The coronavirus outbreak in the U.S. spurred calls to "flatten the curve" to limit the speed of the virus' spread. Now, as states begin to think about how to ease their lockdowns, a new rallying cry has emerged: "test and trace."

The term points to the need for a monumental effort to ramp up testing speed and capacity, as well as an extensive — if not ubiquitous — tracing program. Epidemiologists believe that if it is properly employed, testing and tracing can allow the U.S. to open some businesses and relax social distancing requirements.

But it will almost certainly need to be a slow and methodical process or else the country risks undoing the good from the initial wave of lockdowns.

"There's no doubt that we have to reopen correctly," Dr. Robert Redfield, director of the Centers for Disease Control and Prevention, said Monday on NBC News' "TODAY." "It's going to be a step-by-step, gradual process. It's got to be data-driven. And as I said, I think it would be community by community, county by county."

Health officials in some states are reporting encouraging signs that strict stay-at-home orders are helping slow the rate of coronavirus transmissions. The numbers have spurred discussion of when and how various parts of the country could emerge from lockdown.

President Donald Trump has said he would like to reopen the country May 1, but experts say it must be done carefully to prevent a spike in new cases and a huge second wave of infections.

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West Coast states, such as Washington, Oregon and California, enacted sweeping stay-at-home orders early and have been relatively successful containing the virus. On the East Coast, states such as New York and New Jersey remain the hardest hit, and new cases — and deaths — are surging in pockets of the South.

Before lifting stay-at-home measures, states would need to see sharp declines in new cases, hospitalizations and intensive care admissions, according to Caitlin Rivers, a senior associate at the Johns Hopkins Center for Health Security.

"There will be a time when transmission declines, where we can move to case-based management," she said. "This involves isolation, contact tracing and quarantine."

Testing

To get to that phase of the pandemic, states would first need to ramp up their abilities to test people who may have been exposed and those who are showing symptoms.

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In many states, such as New York, test shortages meant that only patients requiring hospitalization were being tested. That resulted in many mild cases' not being counted in official tallies.

States hoping to loosen restrictions will need to have a clearer picture of how many residents are infected, which will require expanding test capacity so anyone who experiences symptoms can get a diagnostic test, Rivers said.

If a patient tests positive, he or she would then go into isolation, and public health officials would begin a process known as contact tracing, which involves mapping whom the patient has interacted with and therefore who else may have been exposed to the virus.

"We would try to understand where they went, what they did, who they were with when they were symptomatic — or even a bit before — with the understanding that this is the period when they were most likely to be infectious," said Dr. Mike Reid, an assistant professor of medicine at the Center for Global Health Delivery and Diplomacy at the University of California, San Francisco.

Each of the established contacts would be urged to self-quarantine for at least 14 days. The goal of contact tracing, Reid said, is to identify people who may have been exposed as quickly as possible and then intervene early to prevent further spread.

The principle is guided by the virus' "basic reproduction number" — an epidemiological concept known as R0 (pronounced R-naught) — which is an estimated measure of a pathogen's transmissibility. In other words, it estimates how many people are likely to get the disease from one infected person.

For this coronavirus, it's estimated that two to three people will get infected from one positive person. Although that figure doesn't remain static and can vary over time, it offers some clues about the contagiousness of a virus.

"With that reproductive number, after 10 rounds of replication, that's about 59,000 people that could be infected," Reid said. "It's mind-boggling how quickly it spreads, which is why you want to do contact tracing quickly."

Tracing

In a scenario in which a city has relaxed its lockdown, health officials would need to be able to test quickly and then immediately start tracing people who have been in contact with someone who tested positive.

That can be an arduous process even for small outbreaks. For the coronavirus, it will be a herculean task — but one that is already in the planning stages.

The speed at which the coronavirus can spread makes it imperative for health officials to exhaustively map every interaction a positive individual has had in recent days. Public health departments conduct contact tracing regularly for other diseases, but the efforts would need to expand significantly to stem the spread of the coronavirus.

"We do this all the time in public health, but not at this scale," Rivers said. "It's very resource intensive. It takes a lot of time and personnel."

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Some states are already beginning to ramp up contact tracing. Massachusetts Gov. Charlie Baker announced plans April 3 to deploy a program involving 1,000 virtual contact tracers who will work to track and contain the virus across the state.

Technology could also play a major role during the transition period. Last week, Apple and Google announced that they are teaming up to build software into Android and iPhone devices around the world to track encounters with people who are infected. California Gov. Gavin Newsom said he plans to incorporate smartphone contact in his strategy to lift statewide stay-at-home orders.

But as more states approach their peak levels of infection and subsequently start to see signs of progress, it will be even more important to develop strategies to lift restrictions without compromising public health and safety, according to Reid.

"It's a hotly debated topic," he said. "We're clearly going to have to be really smart and sophisticated about how we approach a return."