These are the numbers health officials are watching at this point in the pandemic

The U.S. is now averaging more than 400,000 new COVID-19 cases a day. This comes after a week where new case counts shattered the previous day's records again and again. And even those staggering numbers are probably an undercount.

Rochelle Walensky, the director of the Centers for Disease Control and Prevention, told NPR last week that with so many people testing at home, it is hard to capture the true number of cases.

Many people are wondering how meaningful the case numbers are at this point in the pandemic, but Natalie Dean, an assistant professor of biostatistics at Emory University, says they're still useful because they reflect how much infection is in a community.

"[Case numbers] are one element of multiple numbers that we're tracking," Dean said. "Case counts are always going to reflect how much infection is out there, and even though there are limitations, we still are able to see these trends, these staggering increases in cases over time that really do reflect large epidemics."

Dean specializes in infectious disease epidemiology and spoke with NPR's All Things Considered about what's contributing to case undercounts, the current trajectory of the omicron variant and what other numbers public health officials are looking at.

This interview has been edited for length and clarity and includes some web-only answers.

Interview highlights

On whether the current numbers are an undercount of actual COVID-19 cases

Yes, it's definitely an undercount. I mean, we've had undercounts throughout the pandemic. That's always been an issue and that there's always some fraction of cases that are not reported for whatever reason. And so the truth is just something even larger. And what we're really seeing is these really stark trends.

On what else public health officials are tracking

We're closely tracking also ... the other public health impacts, how many people are getting severely ill, how many people need to be admitted to the hospital, how many people need to go to the ICU and require mechanical ventilation. And we know that those things happen with the lag. So one of the other reasons we really like case counts is because ... you can observe that earlier than you see hospitalizations or ICU utilization. But we're starting to see some trends. We're seeing hospitalizations are going way up. They are increasing quickly. But we're keeping a close eye on mechanical ventilation use. We're not seeing the same rise as we've seen in prior waves with delta. Some of that may be some of the lags, but when we look at data from London and South Africa, and other European countries, there is sort of a picture emerging. So we're tracking all those things together. Those all give us a sense of the impact.

On what other ways levels of infection can be gauged in the U.S.

There are a few other things we can pay attention to. One is wastewater surveillance. So basically, you can track sewage, test it for viral particles. And that's nice because you remove that dependency on who is deciding to get tested and how many tests are available. It's just looking, at a very community level, how much virus is there out there. Another thing, in the U.K., they do random sampling. And that's a really reliable way to get a measure of how many people are infected at any one time. And then that doesn't depend upon, again, who's getting tested and whether tests are available. So that's kind of a scientific way, using statistics, in order to get a sense of community prevalence.

On whether a variant with milder symptoms would change how useful case numbers are as a measure of the seriousness of the pandemic.

Yes. I mean, the public health impact is made up of a lot of different things, and we're most acutely interested in severe disease and death. But, of course, infections have impacts, and we think about the disruption. You know, all the people who are going to need to miss work, including health care professionals and front-line workers. So the numbers have meaning. But it is a different public health impact when someone is mildly ill or doesn't even have symptoms, than when someone is severely ill.

On seeing such large case numbers every day

It's really beyond comprehension. We think about, as statisticians and mathematical modelers, we're used to a certain range of numbers. And when we saw delta, I mean, that was really just seeing the transmissibility of delta was remarkable already. But this is just even beyond that. So I think that's something that's hard to wrap our heads around. Just how many people are likely to be infected over the next month. It's just going to be a lot of disruption, unfortunately.

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