ARI SHAPIRO, HOST:
The U.S. just hit another unwelcome record - nearly 50,000 new coronavirus cases in a single day yesterday. It is especially bad in the South and West. To talk about where the current hot spots are and where new ones could surface, we're joined by NPR global health correspondent Nurith Aizenman. Hi, Nurith.
NURITH AIZENMAN, BYLINE: Hi, Ari.
SHAPIRO: And, Nurith, start by giving us the big picture. What does the U.S. look like right now?
AIZENMAN: OK. Among many scientists who measure this, there is agreement that three states - Arizona, Florida and South Carolina - have likely reached the point where, to get a handle on their spread, they basically have to go back to stay-at-home mode. Less drastic measures aren't going to cut it. But while the governors in those states have been talking some closures, they've been resistant to full shutdowns. And meanwhile, 13 more states are just one notch below that red-alert level, led by Louisiana, Nevada, Mississippi. But this category includes almost the entire South and West and also two Northern states - Iowa and Idaho.
SHAPIRO: Full shutdown - that's a serious warning. What do scientists base that on?
AIZENMAN: The main metric they're looking at is how many new cases are being confirmed each day as a percentage of the local population. And this week, a team led by researchers at Harvard came up with these color-coded thresholds for, you know, if your daily new per capita case count goes above this amount, then you need lockdowns to control the spread; if it's slightly lower, you can probably avoid that through really ramped-up testing and contact tracing. There are four risk categories in all, and right now only two states - Hawaii and Vermont - are in the lowest-risk one.
SHAPIRO: This is such a grim picture after all states did to control the pandemic. What went wrong?
AIZENMAN: The evidence is clear that human behavior has a massive impact on how fast this virus spreads. How much contact do we have with each other? Do we wear masks? And so many places went straight from lockdown to almost full opening up. One way to understand what a challenge this is for states is to drill down to the county-level data. Marynia Kolak of University of Chicago leads a team that's put together a county-level tool. And she says it points to spillover that's happening in border areas, like the Mississippi River area from Memphis, Tenn., into Arkansas.
MARYNIA KOLAK: So there's definite concern there because, unless you have a federally coordinated response, anything that one of those states does may not necessarily have the full impact unless the nearby states also agree.
AIZENMAN: She's also got this tool for flagging clusters in a state that's not in as bad shape overall.
SHAPIRO: And where do we see those clusters popping up?
AIZENMAN: Some counties in Washington state, California, Iowa. It's an early warning for potential future hot spots. Looking at the county also helps tease out - you know, is this spread happening in just one area of the state, or is it statewide? And one of the Harvard researchers, Thomas Tsai, says he's really worried about Florida because, before, the focus was really in the metropolitan areas, Miami. But now we're seeing that it's escalating into central Florida, areas of the panhandle. He says Arizona is in a similar situation.
SHAPIRO: That is NPR's Nurith Aizenman. Thank you for the update.
AIZENMAN: You're welcome. Transcript provided by NPR, Copyright NPR.