Contact tracing will be essential even after Covid-19 vaccines become widely available, because not everyone will get shots and scattered outbreaks will continue to ripple across the country. Quickly identifying who’s been exposed to people with infections can guide efforts to stamp out new hot spots and break chains of transmission.

State and local health departments had fewer than 2,000 workers carrying out contact tracing before the Covid-19 outbreak. A hiring burst over the summer wasn’t nearly enough as once-isolated outbreaks blanketed the country. But building a federal army of disease trackers could cost billions of dollars — and won’t come in time to slow the current wave that’s led to more than a million new U.S. cases a week and left health officials largely in the dark about where the virus is spreading.

“Contact tracing works best and is most effective in settings where there isn’t the level of rampant transmission that there is now,” said Nicole Lurie, a former assistant secretary for preparedness and response at the Department of Health and Human Services and a public health advisor on Biden’s campaign. “There have to be measures to tamp down the level of the virus before it can be effective.”