Omicron Became Dominant COVID Variant in U.S. Faster Than Delta

The Omicron COVID variant is surging in the U.S. and is now thought to account for more than 73 percent of new cases.

Data from the Centers for Disease Control and Prevention (CDC) shows that Delta, which has been the dominant variant in the country for several months, now accounts for just 26.6 percent of new cases.

Omicron's rise has been rapid. In the U.S., the variant is thought to have been around since the middle of November after the first U.S. case was confirmed in a Californian man on December 1. He had returned to the country from South Africa on November 22.

It's unclear if the variant had been in the U.S. before then. The CDC has said a patient developed symptoms earlier, on November 15.

This month, the variant has gone from accounting for less than 1 percent of cases in the week ending December 4 to around 12.6 percent of cases in the week ending December 11, and to 73.2 percent of cases in the week ending December 18.

This appears to be even faster than the spread of Delta in the U.S. back in the summer. On June 8, Biden administration officials said that Delta accounted for 6 percent of new U.S. COVID cases.

By early July it had passed the 50 percent threshold, and on July 20 it accounted for more than 80 percent of new U.S. COVID cases.

It's unclear exactly how much faster Omicron's spread has been due to uncertainties surrounding how long the variant has been in the U.S.

In addition, the current CDC figures are based on Nowcast estimates, which are projections due to reporting lags.

Just days after Omicron was first identified in November experts warned that it might be incredibly infectious, and ongoing data suggests that this is the case.

Much of the initial concern around Omicron was based on how many mutations it has compared to other variants. There are at least 30 in the spike protein alone, and this protein is what the virus uses to enter human cells and make people sick.

Higher Risk of Reinfection

On December 17, the U.K.'s Health Security Agency said in a technical briefing that Omicron poses a higher household transmission risk than Delta and that the risk of reinfection with the Omicron variant may be around three times higher than for other variants.

The U.S. isn't the only country in which Omicron is now dominant despite only coming to the world's attention around a month ago.

Health officials in England, Scotland and Ireland have all said that the Omicron variant appears to be dominant there, and other European nations like Denmark and the Netherlands expect this to be the case for them too in a matter of days, if it isn't already.

omicron testing covid
A person has a COVID test on December 15, 2021 in New York City. The Omicron variant is thought to be dominant in the U.S., based on CDC projections this week. David Dee Delgado/Getty Images