Social Distancing in times of COVID-19 (Part 2): What is the science behind the recommendations?

By Edison Liu and Jill Goldthwait


Social distancing means keeping people apart to minimize the chances of catching COVID-19 and to reduce the chances of spreading the disease. Implementing social distancing can be done at the individual level which is focused on a person not catching COVID-19 or at a population level to slow the spread of the pandemic across large groups of people.

You can get infected by inhaling the virus released by another person coughing, or by touching the hands of an infected individual or surfaces where the virus has been deposited. Social distancing at the level of the individual is about washing hands before and after contact with other people, going to the bathroom, preparing food, eating and going to the grocery store. Washing means using soap and not just a rinse. Hand sanitizers are good, especially those with at least a 60 percent alcohol base, but proper handwashing actually removes the virus from your hands while hand sanitizer kills most of it — but not all of it. This video from the CDC shows how you should wash your hands:

If you should cough, do so into a napkin, discard that napkin and then wash your hands or cough into the crook of your elbow to avoid contaminating your hands. You should avoid all touching: handshakes, hugs and social kisses, and keep a distance of about six feet from the next person in public places. Potentially contaminated surfaces such as door handles, shopping cart handles, and table-tops should be cleaned prior to use. Though not perfect, there is scientific evidence that guides these recommendations.

COVID-19 is primarily transmitted through tiny water droplets called aerosols projected out by coughing. The distance that these droplets can travel is up to six feet with each cough or sneeze. Experimentally, the virus persists in these aerosols in enclosed spaces for up to three hours. The viral dose coming from an infected individual depends on how much virus is in their system. People with symptoms excrete more virus than those who don’t. Since the virus does not survive outside the body well, the dose of that aerosolized virus is reduced by a half in about one and a half hours.

If you are infected, standard surgical masks help by limiting the distance that aerosols project; but these masks do little to prevent the intake of infected particles. “N95” masks can filter much of the viral aerosols out, but they require special fitting and are currently reserved to protect healthcare workers so are not generally available. Because of the limitations of the supply of all masks, the recommendation is not to wear one if you are healthy, but only if you have symptoms.

The virus when sprayed onto surfaces will survive for up to three days but this varies according to the material of the surface: COVID-19 persists longer on plastic and stainless steel than copper and cardboard. When the information on all coronaviruses was reviewed, it was found that they survive better in lower temperatures and higher humidity. Though there is no cut-off of what specific temperature and humidity the virus survives best, this justifies the closure of gyms with communal showers. The virus on such surfaces can be effectively neutralized with solutions containing 70 percent or more alcohol, or solutions of 0.01 percent bleach, or 0.5 percent hydrogen peroxide.

Often, we are careful with social distancing and cleaning when we are not at home but neglect the fact that our loved ones, including children, can bring germs home. While social distancing does not mean social isolation, surfaces should be cleaned at home, handwashing should continue and family members who are ill should not sleep in the same room with those who are well.

At a community and population level, social distancing is accomplished through limiting the formation of crowds or groups of people and by work-at-home and shelter-in-place policies. There is often confusion as to why events like March Madness, the NCAA basketball tournament, should be cancelled since so few people relative to the population size are deemed infected. Some are even indignant that events and social gatherings are cancelled. But the math tells us this must happen.

Proper social distancing means a shorter trajectory of disease. China is now reporting few, if any, new cases and beginning to lift some of the quarantine guidelines. Ignoring CDC guidelines and continuing social contact means a longer period of the epidemic and a higher infection and death rate.

We now know there are asymptomatic carriers of COVID-19. Since there are not enough tests for the infection, we do not know how many people are infected but not showing symptoms. Joshua Weitz and his colleagues at Georgia Tech calculated the chances of an infection spreading at the NCAA basketball tournament. Even if only one infected individual attended March Madness, many would become infected.

This is how the calculations go. Using the estimate that 20,000 Americans out of the 330 million in our country are infected (as of March 20, there are 15,219 confirmed U.S. cases – a gross underestimate), they calculated that every American has a 99.994 percent chance of being free of disease, or 16,500:1 chance to be in the clear. The probability that all 75,000 people attending the tournament will be free of COVID-19 is only about 1 percent. That means there is a 99 percent chance that the NCAA tournament will spark another wave of infection as newly infected spectators return home.

For the COVID-19 pandemic the danger is wherever people congregate in significant numbers. This includes workplaces where people often spend 8 hours a day. This is why the Jackson Laboratory and other employers have implemented a remote work policy to have workers whose jobs can be done remotely, work from home. In this manner all employees, including those who perform essential functions on site, will have the benefit of reduced exposure to people traffic. The workplace is not inherently dangerous. It all depends on how we manage the flow and the concentration of people.

So, what is the “safe” size of an event that makes it unlikely that an infected individual will be present? That depends on how common the COVID-19 infection is in the community. The currently recommended limit of 10 in a group means that you are willing to risk a 1 percent chance of inviting an infected person, assuming the frequency of COVID-19 to be less than 200,000 in our population. Therefore, the more prevalent the disease in the population, the smaller the group size limit.

Complicated? Yes, and the numbers are not only changing every day, but some numbers, like how many Americans have been infected with COVID-19, are still unknown.  To get that number we need widespread testing, which will be the topic of our next conversation.






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