The Omicron variant arrived in the United States right around Thanksgiving. Less than a month later, it’s the country’s dominant coronavirus strain, accounting for 73% of new infections last week, according to the Centers for Disease Control and Prevention.

How did that happen? Infectious disease experts say there are two key factors that determine how quickly a virus will spread: how easily it is transmitted and how well it eludes the body’s defenses.

Early research suggests Omicron has advantages in both areas. But the data also suggest the variant’s higher rate of transmission hasn’t led to more hospitalizations or deaths.

Preliminary results from a Dec. 14 study led by Alejandro B. Balazs of the Ragon Institute in Cambridge, Mass., found that Omicron was twice as infectious as the Delta variant and four times more infectious than the original virus. That study, which has yet to be peer-reviewed, relied on a relatively small sample of 239 patients in and around Boston, so the results may not be representative of Omicron’s behavior in general.