Contact tracing, which involves laboriously trying to find close contacts of people who test positive for COVID-19 and then isolating them to help reduce the virus’ spread, was abandoned on Long Island and elsewhere as the number of coronavirus cases mounted and made the practice unfeasible.

Now experts say contact tracing could be revived as part of an effort to reopen the economy.

When a 42-year-old Nassau County man became the first confirmed Long Island coronavirus case on March 5, a team of investigators began reviewing the man’s routine, Nassau Executive Laura Curran said at the time.

The process is time-consuming. Investigators interview the person who tests positive, ask with whom the person recently had close contact, find those contacts and then tell those contacts to self-isolate, Nassau health spokeswoman Mary Ellen Laurain said. Suffolk County took a similar approach.

Suffolk Health Commissioner Gregson Pigott said that when the county confirmed its initial cases — the first was March 8 — and began locating their contacts and telling them to isolate themselves, “We were hoping that would be the end of it.”

As COVID-19 testing ramped up and the number of positive test results rose, it become clear how widely the virus had spread throughout the county, he said.

By March 22, when the number of confirmed cases in Suffolk had reached 1,034, the county shifted from a “containment” strategy to “mitigation,” with contact tracing only in limited cases, such as with someone who cares for those most vulnerable to serious complications from COVID-19, the health department said in a statement.

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Nassau made a similar shift as the number of cases made contact tracing for every person impractical, Laurain said.

“We have to prioritize," she said. Contact tracing is still done for some people, such as food handlers or residents of group homes, she said.

Both counties tell those who test positive to alert close contacts on their own.

“At the very beginning of a pandemic, the best way to stop it is to track down all the contacts of the initial, indicator cases," said Dr. Bettina Fries, chief of the infectious diseases division at Stony Brook Medicine. “That approach doesn’t work anymore when the genie is out of the bottle and when you have widespread community-based transmission. Then you would put too many resources to trace contacts. By the time you have your contact, that contact has already generated many more contacts.”

A problem early on in the United States was a lack of widespread testing for the virus, she said. Health officials couldn’t perform sufficient contact tracing if they didn’t know who was infected.

South Korea’s early and aggressive contact tracing was more effective because of more early testing, Fries said. South Korea, which reported its first confirmed case on the same day, Jan. 21, as the United States, has under 11,000 cases, according to the Johns Hopkins University Coronavirus Resource Center. The United States has more than a half million.

Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, is advocating for large-scale contact tracing soon. He said on the "Today" show Thursday that widespread testing, contact tracing and isolation of those who are infected should be in place as the country begins to return to normalcy.

Contact tracing could become feasible if social distancing and other measures succeed in markedly reducing the number of cases, and there are enough resources dedicated to it, said Barun Mathema, an assistant professor of epidemiology at Columbia University and an infectious-disease expert.

“Contact tracing works when there is a manageable number,” he said.

On Friday, Apple and Google announced they were jointly developing a high-tech form of contact tracing that would allow apps from public health authorities to notify people whose phones were recently in close contact with someone who tests positive for COVID-19. It’s an opt-in system, but it is raising privacy concerns.