As part of a continuing series of conversations about Covid–19 (see also A High School Q&A About Covid–19 and A Primer On and Conversation About the Biology and Evolution of SARS-CoV-2, the Virus That Causes Covid-19), Applied Ecology Prof. Rob Dunn asked Prof. Linsey Marr, Prof. Matt Koci, Prof. Benjamin Chapman, Dr. Julia Casani, and Prof. Jack Gilbert to chat–virtually–about how viruses spread in general and the origins of the six-foot-rule.
RRD: So what are the main ways viruses can be spread from one person to another (AKA, exposure routes)?
LM: The main exposure routes are contact, either through picking up viruses from contaminated surfaces (also called “fomites”) or someone who is infected and then touching your eyes or nose, and inhalation, breathing in viruses from the air. Contact transmission can also occur if an infected person expels large droplets, through talking, coughing, or sneezing, that land directly in your eyes or nose.
RRD: Then, of course, some viruses are carried from person to person by mosquitoes or other vectors.
MK: Yes, mosquitos or other biting insects can transmit viruses (for example Zika virus or tick borne encephalitis virus). Biting animals can transmit viruses too, think rabies. Viruses can also be transmitted sexually, like HIV. The other major means of transmission is in food or water, like norovirus or hepatitis A virus…. But the good news, at least for SARS, is there is no evidence that we need to worry about getting CoVid-19 from these methods.
BC: Like MK says, with a few other viruses like norovirus and hepatitis A, we have seen the dreaded fecal-oral route as the primary mode of transmission (meaning poop making its way into the gastrointestinal system through food or water). We don’t have any example of this in SARS-CoV-2 as of right now though, which is good as trying to manage sick food employee is something we’re having a tough time with right now (see many food processing plants shutting down due to clusters of illnesses).
RRD: Do we know how long the virus that causes Covid-19 hangs out on, say, carry out food or packages from Amazon?
JAG: It could be up to 72 hours on cardboard, but that is based on a personal communication reference in an editorial that was not peer reviewed in NEJM. The personal communication was from someone at the CDC. What I tend to do is to bring the packages in, open them up and then dispose of the cardboard, then wash my hands.
BC: I don’t think we know exactly how long because the infectivity is related to how much is there in the first place, what the temperature is and what the humidity is (and UV C seems like a factor too). With all that variability, and the lack of examples of illnesses from packages so far, I’d go with handwashing after handling as a decent way to reduce the risk.
RRD: Which of these are relevant in the context of the virus that causes Covid-19?
LM: We don’t know yet. Transmission seems to occur mainly at close contact, and all of the above routes can operate at close contact. It is difficult to disentangle the different routes of transmission. Researchers have been studying this question for decades for the flu, and we still don’t know the answer.
JAG: Exactly, we know so little, however, it is safe to say that personal contact, and aerosolized droplets from breathing and sneezing are high risk factors.
RRD: Do we know which of these is most frequent?
LM: See above.
RRD: We have heard a lot about the six foot rule? First, do we know where that rule comes from? Second, I have read in old studies of flu that talking spreads droplets with the flu virus more than does just breathing, loud talking more than quiet talking, singing more than loud talking and coughing more than any of them. Should the same also be true of the virus that causes Covid-19?
LM: The origins of this guideline are not clear. There were papers in the late 1800s and 1930s showing that bacteria did not grow on plates placed beyond a “few feet” from a person whose mouth was inoculated with bacteria, but this assay captures only those microorganisms that fall to the plates. Others can remain airborne. It is likely that certain activities lead to the release of more viruses into the air. There is also considerable interpersonal variability, in that some individuals release a lot more viruses than do others.
RRD: Those individuals, so-called superspreaders, could release virus farther than other people, in addition to releasing more, is that right? And there is no way to know if someone you are walking past is one of those people?
MK: We know super spreaders are a real thing, so much that it led to this “20/80” idea that’s almost reached meme-like status. The idea is that in a lot of outbreaks, 20% of the people are responsible for 80% of the infections. This basic idea has been documented for lots of viral diseases: Ebola, HIV, influenza, the 2003 SARS, and there’s pretty good evidence of this for the current SARS-CoV-2. That said, we still don’t really understand the mechanism behind what makes someone a superspreader. There’s probably not one answer, and it’s probably different for different viruses. For the SARS-like viruses, the level of virus replication is probably involved. There are reports of at least one person in a cohort in Germany that had virus levels 10-100 times higher than most, but we don’t know if they were a superspreader or not. It could also be where in your body the virus is replicating. For some people it looks like the infection is mostly in the upper airways (nose and throat), and others the infection is more in the lungs. You can imagine that it is easier for the virus to get out and spread from person-to-person with more replication in the upper airways as opposed to lower. Spread from the upper airways could happen with sneezing even just breathing and talking and probably be smaller droplets, while spread from the lower airways would require more deep coughing and hacking and likely be bigger droplets. However, what makes someone a superspreader could also be all about the person and their respiratory anatomy. Some people may just have pipes that are better at making clouds of virus.
RRD: If that is the case, do we actually need more than six feet of distance when talking, singing or, say, exercising?
LM: There is no “safe” distance. The farther the better, since airborne concentrations will fall off rapidly with distance from the source. We need to account for motion of the individuals and the air flow around them, whether indoors or outdoors. At the same time, if you’re far from the source, the chances of being exposed to the virus are very, very low.
RRD: What about aerosols? The spread of virus via aerosol is different than via droplet, right?
LM: This is a false dichotomy.
JAG: LM is very wise.
RRD: I have the idea based on some of Linsey’s work that cold dry environments are better for lots of viruses. If that proves true for the virus that causes Covid-19, is it better to keep our windows open as it gets warmer out?
MK: Maybe, but if you suffer from allergies and get congested from pollen and such, you may need to think about the additional stress that will put on your lungs if you get infected.
RRD: How much distance do each of you try to keep from other people?
LM: 10 feet.
MK: As much as I can and it depends on what I, or they, are doing. If I see people running or biking, I try and make the distance, especially behind them, as big as possible.
RRD: Is it reasonable to assume (based on what we know about coronaviruses in general) that the virus travels farther if you are talking than if you are just breathing, farther if you are talking loudly than if you are talking quietly, farther if you are singing than if you are talking loudly and especially far if you are exercising or coughing?
MK: I think this question is better answered by LM (hopefully she’ll correct anything I get wrong), but in general yes. Anything that projects more air out of your nose and mouth is going to expel more virus and with more force and therefore the further it will travel. And you’re always expelling spittle, bacteria, and stuff. You (RRD) and JAG and others have shown how much we share our microbes with each other, humans are really disgusting creatures 🙂 Viruses are smaller and lighter so it is even easier for them to float away. This is the logic behind the universal masking programs that were implemented in most of Asia, and just starting to be encouraged here. Wearing a mask will help trap some of what you’re breathing out, and hopefully limit your ability to spread the virus to others especially during the time when you may be contagious but you don’t feel sick yet.
RRD: How much distance would you try to keep if you were over sixty or immunocompromised?
LM: I would do everything possible to avoid going to indoor public settings and crowded outdoor settings.
MK: What Linsey said. And if I had to for some reason, I would wear a mask. I know that’s still a little controversial in the US. They aren’t a magical force field, but if used right they can help minimize (not eliminate) exposure.
JC: I think that this question gets at a part of the discussion that is frequently not appreciated. Infection in an individual is as much about the status of the recipient, if you will, of the virus as it is of the “donor”. In the questions above we talked about breathing the virus out but little is studied about how we breathe it in, what physiologic or anatomical issues might the recipient need to have going on to foster infection… other than simply immune status. If I am also running, singing, exercising and opening up all that dead space in my lungs and drawing it in more deeply does that make me more likely to get infected. Is it just a matter of hitting my nasal passages or getting deeper down? Is it different if the virus lands on my tongue than on my nasal passages? Then I have to be “ready” to be infected. The interesting answer to this may lie in looking at asymptomatic immune individuals who now have antibodies, had an immune response but never appeared to be infected or at least demonstrated symptoms.
RRD: Do you plan to leave your windows open as it gets warmer?
LM: Not in my home, where transmission between household members due to long periods of close contact is unavoidable, but this could be helpful in public buildings.
MK: I wish, but not with my allergies. At least not until the worst of the pollen has passed, but unfortunately in North Carolina by then it’s too hot to leave the windows open.
Who we are
LM is Linsey Marr: Linsey is the Charles P. Lunsford Professor of Civil and Environmental Engineering at Virginia Tech. Her research group applies interdisciplinary approaches to study pollutants in indoor and outdoor air. She is especially interested in emerging or non-traditional aerosols such as engineered nano-materials and pathogens and how they are transported and transformed in the environment. Her research on the airborne transmission of infectious disease has focused on influenza, Ebola virus disease, and Legionnaires disease.
JC is Julie Casani: Julie has experience leading statewide (Maryland and North Carolina, USA) response to public health emergencies. She teaches global health at NC State University.
MK is Matt Koci: Matt is a virologist and immunologist working on host-microbe interactions in birds. He is based at NC State University and has also contributed to A High School Q&A About Covid–19 and A Primer On and Conversation About the Biology and Evolution of SARS-CoV-2, the Virus That Causes Covid-19.
JAG is Jack Gilbert: Jack is a microbiologist at the University of California, San Diego and author of the terrific book, Dirt is Good.
BC is Benjamin Chapman: Ben is a professor and food safety extension specialist at NC State’s Department of Agricultural and Human Sciences.
RRD is Rob Dunn: Rob studies the biology of species associated with humans. He is based in the Department of Applied Ecology at North Carolina State University and the Center for Evolutionary Hologenomics at the University of Copenhagen.