Apollo's Arrow: The Profound and Enduring Impact of Coronavirus on the Way We Live

by Nicholas A. Christakis MD PhD

Based on the fundamental R0 of SARS-2, as we saw in chapter 2, up to an estimated 60 to 67 percent of the population could be affected (or roughly two hundred million people in the United States). The necessary percentage could be lower, closer to 40 to 50 percent, given that social network structure means that different people spread the virus to different extents (as we also saw in chapter 2); or it could be higher, if the epidemic moves extremely fast and we overshoot the level required for herd immunity.

As we saw earlier, from a Darwinian point of view, it does not suit a pathogen’s interest to kill its victims, since it would rather that its hosts move around and transmit it to other people. Typically, with time, viruses become less lethal as a result of this preferential spread and survival of milder strains.

In fact, it’s possible that the four coronavirus species that cause the common cold are distant echoes of long-ago pandemics, now domesticated through a combination of herd immunity and genetic change. There is some intriguing evidence that this might have happened with the OC43 coronavirus species that causes the common cold.

But it is one thing to deny the reason for panic and another to urge the recklessness of unconcern.”

After being among us for a century, this virus would have further evolved to be a mild pathogen that just causes the common cold today. In addition to the virus evolving to be milder, our modus vivendi with it reflects an additional factor. Since OC43 is so widespread, most people are exposed to it in childhood and are spared any serious illness (remember the backward-L-shape mortality function). Thereafter, if they are exposed to OC43 again later in life, the virus just causes a mild cold, if it causes anything, because the hosts have some memory immunity. That’s a stark difference from the situation now, as SARS-2 is having a field day in a wholly immunologically naive population. But it’s possible that once we reach herd immunity in the coming years, people will simply be exposed to the SARS-2 virus as children, have a mild disease (most of the time!), get some immunity, and then avoid serious disease if they are re-exposed thereafter. Such a scenario is a quite possible eventual end to the story of SARS-2.