Post #1385: Revised CDC estimate of Omicron incidence, and the Omicron case hospitalization rate.

Posted on January 4, 2022

The CDC issued another weeks’ worth of estimates of Omicron as a fraction of all cases. 

For the week ending 1/1/2022, they estimate that 95% of cases were Omicron. Estimates for prior weeks were revised as well.  Below, the current estimates are in the first column.  And you can see how those numbers have bounced around from week to week by reading across the additional columns.

Source:  CDC COVID data tracker, accessed on various dates ending 4/1/2022.

Assuming this latest estimate is more-or-less correct, this has implications for a lot of things.  Some good, some not so good, some neither/nor.

The worst of which is that the U.S. case hospitalization rate for Omicron is nowhere near as low as one-third that of Delta.  It’s now looking like it’s half, or more.

Implication 1, Good:  This should be as bad as it gets, for case growth.

I think that’s pretty obvious.  If it’s all Omicron now, every case is as infectious as it can be.  We’re no longer substituting the far-more-infectious Omicron for the less infectious Delta.

Implication 2, Neither here nor there:  Omicron has been pushing Delta out all along.

This probably only matters to me, because I went out of my way to estimate the actual number of cases by variant type.  Under the old CDC estimates, it looked like Delta hung around, and Omicron layered on top of that.  Now, it looks like Omicron has been pushing Delta aside from the start.  Like so.

Implication 3, Bad:  The case hospitalization rate for Omicron in the U.S. is nowhere near as low as one-third that of Delta.

In prior posts, to estimate the hospitalization rate for Omicron, from the observed data, I had to back-solve.  I had to use the estimated Delta and Omicron share of cases, and a bit of algebra, to net out an estimate of Delta’s contribution to the observed hospitalization rate.

The surprise here is that, based on this most recent CDC estimate, the Delta contribution was more-or-less zero.  So the raw case hospitalization rate we’ve been observing for the past few days is more-or-less the U.S. Omicron rate.  No deductions for Delta needed.

Best guess, for the seven days ending 1/3/2022, plus or minus some possible timing issues to sync up the two data series, we had:

  • 109,000 COVID-19 hospital admissions
  • 2,621,000 new COVID-19 cases
  • Roughly 97.5% of those were Omicron

That’s a case hospitalization rate of 4.2% for a mix of cases that is very nearly all Omicron, compared to maybe a 6.5% case hospitalization rate under Delta.

At the rate at which we’re seeing new infections, we’re certainly going to fill a lot of acute-care beds with Omicron cases.  Whether or not we fill up the ICUs as a result of that remains an open question.  And it’ll be another week or two before we can even get a rough handle on the death rate.