Social Distancing and Paranoia About Human Interaction

The practice of social distancing has been extolled as effective in mitigating the spread of the coronavirus infection. However, its promotion and enforcement are often accompanied by a message intended to arouse unfounded fear or mistrust of others by overestimating the threat from human contact. For example, one state’s official website describes the current risk of human contact to the extent of equating it with a potential homicide.

To understand why the risk from human contact tends to be inflated, we can calculate the chance of one-on-one physical contact involving one person who is a carrier of the virus and the other who is a non-carrier in a county by using the confirmed virus data in the county. One county in the Pacific Northwest has a total population 382,067 with half residing in two adjacent cities. The public-health office reported a total of 36 positive tests in the county (April 12, 2020).

This analysis uses Bayes’ theorem (e.g., Howell, 2010) to derive the probability of one-on-one encounter where one person is a carrier of the virus and the other is a non-carrier.

1. The hypothesized prior probability of meeting a virus-carrying person is 36/382067 =0.000094 or 9.4 persons per 100,000 population.
2. The hypothesized probability of meeting a non-virus person is (382067-36)/382067= 0.9999.
3. The hypothesized posterior probability of the encounter between the carrier and the non-carrier is: (0.000094) (0.9999) / ((0.000094) (0.9999) + (0.9999) (0.9999)) = 0.000094

Interpretation: If you are a healthy person, the chance that you will meet a person with the virus outside is about 1/10,000 in the county.

Three factors may also need considering:

First, there are some asymptomatic persons who are not counted in the confirmed cases. Therefore, the current case number may not include all infected persons. Second, most infected persons are unlikely to walk on the street or to do grocery shopping. That makes encountering them even less likely. Third, (Update), the CDC has clarified some confusion regarding what constitutes “close contact” as being within 6 feet of someone infected with COVID-19 for at least 15 minutes or more. Namely, merely interacting with a person with the infection is insufficient to catch the infection. The interaction must last at least minimum 15 minutes. For majority of people walking on the street or doing shopping, their contact with strangers will be far less than the duration. Therefore, it is even more unlikely that individuals will get infected by simple interactions with others, most of whom are not the virus carrying persons.

In short, this analysis shows that although social distancing and lockdowns are crucial for slowing the spread of the virus, the probability of getting infected with the virus through an interpersonal encounter needs not to be exaggerated.