As a number of states prepare to loosen stay-at-home restrictions amid the coronavirus outbreak, Microsoft co-founder and billionaire philanthropist Bill Gates says the country will need to aggressively test for and trace new cases of the virus in order to reopen safely.

Gates warned that areas without a major outbreak yet are still at risk, and a second coronavirus wave is possible if states roll back restrictions too soon. He said successful reopening will happen in stages, with the most essential businesses reopening first, along with widespread testing and contact tracing to understand the effects easing restrictions has on the spread of the virus.

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“What we don’t know is, [if] we go slightly back to normal, which activities create the risk of a rebound?” Gates said on CNN’s Fareed Zakaria GPS Sunday.

“We need to put in place a very dense testing regime so you would detect that rebound going back into exponential growth very quickly and not wait for the ICUs to fill up and there to be a lot of deaths. If you see the hot spot, you kind of understand the activities causing that.” Gates said.

Gates said the United States is currently nearing between 400,000 and 500,000 tests per day, but added that’s “just barely enough for really doing the tracking.”

Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases (NIAID) and a key member of the White House task force, said Saturday he’s confident the U.S. will be able to double its current coronavirus testing capacity in the coming weeks. The U.S. is currently conducting between 1.5 million and 2 million tests per week.

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When planning for reopening, Gates said states should focus on “high value” segments of the economy, such as schools, manufacturing and construction, and determine how to reopen those segments with social distancing measures. Gates said careful planning to manage the virus is important as it is likely a vaccine is still one to years away.

“It’s very hard to compress these timeframes and, you know, if everything went perfectly, we’d be in a scale manufacturing within a year. We may not achieve that. It could be as long as two years,” Gates told CNN. The billionaire is funding the production of potential vaccines through the Bill and Melinda Gates Foundation.

Gates' comments come as some states have already partially reopened or are working on guidelines on how to open parts of their economy.

The U.S. has recorded more than 965,000 confirmed coronavirus cases, and nearly 55,000 deaths since the outbreak began, according to Johns Hopkins University data.

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