According to the Centers for Disease Control and Prevention (CDC), the Omicron strain of the coronavirus now accounts for more than 73% of new Covid-19 infections in the United States, up from 13% just a week ago.

The CDC revealed figures for the week ending December 18 on Monday, indicating that Omicron had overtaken Covid-19 as the dominant variety in the United States less than a month after being first identified in South Africa. Almost all other new Covid-19 infections, or approximately 27%, are caused by the Delta strain.

The Omicron figures are released on the eve of President Joe Biden's Tuesday speech, during which he is anticipated to announce new actions to combat the spread of Covid-19. Major cities in the United States have already tightened pandemic restrictions. Boston Mayor Michelle Wu announced Monday that the city would require proof of immunization for entry into indoor businesses. At the same time, Washington, DC reinstated a mask requirement and required city employees to receive booster doses.

Dr. Anthony Fauci, the White House medical advisor, predicted on Sunday that the next few weeks to months would be "tough" as Omicron spreads swiftly across the United States. Covid-19 infections have increased dramatically in recent weeks, and Biden has warned of a "winter of severe illness and death" for those not vaccinated.

However, health professionals in South Africa – where Omicron was first discovered – have stated that the variant is associated with relatively subtle symptoms. Even as the strain spread like wildfire across the country, hospitalizations for Covid-19 remained stable. Over the last week, South Africa has had an average of 30 Covid-19 deaths per day, down from a high of around 600 in January and approximately 400 in July.

Dr. Angelique Coetzee, chairwoman of the South African Medical Association, said Monday that the country is "ahead of the curve" with the new variation with Omicron instances dropping in South Africa. International reaction to the new strain, which includes travel bans on southern African countries, she has dubbed "a storm in a teacup."