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The first in a continuing series.

Science

COVID-19: You’ve got questions, virologist Jim Duehr has answers

The first in a continuing series.

The world is scary right now, even more than usual. But science is a candle in the dark, so we here at AIPT have directed all our questions about COVID-19 to contributor and virologist Jim Duehr, and in a continuing series, we will share those answers with everyone. If you have your own questions, let us know!

Once a person comes into contact with another who has tested positive for COVID-19, how long before they are able to spread the disease?

It’s very hard to evaluate stuff like this, which requires us to understand what’s happening in asymptomatic and yet-to-be-symptomatic (incubating) people. Because healthy people don’t usually come into the hospital! There are at least two studies I know of that have tried to start answering this question. They were very focused on the number of days before the beginning of symptoms in individuals who had virus detectable in their body. See below (emphasis mine):

  • “Assuming an incubation period distribution of mean 5.2 days from a separate study of early COVID-19 cases, we inferred that infectiousness started from 2.3 days (95% CI, 0.8–3.0 days) before symptom onset and peaked at 0.7 days (95% CI, −0.2–2.0 days) before onset.”
  • “In the four clusters for which the date of exposure could be determined, presymptomatic transmission occurred 1–3 days before symptom onset in the presymptomatic source patient.”

And we know that incubation period can range from ~5 days to about 14 days. So from those two numbers, we can presume that people are becoming infectious mostly ~2-3 days after exposure, but with a tail that can stretch up to ~11 days.

I work with someone who is married to a healthcare worker. Someone she works with is positive. Am I in danger?

That’s three degrees of separation from you, right? Bottom line: You are way more likely to contract SARS-CoV-2 from going to the grocery store or on the subway or wherever, and directly coming into contact with an infected person, than you ever would be from these distant relationships. It’s likely that if you explored every single person who has that level of distance from you (your husband’s boss’ wife, your daughter’s classmate’s mom, etc.) there would be quite a few people at that level of distance who are SARS-CoV-2 (+).

In most areas of the United States, your coworker’s spouse should be receiving regular testing. But it’s also likely that your coworker’s spouse and their infected friend wouldn’t have spent all that much time together without a gown, gloves, mask, etc. Much is done in healthcare settings to also reduce transmission between healthcare workers, like making sure everyone knows how to put on and take off their PPE (personal protective equipment).

Now, is every hospital doing this? No! Unfortunately not. It’s a range. And we need to keep hospitals accountable to make sure they’re providing safe working conditions for employees.

That being said, the fact that it’s three degrees of separation means you have some other safeguards as well. As soon as your coworker’s spouse tests positive, your coworker should be self-isolating, because she could also be infected and not yet know. In the experience and knowledge-base of epidemiology professionals, we know that these three+ degrees of separation should be more than enough to keep you personally safe.

We have implemented as much social distancing as we can in our office, but we are essential employees. Should we be wearing masks to work?

Yes. Every person who leaves their home should be wearing a mask. It’s just such a low-risk solution that has such a large benefit, if implemented on a society-wide scale.

Masks are a low-risk solution that has such a large benefit, if implemented on a society-wide scale.

The evidence for surgical masks preventing you from getting an airborne or droplet pathogen is extremely weak. They probably do something beneficial, but not a lot, and only if worn properly. Overall evidence is “mixed.” Honestly a huge part of this is training, and not everyone (even healthcare workers) is trained all that well in how to properly use masks.

On the other hand, all kinds of masks definitely prevent you from breathing out virus particles. They catch many of the respiratory droplets that we cough and breathe out all the time! So arguably, it always made sense for someone who is sick to wear one.

This is also why surgical masks are the type surgeons wear when they cut open non-COVID patients. They’re trying to make sure they don’t breathe out bacteria/viruses that wind up destroying your insides! When they cut open people with HIV/HCV/etc. (and now COVID), they actually wear N95s, to protect both the patient and themselves.

At this point in the pandemic, the asymptomatic may be enough of a problem that not only sick people should be wearing masks. We all should be, just in case. Because a great many of us could be, at this very moment, asymptomatic. Many of us could even be in the incubation period (meaning we will develop symptoms later, but could still be spreading virus — see question 1!).

The main counterargument is that masks are in short supply, and that healthcare workers need them more than the general public. This is still true about disposable surgical masks.

But it isn’t true about fabric masks, which are A) easy to make, B) easy to buy, and C) at the bottom of the totem pole for what hospitals want to use. They need to exhaust their supplies of regular masks first. This is exactly why cloth masks are perfect for public use.

I’ve heard a term I don’t understand. What is “shedding”? Is that the same as coughing or sneezing?

Viral shedding means “putting out infectious virus.” If you’re actively infected, and there’s virus in your lungs and your nose and your mouth, then when you cough or sneeze (or even breathe!), virus is hitching a ride on those little droplets of saliva and mucus.

A person who is infected but has no symptoms can still shed, as we’ve come to know. People appear to shed two or three days before they start to show symptoms (see question 1). And even more worrisome, viral RNA is still being shed in people who have recovered for 3-4 weeks after the symptoms go away. That’s a long time!

But we have to keep in mind that RNA does not always mean virus. RNA is the genetic material that codes for viral proteins. It’s the instructions that a virus uses to make more copies of itself. And, as we’ve come to understand during this pandemic and in past diseases, our bodies seem to have a way to keep RNA around in certain areas of the body long after an infectious virus is present there. This may be an evolved strategy to make bits of that protein and stimulate an immune response, or it could just be left over as our cells slowly destroy every viral particle. At this point, it’s unclear, but it means that even after recovering, we need to be careful for a little bit.

How long does it take for this virus to “inactivate”? I hear it’s days for some things and hours for others. What about on groceries?

The best way to look at this is to figure out what the “half life” of the virus is on a given surface. Check out this paper from the NIH and the figure I’ve excerpted below:

covid-19 chart

For each little graph on the top two rows of this image, there’s a Y and an X axis, right? The Y (vertical) axis is “number of virus particles” (measured in a way that would take a long time to explain, so just assume it says “virus particles.” The X (horizontal) axis is “time from initially smearing this virus mix on the surface.” On the first row, the red lines, they’ve smeared SARS-CoV-2, which causes COVID-19. On the second row, the blue lines, they’ve smeared SARS-CoV-1, which caused the 2003 outbreak in Hong Kong that spread to various places, including Toronto.

The experimenters did this a bunch of times and on different surfaces, and then they measured the amount of virus every few minutes until it got below the dotted line, called the “limit of detection” (LOD). Below that line, we cannot reliably see if there is any virus present at all.

The big black dots are measurements of virus, and the lines are regressions of those dots. There are so many lines because the researchers used a computer modelling technique:

Lines are random draws from the joint posterior distribution of the exponential decay rate (negative of the slope) and intercept (initial virus titer), thus visualizing the range of possible decay patterns for each experimental condition.

Basically, they used math to try and introduce some randomness into the equation, since it doesn’t always happen so perfectly like this in real life. So the cloud of lines shows you kind of what is possible with more or less virus, or more or less drying out, or more or less sunlight, etc.

The third row is the same data, shown a different way. They took their linear regressions, and figured out what the “half life” was for each virus on each surface. Half life, in this case, is the time it takes for the amount of virus to be cut in half on any given surface.

Since half life is a thing you get from the slope of each line, they can show it as a sort of “cloud,” based on those random “decay patterns.” It bears mentioning at this point that the top graph Y axis is logarithmic, so the lines are really logarithmic curves, but plotted on the top graphs they look linear.

So each dot on the bottom graphs is an estimate of how many minutes/hours it takes to only have 50% of the initial amount of virus remaining. So when I say 4-5 half lives is 98.75%, that’s what I mean. Other studies have come up with different numbers, but these are the most reliable data that I’ve seen so far.

In this case, 4-5 half lives is:

  • ~8-10 hours with air or copper
  • ~32-40 hours on cardboard
  • ~48-60 hours on steel
  • ~64-80 hours on plastic

We have no idea what the minimal infective dose is, and we also have no idea how many virus particles actually come out when you sneeze (or cough). So this data is not as useful as you might think. It comes with lots of caveats: This is a controlled laboratory experiment, it may not reflect what happens in the real world. This data is only an estimate, and though it should be useful for your life, it should not be taken as a guarantee that you won’t get any virus. This is just to help you reduce your likelihood of getting infected.

All that being said, I think we can reliably assume that 4-5 half lives with some Purell is enough to not get infected. That would be like a 99.9% reduction in virus (isn’t that what the bottle says?) from the Purell and a 98.75% reduction in virus from just waiting around.

So, how can I translate that into useful tips? Well, for grocery shopping, especially if you’re having groceries delivered or if you’re immunocompromised or elderly, then there are certain things you can do to reduce your risk!

If you get groceries directly from the store or delivered, leave them in the front entranceway of your home, or in a closet. Wash your hands, and then remove only the perishables and put them away in the freezer or fridge. Remove any fruits or veggies as well.

Then spray down the outer plastic bags and boxes, and perhaps leave everything else in that entranceway or closet for a few days before eating it. Or, you could remove all your groceries from the outer packaging and discard all of that outer plastic/ cardboard first.

It’s worth saying that there is no evidence SARS-CoV-2 can be transmitted via food, and it’s especially unlikely that SARS-CoV-2 can be transmitted through food that is cooked properly. Most importantly: before eating, always wash your hands. And stay well!

For more from Jim Duehr on COVID-19, check out a collection of his public Facebook posts on the subject here.

AIPT Science is co-presented by AIPT and the New York City Skeptics.

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