1 Important context

This website was initially create in late March of 2020, during the rapid expansion of the first wave of COVID-19 in the United States. It reflects that context, a time when there was an urgent need everywhere to either “flatten the curve” or keep it from rising in the first place. Stay-at-home orders and social distancing rules were in place nearly universally throughout the country, even as many folks were struggling with following them.

The epidemic is of course continuing to unfold, and we are not through this yet. However, many settings have indeed brought their incidence down considerably, and have relaxed social distancing rules. At the same time other localities are seeing rising numbers. And even as the first wave is not over, prospects for a second are always ahead.

All of this means that the exact trade-offs involved in visiting a friend now vary by location and context. However, the basic idea found in this webpage remains universal - if the virus is present and every household has just a few close face-to-face connections to others, there is a lot more room for spread through the population than if people limit those connections further.

Given all this, we will be keeping the remainder of the content of the webpage as is–but we will aim to periodically update this introduction to keep it up to date with the broad context.

2 The situation

COVID-19 is raging around the world, and you and your family have been asked (OK, told) to stay home and practice social distancing.

For the most part you are doing so, because you are good people.

But, you wonder—or perhaps, more likely, your kids wonder—is there really so much harm in meeting up with just one friend?

After all, there are all kinds of interactions still happening—between health care workers and patients, people in the food services industries and their customers, and so on. Why does one more matter?

This is a question we’ve heard a lot from our own friends and relatives.

To be clear, when we talk about “meeting up” here, we mean doing so in a way that doesn’t carefully follow all the rules about social distancing. That is, you interact directly or at a distance of less than 6 feet, and/or you share various items between you without rigorous handwashing and disinfecting.

For many of the people asking the question, it seems particularly low-risk to meet up with a friend if neither one has an elderly person in their household, or someone else with a compromised immune system, given how concentrated the mortality is within those populations.

As network epidemiologists, this is our effort to explain why these connections matter more than they may seem.

3 The Good Ol’ Days

Let’s start by imagining back to the good old days before COVID-19. People had lots of daily interactions with others outside their household. Let’s say, for the sake of the example, that a community had 200 households, and the members of a typical household together created 15 regular interactions with other households.

What would that look like? We can visualize a network as a set of nodes and links. Here we’ll make one in which the nodes (green dots) represent households, and the links (gray lines) represent interactions between members of two households.

[For those who want more detail—on terms, explanations and numbers—click on the “What’s Going On”? button in each section; if you just want to compare the pictures, continue on!]

3.1 The Network