. If a surface is contaminated with the SARS-CoV-2virus, how long does it pose a risk of infection? The virus is thought to mainly spread throughrespiratory droplets. These are produced in a cloud when a personcoughs or sneezes, or even talks. Some potentially-virus-laden droplets mightend up getting breathed in by other people in the vicinity. But many of them end up landing on objectslike door handles or water faucets.
When that happens, infectious disease expertsrefer to that door handle as a fomite. And if a person then touches the fomite whilethe virus is still infectious, they can then spreadit to new surfaces, or actually infect themselves.
Fomites aren’t just for viruses -- any typeof pathogen can create fomites -- but we’re talking about virusesfor… obvious reasons.
The good news is that SARS-CoV-2 virus particlesdon’t last forever -- or even all that long. Eventually, the protein coat that allows thevirus to actually bind to and infect cells will degrade. This neutralizes the virus and leaves it asa small pile of ineffective protein and genetic material. So the question is, how long does this taketo happen? As is the case with pretty much everythingto do with this virus.
we still don’t know for sure.
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There are still a lot of details to nail down. Some studies on other coronaviruses have suggestedthey’re pretty hardy, by virus standards. The viruses that cause SARS and MERS can survivein some environments for more than a week, and maybe even fourweeks if it’s cold. For SARS-CoV-2 in particular, one of the waysscientists have tested how long the virus might stay activeoutside the body is essentially by re-creating a sneeze.
They spray a watery mixture containing thevirus onto certain surfaces. Then, later, they swab the surface and tryto infect cells in a petri dish or test tube with what theycollected. They can watch to see if infection does occur,and in how many of the cells, and use those numbers to calculatehow much of the virus survived. This is totally different from the nasal swabtest we use to see if someone’s infected.
That test essentially just looks for geneticmaterial.
This test is specifically looking to see ifactive, still-able-to-infect virus is present. One of the things we’ve figured out usingthis method is that what type of surface the droplet falls ontomakes a big difference. One study from the New England Journal ofMedicine, for example, found viable virus, though at low levels,on steel and plastic after 72 hours. On cardboard, the virus couldn’t be detectedafter 24 hours. And copper, which has antimicrobial properties, made it disappear in four hours.
Another paper in the Lancet found viable virusafter a whole six days on steel and plastic. Meanwhile, it seemed to disappear on glassand banknotes after about four days, two days for wood,and three hours for paper. It’s important to note that while the scientistsin both studies found evidence of viable virus.
it’s hardto say how infectious these viruses are. That’s because you need a higher dose ofsome pathogens than others to actually get infected withthem -- and we don’t know what that is for SARS-CoV-2yet. For one thing, the New England Journal ofMedicine study found that while some of the virus lasteda long time, the amount decreased exponentially over time.
This means most of the individual virus particles didn’t last very long. Outside the body, changes in things like temperature,humidity, and UV radiation can quickly degrade the virus. Even though these studies found the virusafter days or even weeks, the amount was generally really small -- atiny fraction of what was there at first, and probably notenough to infect anyone.
The Lancet group’s experimental method alsoused a solvent specifically designed to transport viruses,and they note that this is not exactly comparable to a real-life contactscenario. But why is there such a difference in the amount of time between surfaces? Well there, the field of physics can helpus out. A paper out this week in the journal Physicsof Fluids modeled how long large, respiratory-sized dropletsof water could last on various surfaces and conditions. They assumed that SARS-CoV-2 would die whenthe droplet evaporated, though we don’t know that for sure yet.
They found that temperature and humidity affectedthe lifetime of these droplets in ways you would expect,with higher temperatures and lower humidities drying them out faster. They also modeled how much the droplets canspread out on surfaces, which in physics is called the contact angle. On some surfaces, like glass, the dropletspreads out really wide and thin, which makes it evaporate faster.
On others surfaces, like steel or smartphonescreens, which can be covered in a water-repellentcoating, the droplet stays more spherical, which limitsthe surface area and makes it evaporate more slowly.
That might be part of the reason why the earlierresearchers were able to find viable virus on those surfacesafter such a long time. What’s the good news? Well, all these tests assume nobody’s cleaning. While viral particles might be able to stickaround for a long time in the perfect environment, they are alsostill susceptible to disinfection. Which means - yes, it’s a good idea to wipedown those door handles.
It also hasn’t stopped being important towash your hands after you’ve been out and about, touching potentiallycontaminated surfaces. Which we are all still doing, right? Models like the one we talked about todayrely on understanding probabilities. And one way you can learn more about probability is with Brilliant’s course Probability Fundamentals. Brilliant has tons of courses in math, science,engineering, and computer science. They’re all hands-on and meant to help teachscientific thinking. And they’re designed by experienced educatorsfrom Duke to Caltech and beyond.
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