Someone — let’s call her Person A — catches the coronavirus. It’s a Monday. She goes about life, unaware her body is incubating a killer. By perhaps Thursday, she’s contagious. Only that weekend does she come down with a fever and get tested.

What happens next is critical. Public health workers have a small window of time to track down everyone Person A had close contact with over the last few days. Because by the coming Monday or Tuesday, some of those people — though they don’t yet have symptoms — could also be spreading the virus.

Welcome to the sprint known as contact tracing, the process of reaching potentially exposed people as fast as possible and persuading them to quarantine.

The race is key to controlling the pandemic ahead of a vaccine, experts say. But most places across the United States aren’t making public how fast or well they’re running it, leaving Americans in the dark about how their governments are mitigating the risk.

An exception is the District of Columbia, which recently added metrics on contact tracing to its online dashboard. A few weeks ago, the District was still too overwhelmed to try to ask all of those who tested positive about their contacts. Now, after building a staff of several hundred contact tracers, District officials say they’re making that attempt within 24 hours of a positive test report in about 98 percent of cases.

For months, every U.S. state has posted daily numbers on coronavirus testing — along with charts of new cases, hospitalizations and deaths. So far, only one state, Oregon, posts similar data about contact tracing. Officials in New York say they plan to begin publishing such metrics in coming weeks.

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Recent case spikes in Texas, Arizona, Nevada, South Carolina, West Virginia, Georgia and California will frustrate contact tracing efforts there. But even states seeing declines are finding it hard to measure their success in contact tracing, in part because the effort is often spearheaded by local and county health departments.

In Maryland, Gov. Larry Hogan announced the launch in May of a digital platform for contact tracing that would allow officials to view detailed performance metrics. More than a month later, state health department spokesman Charles Gischlar declined to release the numbers “because the data is under review.”

In Georgia and Colorado, health department officials say they are developing platforms for tracing data but can’t yet pull out numbers.

The U.S. Centers for Disease Control and Prevention posted guidance in April that included various recommended contact-tracing metrics — none of which are being publicly reported by most states.

“Contact tracing is how you prevent cases from becoming clusters, clusters from becoming outbreaks,” said former CDC director Tom Frieden, now with the global health initiative Resolve to Save Lives.

As jurisdictions scale up their tracing operations, he said, transparency is important. “The more openly any of it is shared, the faster progress is made.”

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Contact tracing is not new to public health agencies, which have long used it to curb diseases like tuberculosis, meningitis and measles. But the scale of the coronavirus pandemic presents new problems. While some places are experimenting with phone apps to aid efforts, they depend on people downloading the apps and have raised privacy concerns.

Each positive test result calls for a case investigation, usually a phone call from a health worker that can last an hour or more. The goal is to help the person recall those with whom they’ve been in close contact — generally within six feet of someone for 15 minutes. Contract tracers try to reach each of them.

“We’ve had people that have more than 60 known contacts, because they were very busy, they were very socially active,” said Stephen Haering, director of the Alexandria, Va., health department, which in May was averaging several dozen new positive cases a day.

“That’s a lot of phone calls,” Haering said.

The city temporarily diverted staffers out of family planning centers, immunization clinics and other specialty areas to work in contact tracing.

Thanks to a prolonged shutdown and slow reopening, Alexandria is now averaging under a dozen new positive cases a day. Haering has maintained a tracing staff of more than 40 people, many of them newly hired through a state contract, who attempt to reach people within hours of a positive test report.

“If we get it in the afternoon, we’re making [the call] that day,” said Haering, who is prepared to add more tracers if case numbers spike. “We’re not operating 24 hours, but we are operating seven days a week, including holidays.”

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Many places didn’t attempt full contact tracing or abandoned efforts during the heights of their epidemics, when testing was more limited and health departments were short of tracers.

Even in the nation’s capital, health workers could focus only on priority cases, such as those in nursing homes, until staffing increases last month.

“Now that we’re able to focus on every positive case and get these comprehensive lists of contacts, we’re able to start making connections between places of exposure and types of exposure that individuals have,” said LaQuandra Nesbitt, director of the D.C. health department.

Experts say the United States will need legions of contact tracers to help contain the virus — perhaps more than 100,000.

“While this figure may be stunning,” an April report by the Johns Hopkins Bloomberg School of Public Health said, it’s fewer than half the number per capita that were employed in Wuhan, China, where the virus originated.

State and local officials have since announced the hiring of thousands of contact tracers across the country. But in places seeing big case spikes, even those expanded operations are becoming overwhelmed.

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Texas, for example, has fewer than 3,000 contact tracers statewide, according to the Texas Tribune. The state reported more than 8,200 new cases Saturday.

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The coronavirus isn’t just fast, it’s stealthy, and that creates another major challenge. Researchers aren’t sure what percentage of transmissions — maybe 15 percent, maybe 30 percent, maybe more — come from people who don’t yet feel symptoms.

But some people do appear to spread the disease before they develop symptoms that could alert them to consider isolating themselves.

Often, case investigators, who make initial calls to new positive cases, start out behind because test results typically take days. The pharmacy chain CVS says customers should expect to wait about three days for results, and sometimes more than that. District health officials say results from city testing sites have recently taken as long as seven days but now take three to five.

If Person A’s test result takes four days, then by the time a contact tracer reaches Person B, she is likely to be feeling symptoms. By that point, she may have decided to self-isolate, but only after spending a day or two spreading the virus.

“Just the nature of this virus means you have a really small window to catch that next round of infection events before they, in turn, go on to infect other people,” said Adam Kucharski, a professor at the London School of Hygiene & Tropical Medicine who wrote The Rules of Contagion, a book scheduled for release this week.

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In a modeling study recently published in the journal Lancet, Kucharski and his co-authors concluded contact tracing alone will not likely contain the virus.

“We found that, even with quite rapid detection of cases, even with quite extensive tracing, it’s likely that you’d need some additional measures alongside that,” Kucharski said, including social distancing and restrictions on gatherings.

Still, the better a community manages contact tracing, the fewer restrictions on gatherings and economic activity it may need.

Health officials in North Dakota, which is seeing only a few dozen positive cases per day, say they reach out to people within an hour of when they are notified by their health care provider of the test result. Officials in Maine, which is experiencing similar case numbers, say they begin investigating within hours.

Contact tracers in New Mexico, which has been seeing between 100 and 200 new cases per day, are reaching those people and their contacts a median of 48 hours after test results, short of the state’s target of 36 hours, officials said at a news conference in late June.

Officials in Massachusetts, which has seen between 100 and 300 new cases per day recently, say they contact cases and contacts within 24 to 48 hours. Colorado, with similar numbers testing positive, set a goal of reaching the close contacts of patients within 72 hours of test results, officials say, but won’t have the capability to draw metrics from the data until later this month.

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One sign of success for a contact-tracing program comes when a new positive case is someone tracers have already spoken with, and persuaded to quarantine, after identifying them as another infected person’s close contact.

The proportion of new cases arising among quarantined contacts is a key metric recommended by the CDC and other experts. Frieden calls it “the fundamental outcome indicator of a contact tracing program.”

Frieden said some states aren’t tracking the metric, while others may not want to disclose it. He said he heard one public health leader comment recently, “If we reported those, it would be zero every day.”

The District has created a spot for the metric on its online dashboard with the tag: “Coming soon.”

Conversely, new cases that cannot even in hindsight be traced to a known source — known as “unlinked” cases — demonstrate where contact tracing operations need to improve. Though countries with effective tracing programs, including Iceland and New Zealand, track this metric closely, Frieden said it is rarely reported in the United States.

An exception is Oregon. The state aims to keep its average rate of unlinked cases below 30 percent and did so for much of May and June. But as the state has reopened and new cases have climbed, so has the portion that are unlinked.

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“It’s really a good indicator of how effective our whole statewide approach at suppression is,” said Tom Jeanne, Oregon’s deputy health officer and deputy epidemiologist. “The more that is rising above 30 percent, the more we are saying, ‘Well, there’s just broad community spread of this happening, and we’re not keeping up with it.’ “

Arkansas, where cases have been climbing recently, doesn’t post the unlinked metric online. But a health department spokesman told The Washington Post that 25 percent of the state’s recent cases have been unlinked.

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Sporadic reports of contact-tracing success rates vary widely from place to place. While District investigators make a first contact attempt within 24 hours in nearly every case, only 60 percent of people pick up the phone. Those who don’t answer receive a text message and several more attempts over three days.

New York City began an ambitious contact-tracing program last month, but fewer than half of those who tested positive gave information about their close contacts, a spokeswoman for Mayor Bill de Blasio told The New York Times.

In Fairfax County, Va., the health department seeks contacts only from high-risk cases. Those deemed lower risk are asked to reach out to contacts themselves and encourage them to quarantine.

“This is a system that we had developed several months ago as the pandemic was much hotter in this area,” Benjamin Schwartz, Fairfax’s director of epidemiology and population health. “We are currently in the process of hiring additional contact tracers, so they will be able to do the full contact tracing for every case that’s identified.”

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Schwartz said case investigators have managed to reach 91 percent of probable and lab-confirmed positive cases. He said that number is encouraging, “particularly given that many of the cases occurring in the county are among the more disadvantaged population, who one might expect would be a little bit harder to reach.”

After schools closed in March, officials in Anne Arundel County, Md., enlisted school nurses to become contact tracers, launching far ahead of state efforts.

Lately new cases in the largely suburban county on the Chesapeake Bay have averaged a few dozen per day. The county’s case investigators typically reach out to them in far less than 24 hours.

“If it comes in at 10 o’clock in the morning, we’re calling them in about three or four hours,” said County Health Officer Nilesh Kalyanaraman. “Twenty-four hours is the maximum time between results and calling them. We’re pretty much at 100 percent on that.”

About 20 percent of the cases and contacts need help with food, housing or transportation to isolate and quarantine, Kalyanaraman said. Addressing those needs quickly, he added, is “probably the most important piece to get right.”

Contact tracing alone won’t stamp out the virus in places like Anne Arundel, research suggests. But it may help keep Person B from passing it on, and it will save others from becoming Person D — or Person Z.