The delta variant of the coronavirus is the dominant strain in the United States, accounting for more than 99 percent of new cases of covid-19. Delta was detected in only about 1 percent of U.S. cases in the beginning of May, but within three months had wiped out nearly all competing strains of the virus.

It has also become dominant worldwide. The virus is continuing to mutate as it circulates in humans, and, until recently, many health experts believed that any future “variant of concern” was likely to be a descendant of delta. 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 more than 20 countries — including the United States.

Rochelle Walensky, director of the Centers for Disease Control and Prevention, 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 has declined somewhat as delta has spread. Some of that decline may be due to the natural waning of antibodies, or to 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 and how people can help protect themselves during the pandemic.