No, 73% of US COVID-19 cases aren’t Omicron yet: how the press got it wrong

Something strange happened this afternoon. The press, including the New York Times, Washington Post, CNN, Wall Street Journal, USA Today, and Axios, reported that the CDC had announced that 73% of U.S. COVID-19 cases were now caused by the Omicron variant. Even top public health figures like Ashish Jha, Eric Topol, and Scott Gottlieb shared the news without question. But what actually occurred was far more complex. There was a glitch in an automated system at the CDC known as the "variant proportions Nowcast." This algorithm is designed to estimate current variant prevalence based on older genomic data. It's not a direct measurement, but rather a projection. And in this case, it went seriously wrong. The 73% number didn’t come from a press release or a statement by any CDC official. It came from the CDC’s online data tracker — an automated system that uses statistical models to predict current trends. But this time, the model produced results that don’t align with real-world data or scientific expectations. To understand what went wrong, we need to look at how the Nowcast works. Every week, only a small percentage of U.S. cases are sequenced to determine their variant. Right now, that's about 3.6%, varying by state. The CDC uses this data to create weekly reports on variant distribution across ten regions. However, because sequencing takes time, the most recent data is typically two weeks old. To give a sense of the current situation, the CDC uses the Nowcast algorithm to project forward from that data. In the latest update, the Nowcast suddenly showed Omicron surging to 73% nationwide — a number that doesn’t match the actual data. In the week ending December 4, Omicron was just 0.7%. Then, according to the Nowcast, it jumped to 12.6% the next week and then shot up to 73.2% the following week. That would mean Omicron grew twentyfold in just one week, with a doubling time of about 36 hours — far faster than what we’ve seen in other countries like the UK or Denmark. This kind of growth rate isn’t supported by the data. Experts have estimated Omicron’s doubling time to be between two and four days, which is already extremely fast. A 36-hour doubling time over two weeks is unrealistic and inconsistent with the observed patterns of case numbers and regional spread. Even more concerning is the discrepancy in regional data. For example, Region 10 (which includes Oregon, Washington, and Idaho) had zero Omicron cases in the latest data, yet the Nowcast claimed it had reached 96.3% prevalence. This suggests the algorithm may be overestimating growth in areas where the actual data shows little to no change. What’s clear is that the Nowcast has a bug — and it’s causing confusion. While the CDC hasn’t officially commented, it’s likely they’ll issue a correction soon. Meanwhile, the media and experts who repeated the 73% number will have to backtrack. This incident highlights a bigger issue: the pressure of real-time data during a rapidly evolving pandemic. The systems in place, while generally reliable, haven’t been tested under such extreme conditions. As Omicron spreads, these tools will need to adapt quickly. So, what should you do? First, continue to prepare for Omicron to dominate in your area soon — it still will. Second, stay informed but skeptical of sensational headlines. Third, keep practicing caution and follow public health guidelines. Omicron is coming, and it’s going to be big. But this story is just a temporary misstep — not the full picture.

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