A Virologist Explains Why COVID-19 Coronavirus Isn’t Really Dangerously Lingering On Surfaces For Weeks

You may have heard reports that researchers found RNA material from the coronavirus on the Diamond Princess cruise ship 17 days after the ship was vacated. Or maybe you heard that there was widespread environmental contamination of viral RNA in COVID-19 patient rooms at the University of Nebraska Medical Center. Or perhaps you read an article saying that viral RNA could be detected up to 37 days in patients following the onset of COVID-19 disease. This might put in your head that COVID-19 patients are effectively clouds of contagion for months, shedding the virus for days before they knew they were sick and long after they recovered.

Calm down. And rest assured that this is not necessarily the case. That’s because the test used to detect the virus in these studies doesn’t actually measure whether it’s still infectious. Let’s break down some of the science. 

The test in these studies uses a very well-established technique called quantitative reverse-transcription polymerase chain reaction, or qRT-PCR. This test demonstrates whether a person is infected or not by detecting the presence and relative quantity of viral RNA. Basically, “Is the virus in your system?” So what you might think about qRT-PCR tests is that measurements of how much viral RNA is detected corresponds directly to how infectious a person is. But that’s not what those tests are looking for.

Here’s the thing. RNA viruses, like the one that causes COVID-19, make a lot of mistakes when replicating their genomes, and sometimes these mistakes result in viral genomes containing fatal mistakes, or mutations, that render a critical viral gene non-functional – meaning they won’t infect people who come into contact with them. (In fact, these kinds of mistakes are why plants, animals, bugs, people – basically anything more complicated than these viruses – store genetic information in DNA instead, because far fewer mistakes are made when DNA replicates.) But when you run a qRT-PCR test, those viral genes with mistakes are indistinguishable from the ones that aren’t. And either way, the cellular machinery that makes more viruses will package up both functional and non-functional RNA. These packages, called virions, is what the qRT-PCR test is looking for. 

For reasons that are not fully understood, patients that have recovered from a viral infection have cells that can continue to produce viral RNA without actually making infectious virus particles. That means it is not only possible but common to detect viral RNA without there being any infectious virus present.

Virologists use other tests to detect infectious viruses – the ones we need to worry about actually making people sick. The most classical of these, plaque assays and 50% tissue culture infectious dose (TCID50) assays, are based on the ability of viruses to kill infected cells in culture. These methods are much better for assessing how much potentially transmissible virus would be “shed” from recovered patients or in the environment. (“Shed” is a bit of a jargon term virologists use – but it’s a good metaphor for thinking of viruses moving out of you and into the environment when you cough or sneeze.) 

So why don’t we use these tests all the time? The problem with them is that they are also more time-consuming to perform and require specialized biocontainment. So they’re not practical for performing clinical diagnostic testing or broad surveillance. qRT-PCR can be performed in hours in standard laboratory conditions (biosafety level 2, or BSL-2), while plaque and TCID50 assays for SARS-CoV-2 take several days and must be performed in BSL-3 containment. Working in BSL-3 containment labs requires specialized training, and many clinical sites are not near a BSL-3 lab. So, most of the studies you read in the news about viral shedding and environmental contamination is just measuring the amount of viral RNA – but they’re not necessarily saying much about whether the virus is still contagious. 

Fortunately, two studies have investigated the ability of virus to remain infectious on different materials in the environment. Although the length of time that virus on a surface remains infectious is dependent on environmental conditions such as temperature and humidity, no virus remained infectious on surfaces for anywhere near 17 days. Furthermore, in both studies, the amount of infectious virus was greatly reduced after several days. This suggests that risk of infection from virus on objects or surfaces in the environment can be minimized by diligent cleaning and disinfection practices. 

Going forward, testing for infectious virus using plaque and TCID50 assays will be essential for accurately determining how long recovered patients remain a transmission risk. Undoubtedly, those studies are in progress by virologists around the world using samples from both patients and animal models that are actively under development.

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