Here’s a Covid-19 Number Worth Watching

It’s easy to see why people might be frustrated with the mathematical models that the world’s leaders have been using — and abusing — to decide how to respond to the coronavirus pandemic. First they said the U.S. might have 2.2 million deaths, then 60,000, then 100,000. One day it seems we’re all going to die, and the next it’s time to reopen.

That said, the better models contain a valuable element: the reproduction rate, or the number of people each sick person is likely to infect. Known as R(t), it’s important for understanding what might happen and how our behavior can change the outcome.