Millions of coronavirus cases likely went undiagnosed

As the U.S. reaches a toll of 500,000 lives lost to COVID-19, a new study suggests millions of infections last year went undiagnosed. The study's senior author, Kaitlyn Sadtler, an investigator at the National Institute of Biomedical Imaging and Bioengineering, joins CBSN to discuss what the findings mean for potential immunity.

Video Transcript

- The US will hit a coronavirus milestone many thought would be impossible. Roughly half a million American lives have been lost in the 11 months since the pandemic began, but that number may actually be higher. A new study from the National Institute of Health estimates, last summer, there were 17 million undiagnosed cases of COVID-19. To put that into perspective, today, the US has just under 29 million confirmed cases.

The study's senior author, Kaitlyn Sadtler, joins me now. She's an investigator at the National Institute of Biomedical Imaging and Bioengineering. Kaitlyn, thanks for joining us. So public health officials have been saying for months the actual number of COVID-19 cases may be higher than what we know, but 17 million more people is a huge number. Why are there so many who have not been diagnosed?

KAITLYN SADTLER: So first of all, thanks so much for having me. And the reasons why there are so many people that are potentially undiagnosed for COVID-19 or SARS-CoV-2 infection, there's a variety of factors that can affect that, one of which is the range of symptoms associated with this disease, as we see some people requiring going to the hospital, but we also see some people that have completely asymptomatic infections. So we can miss those cases.

And then, of course, also we have the timing of when the SARS-CoV-2 virus actually came to the United States. It will be a bit of a lag in terms of catching up to figuring out, OK, how many cases have there actually been? Because once you find one, there's more likely to be more than one. And of course, early on, there were shortages of testing supplies, and so not everybody that had a cold could get tested for SARS-CoV-2 infection. So all of these factors together can add to that undiagnosed proportion.

- So if more Americans have been infected, does this translate to more Americans actually being immune to the virus? Does this mean we may reach what some call herd immunity a little bit faster?

KAITLYN SADTLER: That's an important question, and it's one that quite a few researchers, including our group, are interested in. So shortly, yes, there's quite a possibility that people are out there that have some level of immunity to COVID-19 that aren't aware of it. We don't know for certain, for some of these asymptomatic or low, mild cases, what exactly their immunity could be in terms of how strong it is or how long their immunity might last. So there's still some more questions to answer there, but the presence of extra cases that were undiagnosed do suggest there could be a proportion of the population that has some level of immunity to the virus.

- Research can be a slow process. So how can health officials get a more clear picture of just how many people are out there carrying COVID-19 today?

KAITLYN SADTLER: Yeah, research is-- it takes time and takes effort to do things properly. And really, what we benefit from is an integration of different research fields. So for example, our teams here are working in the laboratory. We are working with actual physical samples, but we also have groups, epidemiologists and people that are way better at math than I am, that are able to integrate these points of information and make estimations based off of the experimental data as to what might be happening today, what might be happening tomorrow, what might be happening months from now. So we can form estimates, like, you know, predicting the weather a bit. So can we estimate how things might look now based off of prior information, or in the future based off of the data we have.

- It's really fascinating. Kaitlyn Sadtler, thank you very much.

KAITLYN SADTLER: Of course.