Test inaccuracies, sporadic shedding, immunity: What scientists know about COVID-19

As the U.S. approaches 1 million coronavirus cases, doctors and scientists are still struggling to figure out how it works, how and when it started to spread, what all the symptoms are, and why many people don’t seem to show symptoms at all. 

To get an update on what scientists have learned so far, KCRW turns to epidemiologist Janet Hamilton. She’s Executive Director of The Council of State and Territorial Epidemiologists, an organization aimed at advancing public health policy nationwide. 

How does this coronavirus behave differently from other viruses in the past? 

Hamilton says COVID-19 symptoms are different from traditional respiratory symptoms: 

“In addition to a shortness of breath and cough or difficulty breathing … some individuals [are] reporting a loss of taste or smell or even a partial loss. And then, of course, fever and chills can also be a part of that symptom presentation. 

… Some of these symptoms can be very mild. It's hard sometimes to distinguish: Did I have those symptoms (such as a headache or slight sore throat) and that was due to allergies, or was that due to coronavirus infection? Is the person truly asymptomatic? Do they have some mild symptoms that maybe they didn't recognize or attribute? 

But all of it makes it much harder to control. The hardest viruses to control are those actually that have less severe presentations. Because the less severely ill you are, the more likely you are to continue your regular activities or come in contact with others.”

How accurate are tests? There are reports of false negatives.

“There are now many tests that have been manufactured — and tests of different types. And it's true that the performance of those tests are not necessarily the same. I often tell people that not all laboratory tests are created equal. 

Issues that also impact the poor performance of the test are issues around the specimens themselves. So the way that your sample is collected can also play a role, as well as the transport of that specimen to the laboratory, how it's stored, the type of specimen that's collected, and even the type of swab that’s used — can play a role in how likely the virus is able to be found in that specimen when the laboratory test is actually run. 

And another thing, of course, is when the specimen is collected within your illness. So specimens that are often collected maybe very early on — may have a different rate of return on those positive results than those that are collected [in the] middle within your infection, or than those often collected at the end.”

What have we learned about immunity? Some people have tested positive after they’ve recovered. 

“The biggest question I think that we don't have answered is: Even if you have immunity for a short time, how long do you really have immunity for? And that's going to be really important when we think about issues in the future about the vaccine, and how long maybe after you've been vaccinated, can you continue to have protection? We just haven't had enough experience with this virus thus far to know how long people may be immune for after they have been infected. 

… There's actually something called sporadic viral shedding. So you'll shed a lot of virus, and then stop shedding it, and then you might shed again. And that's where … people had originally tested positive, and then negative, and then positive again. 

If you have symptoms that track closely with COVID-19, but you received a negative test result, should you believe that result? Or could it be a false negative?

“There are now over 100 different tests out there, and the tests perform in different ways. And so what I will tell you is that if you get a positive result, the performance or the predictive value [of] positive is very high. 

Just like with influenza, on some of the tests that are available for flu, a negative result does not necessarily mean you're negative. 

… It's also true for a lot of tests that they perform better when there's a lot of virus circulating. And so I think particularly if we see the virus circulating less, it will probably also result in a higher proportion of people having negative results when actually they are infected.”

What’s the role of air quality in spreading this virus?

“Air quality is a regular concern and issue when it comes to any virus that can be transmitted via droplets or even in the air. And one of the issues is how far that virus can travel or spread. And the more likely it’s able [to] travel and spread is if there's particulate matter. Therefore, air quality is an important consideration.”  

… If potentially you're in lower air quality … you might be irritated in some way [that] could lead to additional coughing and droplets. 

When we talk, all of us, we have tiny droplets. And those often travel at least three feet. 

But then it's also true that the virus can be found on some of the particulate matter. So it's not just the fact that you're coughing, but now that particulate matter gives … a virus [a place] to travel [to]. 

The virus survives better in colder temperatures? 

“That's one thing that we will probably learn a lot more. We certainly saw the emergence of the virus in colder temperatures. … Looking around the world at how countries that have had warmer temperatures have been impacted by the virus … will give us important information as to what we might see and expect in the summertime. 

… How well can it still survive in warmer temperatures? … That's something that we still have a lot to learn about. 

Do we know when this virus came to the United States?

“Given the rapid spread of this virus, I think the scientific community recognizes that it was probably here sooner than what we originally thought, complicated (or masked, one could say) by the fact that we were having our traditional ‘influenza season.’ 

And that's where some of the laboratory diagnostics are going to be so critical — that are about additional genetic sequencing of that virus, where we can have a better and better understanding of exactly when it was introduced, and what it looked like at the time of introduction.

That's going to be a challenge to pinpoint it. Because to do that well, what you need is samples from early on.”

—Written by Amy Ta, produced by Angie Perrin



  • Janet Hamilton - The Council of State and Territorial Epidemiologists