Friday, February 03, 2023

CDC Nowcast: XBB.1.5 Estimated To Be 2/3rds Of All COVID Cases In the U.S.



The CDC has published their updated COVID Nowcast estimates, which continues to show the slow erosion of BQ.1/BQ.1.1 and the rise of XBB.1.5 (now estimated at 66.4% of cases). The prevalence of the CH.1.1 variant - which has been making strides in the UK - appears little changed.

All of these numbers are estimates, and revisions to previous week's numbers are common.  As an example, last week's estimate on the BQ.1.1 variant was that it made up 21.8% of cases, but today last week's number has been revised upward to 25.2%. 

The estimates are more useful for spotting trends than nailing down exact percentages. The CDC has recently added the following caveat to their report.

Projections for an emerging lineage with a high growth rate may have a higher degree of uncertainty (wider predictive interval) when it is just beginning to spread and still has low weighted estimates. Projections may also be biased during times of delayed reporting (e.g., around holidays). CDC performs frequent evaluations of Nowcast to inform performance improvements. 

Both XBB.1.5 and CH.1.1 were the subject of a preprint we looked at earlier this month  (see Preprint: Extraordinary Evasion of Neutralizing Antibody Response by Omicron XBB.1.5, CH.1.1 and CA.3.1 Variants), that found `a nearly complete escape of these variants from neutralizing antibodies stimulated by three doses of mRNA vaccine'.  

The data, however, suggests that neutralization can be improved (2~8-fold higher nAb titers) by receipt of the bivalent booster.

This weeks (estimated) breakdown of COVID variants circulating in the United States follows.

The revised trend chart (below) shows the changing of the guard from BQ.1.x to XBB.1.5 over the past few  months.

While XBB.1.5 appears to be be king of the viral hill right now, how long it will reign is unknown. Omicron is highly mutable, and another more transmissible or immune evasive variant could emerge at any time.