Personal Immunity and Herd Immunity

When I first heard friends getting excited about T-cell immunity to COVID-19, I was non-plussed.

“This means the disease is less contagious than we thought!,” they said.

And I replied, “You’re double-counting!  I If some people are immune, that will already be reflected in existing estimates of R0.”

As it turns out, however, my friends were right for the wrong reason.  While immunity doesn’t matter for initial estimates of R0, it is crucial for estimating the path of R0.  This in turn is crucial...