The highly transmissible omicron variant of the coronavirus has taken over as the dominant strain in the United States.

Omicron has sparked alarm both internationally and in the United States, where it accounted for more than 98 percent of new infections during the week ending Jan. 8, according to the most recent data from the Centers for Disease Control and Prevention. The variant has an unusually high number of mutations that make it significantly more contagious and capable of eluding the body’s first line of immune defenses.

Amesh Adalja, a senior scholar at the Johns Hopkins Center for Health Security, said omicron “is not something that can be avoided.”

“Most people are going to get infected with omicron — including those who are vaccinated, including those who are boosted,” he said. “We now have to think about covid-19 in two different ways — as a mild disease in the vaccinated and as something that is still a problem for the high-risk unvaccinated.”

World leaders have responded by placing restrictions on travel, ramping up vaccination and booster drives, increasing testing and encouraging an already pandemic-fatigued public to double down on measures including wearing masks and social distancing. Though initial studies indicate that omicron is likely to produce milder infections than other variants, public health experts say the sudden swell of cases and large number of people who remain unvaccinated could still overwhelm strapped health-care systems.

That said, much is still unknown about omicron, and U.S. leaders note that there are many more tools to combat the surge in cases than at the start of the pandemic.