As always, the opinions expressed in this post are my own, and should not be considered to be medical advice.

What is Social Distancing all about? A lot has been said and written about social distancing, but in brief – it’s a way for us to stop the transmission of disease from one person to another. If you only take one concept away from this post, let it be this- we are facing a disease that we have no specific treatments for (yet), is potentially fatal (mostly for those populations at risk, but the whole picture is yet to be revealed) and our only way to stop it is to slow down its spread; and the only sustainable instrument in that toolkit is social distancing.

Social distancing isn’t new. It’s been studied and performed previously, and examples go back to other diseases we’ve combated before – influenza (Flu), leprosy, plague, etc. In fact, more than a decade ago, in an effort to assist the mathematical modeling of the spread of disease, researchers had subjects in eight European countries log a diary of contacts, including close physical contact. It turns out that the 7,290 participants logged contact with 97,904 (!) different individuals in one day. That’s a whopping average of 13.4 contacts per person, per day; this is the number of people that are at put at risk, each day by each person. Turns out that there weren’t a lot of difference in contact patterns between individuals in the different eight countries, and the data is likely just as applicable to the US and the rest of the world. And this is increasingly important for respiratory pathogens, such as SARS-CoV-2 (the virus that causes COVID-19 disease; data shows that the virus likely first inhabits the upper respiratory tract – but that’s for another post).

Importantly, direct transmission isn’t the only possible for SARS-CoV-2 to transfer between individuals. Researchers recently published a study investigating how long virus particles remain viably infectious on different surfaces and compared that with SARS-CoV-1. The researchers aerosolized 10^5.25 50% tissue-culture infectious dose, which was found to be an equivalent amount to that detected in the respiratory tract of infected human (or in other words, what an infected person is likely to leave behind when they sneeze), and then checked how much viable virus was left for seven days. It turns out that the infectious viral particles remained viable up to 72 hours on plastic and 48 hours on steel. So, theoretically, if an infected person sneezes on a steel surface and a healthy person touches that surface the day later, and then touches their mouth – they may potentially be infected, even though a direct contact never occurred between the two. It’s important to understand that this is preliminary data only, this experiment was done under controlled temperature and humidity control and used highly sensitive test methods. Also, we still don’t know what the infectious dose needs to be to generate disease, and it’s unclear whether the detection limits used here will actually trigger transmission. But the concept is important – transmission is possible even without direct contact. So if you’re wondering why playgrounds are shut down – this is why.

In trying to understand the dynamics of SARS-COV-2 transmission, epidemiologists have been studying the epidemic in China, since that’s where it all began, and the most mature data we have comes from China and the Wuhan province, the epicenter of the pandemic. As more data becomes available, we may reveal that patterns differ worldwide, but right now, this is what we have to look at. And all sources and models currently show the US and most of the world follows in a similar trajectory so far (a nice graphical representation of this can be found in the link Oded shared in his comment yesterday, here).

A fundamental figure in modeling the spread of disease is the “Basic Reproduction Number“, labeled R₀. What this number is, basically, is an estimation of how many uninfected people will get the virus from a single infected individual (infected does not necessarily means sick). So, on average (from a population viewpoint), if R₀=2 for example, each disease carrier will infect two healthy individuals. As long as R₀>0, that means the disease continues to spread. The higher R₀ is, the faster it spreads. And R₀ is agnostic to how transmission occurs, it is simply a mathematical approximation of how fast a disease spreads.

And where are we today? A recent study published in the New England Journal of Medicine looked at transmission dynamics in Wuhan, again turning to the epicenter where it all began. They estimated that R₀=2.2. However, that may be a bit of a conservative estimate, and other studies estimate that R₀ is as high as 4.5 in Wuhan and 4.4 in all of China. So for a simple illustration, let’s imaging that R₀=3, an arbitrary figure between these estimates. What does that mean? If we do nothing to change that, each infected individual will transmit the virus to 3 healthy individuals; who each will transmit it to 3 others, and so on and so forth. So if Sally goes to the market today, she may transmit the virus to three individuals, who will then go to work to transmit it each to three more, each will pick up their kids at school and transmit to three more, and so on and so forth; with only four generations of this equation we already went from one individual to 40. The fifth iteration will bring us to 131. And so on. Remember those 13.4 contacts we make on average each day? This is where they come to play.

So why is this important? Are we all doomed? Well, no. Turns out that R₀ is not a constant and it depends on many factors. Remember, it is only a mathematical estimate. If we slow down transmission, we lower R₀ and once R₀<1, that means the disease is R₀ depends on factors that are intrinsic to the virus – such as how infectious it is, the environment – such as temperature and humidity – but most significantly to contact between healthy and infected individuals. The less infected individuals meet with those who have not been exposed, the slow the disease will spread. If we put this in mathematical terms, if f is the proportion of the population that engages in social distancing, and they each only make contact with a fraction a of their normal contacts, we can modify R such that R=[1-(1-a²)*f]*R₀. Notice how a is squared in this equation – the less people you see, the further your contribute to slowing down the disease. If 50% of the population limit their contacts to 50% of normal – we will decrease R by 37.5%. If the same 50% limit their contacts to 25% of normal, R decreases by about 81% (there are much more complex ways to estimate R and R₀ that are likely more accurate but are beyond the scope of this post). This concept is illustrated extremely well by the animation below by Siouxsie Wiles and Toby Morris:

So how does this all tie in? It all seem a little futile, COVID-19 is spreading despite social distancing, right? Well, not quite right. There are two important concepts to remember. First, we see the effects of our efforts in a delayed fashion, most likely about two weeks but possibly even more. So right now, we’re seeing the effect of not having social distancing put in place until about two weeks ago. In 2-3 weeks we’ll have a better understanding of how well we’ve implemented it. More importantly, remember, this isn’t about stopping the virus completely – this will take a lot of time and effort, discovery of new therapies and hopefully the development of a vaccine. All of our efforts right now are geared towards preventing the healthcare system from collapsing. Hospitals and ICUs can only take care of so many people at any given point; the slower this spreads, the more likely that more patients will receive medical care and have access to healthcare resources. It will also make it more likely that less healthcare workers are sick with COVID-19 and our incapable of caring for those who need it, or worse.

“I only believe in statistics that I doctored myself”

Winston S. Churchill

This post ended up being more technical that I originally envisioned it to be. I’ve thrown numbers and formulas at you, and by now you much be thinking of Winston Churchill and his quote from above. Some of you might even say – why should I worry? I’m not sick, I can’t infect anyone! Others might say, I’m not at risk, so what’s the worst that can happen? I’ll have a bit of a cold and this will all blow over. To both I say – you are absolutely wrong.

The second part is easier to rebuke. Yes, this disease mostly endangers those with pre-existing conditions and risk factors, and yes it is rare, but there are reports of young, healthy patients dying from this. This is a virus that we, as a species, never encountered before and we have no immunity to it. Most of us will make it safely to the other side, the world isn’t going to end and civilization will ultimately prevail. But that doesn’t mean there is no risk.

The first part requires us to remember two important facts. First, as I just mentioned, most young and healthy individuals that are exposed to SARS-CoV-2 do not develop a disease and remain completely asymptomatic. Others will only experience mild viral symptoms. These individuals are still spreading the virus, even if they do not experience any symptoms at all. In fact, some of these are super-spreaders that may infect other dozens of others. Don’t let that be you. Additionally, the incubation period of the virus is estimated to be anywhere between 3 and 14 days (likely 3-5 days in most). That means there is a “window period” where the virus is already being shed, but the symptoms have no yet begun.

So, what can we do? It’s actually fairly simple.

• Help us help you. We have to keep going to work. If you don’t – stay home. Keep a distance of 6-feet from others when you have to go outside, and practice common sense hygiene.
• Protect us if you are sick. If you need to see a medical professional, call in first – if you can. Telemedicine has been doing wonders in the ability to continue to provide services to individuals that don’t necessarily need to physically come into a clinic or hospital. And if you are sick or feel that you may have been exposed, if you can, wear a mask before coming in.
• Help others limit their exposure as well. Everybody needs food. Going shopping? call your friends and see if you can buy groceries for them as well, and leave it on their doorstep. There are numerous ways to split the balance afterwards. You just went grocery shopping for 3 of your friends with your own shopping? You just reduced the number of people who may have been exposed by 75%.
• Don’t visit your elderly parents and chronically sick relatives. They are at even greater risk. But call them. See if they need anything. See the point above and help in any way that you can. Remember, we draw our strength from each other, especially in difficult times like this. So check in on your friends and neighbors.
• Educated yourself. Get your data from a reliable source. I recommend starting with your local or national disease control agency, such as the CDC in the US. Follow guidelines, they sometimes change daily. Get your data from verified sources, not random bloggers on the internet (such as this one).
• Listen to instructions by your local healthcare agencies, local, state and national government agencies.

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Sources and further reading:

• Social distancing – Wikipedia. https://en.wikipedia.org/wiki/Social_distancing. Accessed March 28, 2020.
• Mossong J, Hens N, Jit M, et al. Social contacts and mixing patterns relevant to the spread of infectious diseases. PLoS Med. 2008;5(3):0381-0391. doi:10.1371/journal.pmed.0050074
• van Doremalen N, Bushmaker T, Morris DH, et al. Aerosol and Surface Stability of SARS-CoV-2 as Compared with SARS-CoV-1. N Engl J Med. March 2020:NEJMc2004973. doi:10.1056/NEJMc2004973
• Riou J, Althaus CL. Pattern of early human-to-human transmission of Wuhan 2019 novel coronavirus (2019-nCoV), December 2019 to January 2020. Euro Surveill. 2020;25(4). doi:10.2807/1560-7917.ES.2020.25.4.2000058
• Li Q, Guan X, Wu P, et al. Early Transmission Dynamics in Wuhan, China, of Novel Coronavirus–Infected Pneumonia. N Engl J Med. January 2020. doi:10.1056/nejmoa2001316
• Liu T, Hu J, Kang M, et al. Time-varying transmission dynamics of Novel Coronavirus Pneumonia in China. bioRxiv. January 2020:2020.01.25.919787. doi:10.1101/2020.01.25.919787
• Basic reproduction number – Wikipedia. https://en.wikipedia.org/wiki/Basic_reproduction_number. Accessed March 28, 2020.
• Delamater PL, Street EJ, Leslie TF, Yang YT, Jacobsen KH. Complexity of the basic reproduction number (R0). Emerg Infect Dis. 2019;25(1):1-4. doi:10.3201/eid2501.171901
• Anderson RM, Heesterbeek H, Klinkenberg D, Dã T, Hollingsworth irdre. How will country-based mitigation measures influence the course of the COVID-19 epidemic? Lancet. 2020;395:931-934. doi:10.1016/S0140­6736(20)30567­5
• Lipsitch M, Swerdlow DL, Finelli L. Defining the Epidemiology of Covid-19 — Studies Needed. N Engl J Med. 2020;382(13):1194-1196. doi:10.1056/NEJMp2002125

The post’s featured image, “Flatten the Curve” and the COVID-19 spread animation both originally from Spinoff.co.nz, the former by Siouxsie Wiles and Toby Morris and the latter by Toby Morris, used under CC BY 4.0 license.