According to the updated December 25, 2021 Nowcast website from the Centers for Disease Control and Prevention (CDC), the Omicron variant now makes 58.6% of COVID-19 cases in the United States, down from the December 18, 2021 report of 73.2 percent. The Delta variant was reported as 41% of the cases.
Nowcast is a model that estimates more recent proportions of circulating variants and enables timely public health action. CDC is providing weekly Nowcast estimates which are updated every week on Tuesday.
The CDC says the Nowcast estimates may fluctuate because it can take 2-3 weeks from the time a specimen is collected to when its sequence data is available for analysis, Nowcast is an important tool that can estimate variant proportions for more recent time intervals.
Nowcast does not predict future spread of the virus, but it does help estimate current prevalence of variants, based on genomic surveillance data from previous weeks. Estimates of variant proportions for previous weeks may change as more data are reported.