How are things going with Omicron? Hard to tell if you listen to the CDC.

The holidays have brought a surge in COVID-19 infections as well as an increase in the percent of infections with the Omicron variant. Most people don’t need me telling them this since almost everyone knows many people who are currently infected.

New COVID-19 cases in the USA (New York Times graphic). A tremendous rise is seen in December, equalling the peak seen last winter.

COVID-19 infections also seem to be much more insidious, and people often can’t identify when they might have been exposed to the virus. This is likely because of the impressive degree of contagion associated with Omicron, making much more casual exposures likely to cause an infection. Therefore, one doesn’t need a large gathering or other “event” to acquire COVID-19.

New cases, hospitalizations, and deaths in the USA.

While this rise in cases is impressive, the concurrent rise in hospitalizations and deaths observed with prior waves has not materialized. Hospitalizations have increased to levels last seen in October, when we were dealing with Delta, but that was a time when infections were 3-4 times lower than current levels. Daily deaths have been stable or decreased during the current period in December, which is unexpected even with the lag in deaths after COVID-19 infection.

The reasons for these trends are multiple. They include increased immunity from vaccines and prior infections, better treatments, and potentially less severe disease with the Omicron variant. I wrote about this in last week’s blog

Figuring the impact of Omicron on U.S. infections has been difficult, due to problems with CDC monitoring of the prevalence of different SARS-CoV-2 variants in the United States. The rise of the Omicron variant in the U.S. was significantly overestimated by the CDC. The agency reduced the estimate of the percent of Omicron from 73% to 23% for the week ending December 18th. This means most of the infections that week were due to Delta and not Omicron.

The CDC has dismissed this overestimate as being “within its confidence limits” for virus variant tracking. However, the press releases that gave out the 73% number did not focus on confidence limits. More importantly, there are significant implications for this change.

Deaths and serious hospitalizations have reportedly been more common with the Delta variant, which remained the primary source of infection in the U.S. during the last two weeks. Therefore, the CDC’s incorrect estimate gave the implication that Omicron might cause more serious infections than have been reported in South Africa.

The CDC’s tracking of variant viruses had been criticized as inadequate when compared with other countries, such as the UK, Israel and even South Africa. To identify and track SARS-CoV-2 variants, the CDC uses genomic surveillance identical to these other nations. Virus genetic sequences are analyzed and classified as a particular variant. 

The proportion of different variants in the U.S. population are then calculated nationally, by HHS region, and by jurisdiction. The problem is that the CDC samples proportionally fewer cases which results in greater variability. Hence the 73% vs. 23% error.

CDC estimates of Omicron (purple) vs. Delta (orange) COVID in different areas of the U.S. Note that Delta infection remains close to half of the infections Christmas week.

Omicron is now dominant overall in the U.S., reportedly at 53% of COVID-19 cases for the week ending December 25. However, significant areas remain Delta dominant. In areas where Omicron is dominant, hospitalizations have not seemed to increase significantly. Given the lower percentages we now see with the new CDC numbers, we still don’t know the overall impact of Omicron on the U.S. 

Published by jbakerjrblog

Immunologist, former Army MD, former head of allergy and clinical immunology at University of Michigan, vaccine developer and opinionated guy.

2 thoughts on “How are things going with Omicron? Hard to tell if you listen to the CDC.

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