As is true of a great many facets of the COVID-19 pandemic, our understanding of coronavirus transmission is incomplete. Yet, in an era when a vast internet of information and misinformation is freely available, knowing what is unknown can be nearly as valuable as possessing concrete information. Indeed, this is often true in science (albeit with lower stakes). In this week’s installment, we’ll discuss what we know, what we don’t know, and what you can do about coronavirus transmission.

Article by Rachel Jones, Little Shop of Physics Science Writer (rrjones@rams.colostate.edu)

The take-home message

Right now, I know people who are wearing bandanas and vinyl gloves to the grocery store and who are nervous to bring that box of toilet paper they ordered inside. Perhaps you do too, and you may be wondering just how much of this is reasonable and necessary. How exactly does COVID-19 spread? Just how likely are you to catch the coronavirus from that doorknob?

These worries are understandable — COVID-19 seems to be quite contagious (each person who gets it can probably give it to about two other people), and may be substantially more deadly than the seasonal flu. (But keep in mind that we don’t have a very sound estimate of fatality right now, and COVID-19 generally isn’t nearly as dangerous as tuberculosis or Ebola, especially if healthcare systems aren’t overwhelmed.) Unfortunately, we do not have clear scientific answers to many questions about coronavirus transmission at the moment. But we do have some data you can use as a guide, and some strong, evidence-based steps you can take to protect yourself and others in your community.

Let’s talk about surfaces first. Unfortunately, those headlines are right: Researchers have detected infectious coronavirus on plastic and metal after several days. They could also measure infectious coronavirus on cardboard and fabric after about 24 hours. However, it’s important to remember that those studies were done in a lab, and we’re not sure exactly how the results translate to our daily lives. Evidence does suggest that household cleaners like ethanol and dilute bleach can inactivate the coronavirus, as can high temperatures (~160˚F and up). So, at this point, it seems possible that there could be coronavirus on that doorknob. However, we don’t know how much, and we also don’t know how much virus would have to reach your nose, mouth, or eyes to make you sick.

And that’s another critical point: Even if your hands are dirty, surfaces really start to become a problem when you touch your face with those hands, as the virus enters the body through the nose, mouth, or eyes. While it makes sense to clean “high-touch” surfaces when you can (the CDC has advice on doing this) and try to avoid them when you can’t, it makes even more sense to get really good at washing your hands (CDC’s technique advice). Do keep in mind that you may touch your face without knowing it, and keep your hands clean.

You can, however, almost certainly catch COVID-19 from being close to someone who has it. When people cough, sneeze, and talk, they expel little bits of fluid of various sizes. Larger bits are called respiratory droplets; these are the main way public health authorities think the coronavirus spreads. Respiratory droplets are relatively heavy, and fall to the ground before getting very far. This is why you’re constantly being told that you need to keep 6 feet away from everyone else. There’s currently some evidence that the coronavirus may be truly airborne — spread in tiny aerosols that can float a long way — but scientists aren’t sure about this yet. In addition, there’s increasing evidence that people who aren’t showing any symptoms can spread COVID-19.

Uncertainty is certainly unsettling. However, regardless of whether you can catch COVID-19 from aerosols, or spread it without symptoms, or what exactly is up with that doorknob, you can follow public health advice that’s based on the best evidence we have. You can wear cloth face coverings in public, wash your hands well, stay home except for essential tasks, and keep 6 feet away from others when you go out. Some preliminary evidence does suggest that homemade masks, which are now recommended by the CDC, could save many lives. Scientists are working hard to learn quickly about the coronavirus, and you can keep yourself and others as safe as possible in the while they do.

This image shows two cartoon figures standing at some distance from one another. A double-headed arrow separates the figures, and "Social Distancing" is written above them; social distancing is very important to slow the spread of COVID-19. The image is provided by the CDC and bears the agency's logo.
This image shows two cartoon figures standing at some distance from one another. A double-headed arrow separates the figures, and "Social Distancing" is written above them; social distancing is very important to slow the spread of COVID-19. The image is provided by the CDC and bears the agency's logo.

The deep dive

Although research is progressing rapidly, scientists are currently uncertain about number of important details regarding SARS-CoV-2 transmission. Some facts, though, are beginning to crystallize. For instance, it seems that, on average, each person who catches the virus passes it on to 2-2.5 other people (a much-debated value called the basic reproduction number, R0) [1]. This range of R0 values seems to be collapsing into an average of about 2.2 [2]. Data additionally suggests that the COVID-19 case-fatality ratio (aka case fatality rate) may be in the neighborhood of 3% [1]. However, given our currently poor detection of asymptomatic and minimally symptomatic cases, this value may in fact be closer to 1%, or perhaps even lower [2].

If these values are roughly accurate, most people could probably more easily contract, and would be more likely to die from, from COVID-19 than from the seasonal flu. However, COVID-19 is probably not more transmissible than the 1918 pandemic flu, and is, in relative terms, only somewhat more deadly than that flu. It is our current understanding that, outside extreme or unusual conditions, COVID-19 is substantially less dangerous than SARS [1] [2], tuberculosis, and Ebola [1].

You may have noticed a heavy dose of provisional statements and qualifiers in the prior paragraph. While the numbers above are fundamental to understanding the pandemic, they are also very “squishy”. We may see the case-fatality ratio truly lowered with good medical treatments, or find that the present value is an overestimate due to lack of testing. We may see R0 fall substantially with strict social distancing, hand-washing, and other preventative measures, or it may rise if we find higher rates of asymptomatic transmission or longer contagious periods than were originally modeled. Additionally, different regions or populations may see wide variations in these numbers — lack of access to sanitation could inflate R0; an overwhelmed healthcare system could elevate the case-fatality ratio. Indeed, the Johns Hopkins Coronavirus Resource Center currently (April 2020) reports that, in the countries with the most cases of COVID-19, the case-fatality ratio ranges from about 3% (Germany) to over 14% (Belgium). Even within the U.S.A., recently reported case-fatality ratios are as low as 0.7% (Utah) and as high as 5.7% (Kentucky) [3].

This image shows a healthcare worker in blue scrubs sitting with knees on elbows and hands folded. The healthcare worker looks pensive and concerned. The image is provided by the CDC and bears the agency's logo.
This image shows a healthcare worker in blue scrubs sitting with knees on elbows and hands folded. The healthcare worker looks pensive and concerned. The image is provided by the CDC and bears the agency's logo.

While we need to carefully track and study these numbers, the most pressing questions for most citizens of affected areas are probably along the lines of the questions sent to me requesting this topic: How worried should I be about touching a box delivered to my home? Should I disinfect everything? Can I catch COVID-19 from just talking to someone who’s infected, even if they don’t cough or sneeze? Unfortunately, these are not questions to which science can currently provide clear answers. Still, researchers are beginning to build answers.

First up, surfaces. It is likely that SARS-CoV-2 can be transmitted via surfaces [4], and researchers have recovered infectious viral particles from various surfaces they’ve inoculated with SARS-CoV-2 in the lab [5] [6]. Although I’ll discuss these researchers’ findings in some detail, it is critical to remember that, to the best of our knowledge, you cannot catch COVID-19 solely by touching a contaminated surface. The risk arises when you touch a contaminated surface and then touch your face (which you may do without noticing!), giving the viral particles access to the delicate mucous membranes of your eyes, nose, and mouth. This is why proper hand-washing (technique advice from the CDC) and avoiding face-touching are such important strategies in disease control. (Though cleaning “high-touch” surfaces in your home is also a good idea; the CDC has advice on this too.)

You may have seen some troubling headlines recently to the effect of “SARS-CoV-2 virions can remain infectious on plastic and metal for days on end!” This, unfortunately, seems to be true — research groups have found infectious SARS-CoV-2 virions on plastic and stainless steel three [5] to four [6] days after inoculation. However, relatively porous surfaces don’t seem to be as hospitable: Infectious virions were undetectable after at most 24 hours on cardboard [5] and cloth [6], making long-distance mail and last week’s laundry relatively low-risk surfaces (though you still ought to be careful, of course). These results are noteworthy and important, but it is critical to remember that they were obtained under laboratory conditions. It is unknown if infectious periods and transfer rates found in the lab translate directly to surfaces in the “real world”. We also as of yet (April 2020) have not determined the amount of infectious virion initially deposited on, e.g., a shopping cart handle an ill person touched after coughing into their hand. Nor, relatedly, do we know the minimum “dose” of virus a person must receive to fall ill (n.b.: the amount of infectious SARS-CoV-2 on a surface seems to decrease exponentially with time [5]). Importantly, it seems that regular household disinfectants such as ethanol and dilute bleach, as well as high temperatures (over ~160˚F), probably quickly inactivate SARS-CoV-2 [6].

Now, the ability of SARS-CoV-2 particles to remain infectious on surfaces for extended periods may be a critical issue for healthcare workers, many of whom in the U.S.A. lack adequate supplies of personal protective equipment (PPE). One study found infectious SARS-CoV-2 virions a surgical mask about a week after inoculation [6]; many healthcare workers must currently wear masks designed for use with just one patient for a full day or more (though work is ongoing to disinfect these masks; see, e.g., this non-peer-reviewed article). Additionally, preliminary evidence suggests that surfaces in hospitals within Wuhan were widely contaminated by SARS-CoV-2 during the outbreak there [7] [8]. Although, as expected, contamination was most pronounced in areas where COVID-19 patients were treated, SARS-CoV-2 RNA was detected even in areas not housing patients with known COVID-19 [7] [8]. However, these “real-world” studies share a major limitation: When their authors report contamination, they have found the genetic material of the virus, not necessarily infectious virions. Determining whether the contamination is infectious SARS-CoV-2 or its harmless genetic shadow would require time-consuming cell culture assays. Though these are hopefully underway, we should still move to protect our healthcare workers on the strength of these early findings.

This is a close-up image of a person with blue gloves using a cotton swab to transfer a sample to/from a small vial in the process of testing for SARS-CoV-2. The image is provided by the CDC and bears the agency's logo.
This is a close-up image of a person with blue gloves using a cotton swab to transfer a sample to/from a small vial in the process of testing for SARS-CoV-2. The image is provided by the CDC and bears the agency's logo.

Another question of great concern to both healthcare workers and the general public is this: Just how “airborne” is SARS-CoV-2? Initially, health authorities thought SARS-CoV-2 was transmitted through respiratory droplets, which are larger (5 to 10 microns or more in width) than aerosols (less than 5 microns in width) — what we’d think of as truly airborne particles [4]. This is an important distinction; though droplets are too dense to travel very far from their source (hence the 6-foot radius we’re asked to keep around ourselves), aerosols can stay suspended for some time and float considerable distances [9]. Although authorities still think droplets are the main mode of transmission [4], some early evidence suggests that SARS-CoV-2 virions are present [8] and infectious [5] in aerosols. (Though other preliminary evidence, e.g. [10] [11], does not indicate substantial aerosol transmission of SARS-CoV-2.) To varying degrees, people may produce aerosols and aerosol-like particles just by speaking [9] [12]. People with COVID-19 but no or few symptoms thereof may be producing just as much SARS-CoV-2 in their respiratory tract as those with symptoms [13]. However, the sample size in the cited study was small, and how SARS-CoV-2 load relates to contagiousness has not yet been determined. Evidence is growing, though, that those without symptoms can spread SARS-CoV-2 [14] [15].

These are concerning results, but it is important to remember that there is much we have yet to understand about SARS-CoV-2, and while we are learning, we can take steps to protect ourselves and our communities. Whether or not SARS-CoV-2 is spread by aerosols and asymptomatic people, we can take the simple, low-risk step of wearing cloth face coverings in public, as recommended by the CDC. (Medical-grade masks need to be in healthcare settings right now.) Preliminary modeling suggests that even if homemade masks prevent only 50% of infections and transmissions, they could substantially reduce the number of deaths from COVID-19 so long as a large majority of people wear them [16]. We can take care to keep at least six feet from others, and always cover our coughs and sneezes (and, of course, stay home when sick). No matter how long SARS-CoV-2 virions can remain infectious on a surface, we can maintain good hand hygiene and avoid touching our faces.

Though what we know about SARS-CoV-2 is worrying, and uncertainty compounds our fear, it is important to remember that we are moving forward. Scientists continue to learn more about the virus and the disease it causes. Citizens continue to protect themselves and others by following the current best public health practices that flow from increased scientific understanding.

This image shows a row of colorful, home-made fabric face coverings. The CDC recommends people make and wear such face coverings in public. The image is provided by the CDC and bears the agency's logo.
This image shows a row of colorful, home-made fabric face coverings. The CDC recommends people make and wear such face coverings in public. The image is provided by the CDC and bears the agency's logo.

References

[1] Callaway E, Cyranoski D, Mallapaty S, Stoye E, Tollefson J. 2020. The coronavirus pandemic in five powerful charts. Nature 579: 482-483. Available from https://doi.org/10.1038/d41586-020-00758-2

[2] Fauci AS, Lane HC, Redfield RR. 2020. Covid-19 — navigating the uncharted. New England Journal of Medicine 382: 1268-1269. Available from https://doi.org/10.1056/NEJMe2002387

[3] CDC COVID-19 Response Team. 2020. Geographic Differences in COVID-19 cases, deaths, and incidence — United States, February 12-April 7, 2020. Morbidity and Mortality Weekly Report 69: 465-471. Available from https://www.cdc.gov/mmwr/volumes/69/wr/mm6915e4.htm?s_cid=mm6915e4_w

[4] World Health Organization (WHO). Modes of transmission of virus causing COVID-19: implications for IPC precaution recommendations. Scientific brief 29 March 2020. Available from https://www.who.int/publications-detail/modes-of-transmission-of-virus-causing-covid-19-implications-for-ipc-precaution-recommendations

[5] van Doremalen N, Bushmaker T, Morris DH, Holbrook MG, Gamble A, Williamson BN, Tamin A, Harcourt JL, Thornburg NJ, Gerber SI, Lloyd-Smith JO, de Wit E, Munster VJ. 2020. Aerosol and surface stability of SARS-CoV-2 as compared with SARS-CoV-1. New England Journal of Medicine 382: 1564-1567. Available from https://doi.org/10.1056/NEJMc2004973

[6] Chin AWH, Chu JTS, Perera MRA, Hui KPY, Yen H-L, Chan MCW, Peiris M, Poon LLM. 2020. Stability of SARS-CoV-2 in different environmental conditions. Lancet Microbe 20: 411-412 [and online appendix]. Available from https://doi.org/10.1016/S2666-5247(20)30003-3

[7] Ye G, Lin H, Chen L, Wang S, Zeng Z, Wang W, Zhang S, Rebmann T, Li Y, Pan Z, Yang Z, Wang Y, Wang F, Qian Z, Wang X. 2020. Environmental contamination of the SARS-CoV-2 in healthcare premises: an urgent call for protection of healthcare workers. Preprint posted to medRxiv 16 March 2020. Available from https://doi.org/10.1101/2020.03.11.20034546

[8] Guo Z-D, Wang Z-Y, Zhang S-F, Li X, Li L, Li C, Cui Y, Fu R-B, Dong Y-Z, Chi X-Y, Zhang M-Y, Liu K, Cao C, Liu B, Zhang K, Gao Y-W, Lu B, Chen W. 2020. Aerosol and surface distribution of severe acute respiratory syndrome coronavirus wards, Wuhan, China, 2020. Emerging Infectious Diseases 26: [early release, cited 21 April 2020]. Available from https://doi.org/10.3201/eid2607.200885

[9] Meselson M. 2020. Droplets and aerosols in the transmission of SARS-CoV-2. New England Journal of Medicine [online, cited 21 April 2020]. Available from https://doi.org/10.1056/NEJMc2009324

[10] Ong SWXO, Tan YK, Chia PY. 2020. Air, surface environmental, and personal protective equipment contamination by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) from a symptomatic patient. Journal of the American Medical Association [online, cited 21 April 2020]. Available from https://doi.org/10.1001/jama.2020.3227

[11] Xu P, Qian H, Miao T, Yen H-L, Tan H, Kang M, Cowling BJ, Li Y. 2020. Transmission routes of Covid-19 virus in the Diamond Princess cruise ship. Preprint posted to medRxiv 14 April 2020. Available from https://doi.org/10.1101/2020.04.09.20059113

[12] Anfinrud P, Stadnytskyi V, Bax CE, Bax A. 2020. Visualizing speech-generated oral fluid droplets with laser scattering. New England Journal of Medicine [online, cited 21 April 2020]. Available from https://doi.org/10.1056/NEJMc2007800

[13] Zou L, Ruan F, Huang M, Liang L, Huang H, Hong Z, Yu J, Kang M, Song Y, Xia J, Guo Q, Song T, He J, Yen H-L, Peiris M, Wu J. 2020. SARS-CoV-2 viral load in upper respiratory specimens of infectious patients. New England Journal of Medicine 382: 1177-1179. Available from https://doi.org/10.1056/NEJMc2001737

[14] Rothe C, Schunk M, Sothmann P, Bretzel G, Froeschl G, Wallrauch C, Zimmer T, Thiel V, Janke C, Guggemos W, Seilmaier M, Drosten C, Vollmar P, Zwirglmaier K, Zange S, Wölfel R, Hoelscher M. 2020. Transmission of 2019-nCoV infection from an asymptomatic contact in Germany. New England Journal of Medicine 382: 970-971. Available from https://doi.org/10.1056/NEJMc2001468

[15] Wei WE, Zongbin L, Chiew CJ, Yong SE, Toh MP, Lee VJ. 2020. Presymptomatic transmission of SARS-CoV-2 — Singapore, January 23-March 16, 2020. Morbidity and Mortality Weekly Report 69: 411-415. Available from http://dx.doi.org/10.15585/mmwr.mm6914e1

[16] Eikenberry SE, Mancuso M, Iboi E, Phan T, Eikenberry K, Kuang Y, Kostelich E, Gumel AB. 2020. To mask or not to mask: modeling the potential for face mask use by the general public to curtail the COVID-19 pandemic. Preprint posted to medRxiv 11 April 2020. Available from https://doi.org/10.1101/2020.04.06.20055624

Image credits

All images on this page are part of the CDC’s public domain Social Media Toolkit. Available from https://www.cdc.gov/coronavirus/2019-ncov/communication/social-media-toolkit.html