Breathless: The Scientific Race to Defeat a Deadly Virus

by David Quammen

The number R0 plays a big role in determining the supposed threshold at which herd immunity takes effect. That threshold is a percentage level—this much of the population has passed from susceptible to infected. And what does the threshold mark? It marks the point at which R has fallen to 1.0. Transmission has slowed because susceptibles have become fewer, and therefore harder for the virus to locate. Chains of infection come to dead ends. R keeps falling. From then on, so long as the reproduction number remains below 1.0, the outbreak subsides. All things being equal, it will decline to a tolerably low level of endemic disease or, best-case scenario, the virus will disappear entirely from that population because it can’t find anyone new to infect. That’s how smallpox eradication was achieved. The last active smallpox strains in the last few human cases were not killed by antiviral drugs. They died of isolation and loneliness, without leaving offspring.