The delta variant of the coronavirus has been overtaken by omicron — now the dominant strain in the United States.

Until recently, delta had been accounting for more than 99 percent of new cases of covid-19. That number dropped to less than 2 percent the week ending Jan. 8, as omicron has continued to surge across the country, according to data from the Centers for Disease Control and Prevention.

However, that does not mean delta will not continue to circulate.

“Right now, delta is still what’s crushing hospitals,” Amesh Adalja, senior scholar at the Johns Hopkins Center for Health Security, said in late December. “It’s unclear if delta will completely get replaced by omicron, or if this omicron wave will come and go and delta will be left still circulating. Only time is going to tell how these two variants will compete with each other.”

Many health experts had believed that any future “variant of concern” was likely to be a descendant of the delta variant. But the newest variant, known by the Greek letter omicron, is not a delta offspring, coming from a different portion of the virus’s family tree. It was identified in November in southern Africa and cases have since been confirmed in dozens of countries — including the United States.

CDC Director Rochelle Walensky has said that all three coronavirus vaccines available in the United States can provide strong protection against severe disease and death from covid-19, the disease caused by the virus, including the delta variant. However, vaccine effectiveness declined somewhat as the delta variant was surging — and early research suggests vaccines may be less effective against omicron as well. Some of that decline may be because of the natural waning of antibodies, or because of changes in behavior as people have been less likely to wear masks or engage in social distancing.

“The delta variant is showing every day its willingness to outsmart us and to be an opportunist in areas where we have not shown a fortified response against it,” Walensky said.

Here are answers to common questions about the delta variant.