affected by this variant haven't reporting increased severity.
For now, BA.1.1 remains dominant, but BA.2 is expected to erode its market share in the weeks to come, and eventually take the lead. Assuming something more `biologically fit' doesn't emerge in the meantime.
The CDC has also revised and updated their estimates of variants by districts, eliminating many of the variants no longer circulating and concentrating (for now) on only Omicron and Delta. Unlike the weekly estimates above, these numbers are based on data gathered over the 4 weeks ending 1/29/2022.
Given the limits of surveillance, testing, and genomic sequencing around the country, all of these numbers should be viewed as rough estimates which are mostly useful for spotting trends.
