Almost two years after the director of the Centers for Disease Control and Prevention called for 100,000 contact tracers to contain the coronavirus, the C.D.C. said this week that it no longer recommends universal case investigation and contact tracing. Instead it encourages health departments to focus those practices on high-risk settings.

The turning point comes as the national outlook continues to improve rapidly, with new cases, hospitalizations and deaths all continuing to fall even as the path out of the pandemic remains complicated. It also reflects the reality that contact-tracing programs in about half of U.S. states have been eliminated.

Britain ended contact tracing last week, while Denmark and Finland are among other nations that have scaled back the use of contact tracers. New York City announced on Tuesday that it was ending its main contact-tracing program in late April and moving toward treating the coronavirus as another manageable virus.

“This is a big change,” Crystal Watson, a senior scholar at the Johns Hopkins Center for Health Security, said in an interview on Tuesday. “It does reflect what’s already happening in states and localities, particularly with Omicron. There was no way contact tracing could keep up with that. Many of the cases are not being reported, so there’s no way of knowing the incidence.”

The original goal of contact tracing in the United States was to reach people who have spent more than 15 minutes within six feet of an infected person and ask them to quarantine at home voluntarily for two weeks even if they test negative. The aim was to reduce transmission while Americans who tested positive monitored themselves for symptoms during their isolation. Case investigation is used to identify and understand cases, clusters and outbreaks that require health department intervention.

But from the start of the pandemic, states and cities struggled to detect the prevalence of the virus because of spotty and sometimes rationed diagnostic testing and long delays in getting results.

Now the C.D.C. is pushing health departments to focus solely on high-risk settings, like long-term care facilities, jails and prisons, and shelters. Many immunocompromised Americans, though, feel left behind by the lifting of precautions and restrictions across the country.

“The updated guidance is in response to changes in the nature of the pandemic and the increasing availability of new tools to prevent transmission and mitigate illness,” Kristen Nordlund, a spokeswoman for the C.D.C., said Tuesday.

She said that the dominance of variants with very short incubation periods and rapid transmissibility combined with high levels of infection- or vaccine-induced immunity and the wide availability of vaccines for most age groups made the change possible.

Dr. Watson, who was the lead author of a 2020 report recommending that the country have 100,000 contact tracers, said that she was worried that the new guidance might lead to a dismantling of the infrastructure that was put into place to support as many as 70,000 contact tracers, the peak number the country reached during the winter surge of 2020.

“We anticipate that there will be a need for contact tracing,” she said, “so some of the investments made in rebuilding the public health work force should be used more broadly so we can call on them in the next emergency.”

More than 20 states still have statewide contact-tracing programs, according to Hemi Tewarson, the executive director of the National Academy for State Health Policy.

“I actually think that the federal government move is consistent with what states are doing,” she said in an interview on Tuesday. “They’re already concentrating contact tracing on high-risk settings.”

Ms. Tewarson said that contact tracing could not keep up with the Omicron surge, and that it was no longer as effective a tool if people are testing at home and not reporting results.

“As a longer term plan, this is going to be more sustainable,” she said. “We’re at a different stage of the pandemic.”