Dr. Fauci on why Contact Tracing isn't working in the U.S.

He said in areas where the virus is spreading in the community, 20% to 40% of those who are infected are asymptomatic.

"When you have community spread, it's insidious because there are so many people in the community who are infected but asymptomatic," he said. "So the standard classic paradigm of identification, isolation, contact tracing doesn't work no matter how good you are because you don't know who you're tracing."

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https://www.cnn.com/2020/06/28/health/fauci-coronavirus-vaccine-contact-tracing-aspen/index.html