Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases and member of the White House Coronavirus Task Force, says “we are not in a situation where we can say we are exactly where we want to be with regard to testing” capacity for COVID-19 in the U.S.

Fauci, in a discussion for TIME 100 Talks: Finding Hope on Thursday, says that the U.S. needs to not only increase the number of tests, which is happening as commercial testing companies increase production and the Food and Drug Administration continues to clear tests using different types of samples (including ones from the nose and saliva, as well as blood). But, he says, we also need to make sure tests can actually be run the way they should.

“We need to significantly ramp up not only the number of tests, but the capacity to perform them, so that you don’t have a situation where you have a test but it can’t be done because there isn’t a swab, or because there isn’t extraction media, or not the right vial,” says Fauci. “I am not overly confident right now at all that we have what it takes to do that. We are doing better, and I think we are going to get there, but we are not there yet.”

Fauci has become the scientific soul of the U.S. coronavirus response, and his focus on evidence and fact-based science has often put him at odds with President Donald Trump. Fauci has consistently provided a sober, objective view of the risks the pandemic raises for the health of the American public, as well as highlighted concerns about the country’s response.

Fauci noted that there is still a lot about the COVID-19 virus that doctors still don’t know, but they are slowly building on that knowledge daily. He mentioned early hints that there seem to be two different strains of virus dominating in the U.S., one on the East Coast and the other on the West Coast — and that they might have different virulence. That could explain why East Coast cities like New York were hit hard by COVID-19 and saw more people needing intensive care and breathing support. He stressed that the data is early, but that understanding the science behind the virus could help to guide doctors on how best to treat those who are infected.

While Trump has repeatedly discussed reopening the country’s economy sooner rather than later, Fauci cautions that “we better be really careful as we move forward.” Not all parts of the country may be returning to pre-COVID-19 activities at the same time, or in the same way, he says. It will take meticulous testing to identify people who might be infected, trace their contacts, test those individuals an infected person came in contact with and isolate them from the public to reduce the chance they will spread the virus.

“Understandably, people might think that when we get back to normal, it’s a light switch that you turn on or off. It isn’t really that at all,” Fauci says. “We must be in a place that we have the capability when we do start to see cases come back — and I guarantee that they will come back as we pull back on social distancing — that we can identify then, isolate them, contact trace people they have been in touch with, and get people out of circulation who are infected. If we are capable of doing that effectively, then we should feel some good confidence that we could slowly move on. This is a very critical time right now.”

Fauci also addressed the tension between his scientific approach and Trump’s eagerness to get the economy started again, which led recently to Trump tweeting the hashtag #FireFauci. “I meet with the President literally every day,” he says. “There was no way he was going to do that, because he had no intention to do that. As he said publicly when he was asked about it, that’s not even on the table. My job always is, and I’ve always done it and will continue to do it, is to give advice on the basis of evidence and science.”

This article is part of #TIME100Talks: Finding Hope, a special series featuring leaders across different fields sharing their ideas for navigating the pandemic. Want more? Sign up for access to more virtual events, including live conversations with influential newsmakers.

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