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The Omicron variant is starting to eat into Delta’s dominance in the United States.

The new variant accounted for 2.9% of sequenced Covid-19 cases in the United States in the week ending Dec. 11. The week before, 0% of cases were from Omicron. Delta accounted for essentially all of the other sequenced cases, according to data from the Centers for Disease Control and Prevention.

The new figures, updated Tuesday, indicate that Omicron started circulating before that week, given how long it can take for infections to be sequenced and reported. They show that Omicron’s advantage over the highly transmissible Delta variant is becoming noticeable in this country.

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The figure is likely to increase dramatically, given the pattern in other countries. When better-spreading variants enter a new area with other viral iterations circulating, it can take weeks for them to account for even a few percent of all cases. But from there, their prevalence can skyrocket.

This week, virus trackers in Washington state said that clues from testing results suggest Omicron is rapidly on the rise there. In a tweet Tuesday, CDC Director Rochelle Walensky said Omicron was accounting for about 13% of sequenced cases in New York and New Jersey.

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And in recent days, scientists in Europe — from Scotland to Denmark to Norway — have also reported how fast Omicron was growing in their countries and warned about a coming surge of infections. Some places in Europe have mostly vaccinated populations and were in the midst of Delta outbreaks as Omicron began to take hold — similar landscapes to the U.S. situation.

Experts have said it appears Omicron is taking over faster than Delta did as it became dominant globally earlier this year.

Current data from South Africa — which had some of the first detected Omicron cases — indicate that Omicron is causing milder infections than other forms of the coronavirus, perhaps not because of a change in the virus’ inherent virulence, but because prior infections and vaccinations are keeping people from developing serious illness. But experts caution that by leading to more cases overall, better spreading viruses can cause higher numbers of severe infections, even if the rate of serious disease is lower.

Some U.S. health systems are already inundated by Covid-19 cases from Delta and are facing staff shortages. Even a small increase in severe Covid-19 infections could threaten care in more places.

Omicron appears to get its spreading advantage over Delta in large part because it is better at circulating among people who’ve been vaccinated or had previous Covid-19 cases. It might also get a boost from intrinsically being more effective at transmitting.

Early studies suggest that people who’ve had Covid-19 or are fully vaccinated become widely susceptible to infection from Omicron, but that most should remain protected from serious outcomes. It appears that having been infected and then vaccinated, or receiving a booster shot on top of the primary Covid-19 vaccine series, can restore much of the protection that’s lost in the face of Omicron.

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