April 10, 2020

Can I catch COVID-19 from that?

NCSE Executive Director Ann Reid, who as a research biologist helped sequence the 1918 flu virus, digs into the science behind how long the novel coronavirus remains infectious on surfaces.

two hands wearing gloves and shaking

Check out our entire series explaining the science involved in the coronavirus pandemic. Sign up to receive our coronavirus update each week.

How do scientists figure out how long the novel coronavirus remains infectious on surfaces?

Just when you thought you had enough to worry about, news comes out that the virus can remain infectious on surfaces for hours or days.

Okay, so how do scientists figure out how long coronavirus remains infectious on surfaces?

So glad you asked! Turns out, it’s all about serial dilution.

Ready?

Here’s the first part of the experimental procedure from the recent New England Journal of Medicine article that’s caused everyone to be anxious about touching anything outside their own homes:

Aerosols (<5 μm) containing SARS-CoV-2 (105.25 50% tissue-culture infectious dose [TCID50] per milliliter) or SARS-CoV-1 (106.75-7.00 TCID50 per milliliter) were generated with the use of a three-jet Collison nebulizer and fed into a Goldberg drum to create an aerosolized environment.

Yikes. I’m not going to try to define all those terms. In plain language, the researchers took a whole lot of two different coronaviruses: SARS-CoV-1, which causes the disease SARS, and SARS-CoV-2, which causes the current pandemic of COVID-19. The researchers put a very specific amount of each coronavirus into a machine that mixed the virus with water and sprayed the resultant solution on various surfaces—copper, stainless steel, cardboard, and plastic. Then they took samples every hour and tested them on cultures of human cells. If cells died, the virus was still infectious.

That’s pretty simple, right? But you might wonder how they figure out how much of the virus is left at each time point.

To understand that, you have to dig into the term “TCID50,” which stands for Median Tissue Culture Infectious Dose. TCID50 is the amount of virus that kills 50% of the cells in a test tube or plated on petri dishes. When the description says the researchers started with 105.25 TCID50 per milliliter of SARS-CoV-2, that means they started with almost 200,000 times as much virus as they needed to kill 50% of your target cells.

Logarithmic dilution

When they took samples from the various surfaces, they made serial dilutions (check out the figure above) of each sample and tested each dilution. (As an aside, if you want to help your students understand serial dilution, check out this cool activity they can do at home. I love that it starts out with, um, cereal dilution.) When they got to a dilution that killed 50% of the cells, they knew they had reached TCID50. Working backwards, they could figure out what the concentration of the virus was in the undiluted sample.

In a nutshell, here’s what they found. On the graph below, SARS-CoV-2 is shown in red. The first symbol, at 0 hours on the x-axis, is the amount of virus (which is called the titer, shown on the y-axis) when the surface was just sprayed. You can see that the amount goes down over time on all the surfaces. It goes down fastest on copper (after just one hour, the titer has gone down by 90% and by about 8 hours, there is less than 5 TCID50—the limit of detection of the test.) It goes down slowest on plastic—after 24 hours, there is still around 500 TCID50 on the surface. The levels of virus don’t go down to the detection limit until 72 hours—that’s three full days!

graphs

So what’s the take-home message? Do we need to be concerned about catching COVID-19 from surfaces?

Not really. It’s important to know that virus can remain viable on plastic for a long time, but this study started out with extremely large amounts of virus. The average cough (much less regular breathing) will not contain 200,000 infective doses of virus. And only an infinitesimal amount of that virus will survive for 72 hours; after around four hours, the amount of viable virus on plastic will have dropped by about 50% and only 10% will remain after 24 hours.

Also, these tests were done under controlled laboratory conditions at temperature and humidity levels conducive to viral survival; real-world conditions would likely be harder on the virus.

This is really important information, to be sure. It will help those at hospitals, grocery stores, and other places where people still gather determine where it is most important to clean, and how often. But in your day-to-day life, here’s what you and your students need to know (and you likely already know these things).

  • wearing a maskYou will only get sick if you touch a contaminated surface and then touch your mouth or eyes. Wash your hands as soon as you arrive home!

  • You don’t need to freak out about touching packages at the store, as long as you adhere to the first point above.
  • Wearing a mask mostly protects others from you if you’re sick, not the other way around. But if you do wear a mask, you don’t need a medical grade version; only healthcare workers do. Make your own mask at home. Here are a whole bunch of ideas on how to do that. You can see mine in the photo to the right. I made it out of an 18”x18” piece of an old sheet and the elastic bands from the top of an old pair of socks.
  • Social distancing is the only proven way to reduce spread.
  • Stay home, stay safe.

Check out our entire series explaining the science involved in the coronavirus pandemic. Sign up to receive our coronavirus update each week.

NCSE Executive Director Ann Reid

Ann Reid is a former Executive Director of NCSE.

reid@ncse.ngo