Amid all of this, one particular difference stands out between the American and European approaches. Many states were happy to reopen after simply “bending the curve” — that is, slowing upward growth and ensuring spare hospital capacity. These states went on to expand economic activity at an elevated plateau with lots of ongoing transmissions. In contrast, European countries mostly waited to reopen until they crushed the curve or reached its far slope, with substantially lower incidence or dramatic reductions in the viral spread. It’s not the only explanation for a growing gap, but it’s a compelling one…

So why is low incidence so crucial to successful reopening? It’s simple math. More virus circulating in a community means more opportunities for it to spread. It makes every precaution individuals and officials take a bit less effective, and every activity riskier. This doesn’t necessarily translate to immediate outbreaks, as people came out of lockdown quite cautiously. But as activity expands to include things such as indoor service at bars, a high base level of infection becomes increasingly likely to cause problems.

Persistently high case levels amid a substantial reopening also make it far more challenging to identify and isolate a high percentage of infected individuals — again, a numbers problem. At a certain point, there are too many cases and contacts to have a hope of tracing them.