The Coronavirus

[Bob Butler 54]

I have come to view population density of states or nations, social distancing, masks, gloves, other PPE and closing borders as various forms of isolation.  All are attempts to keep carriers from contaminating the uninfected.  They complement each other.  If one approach was carried on to the extent of providing perfect isolation, there would be no need for the others.  Unfortunately, all as generally practiced are flawed.  You often employ multiple methods to complement one another and thus improve isolation.

Closing borders is one of the techniques that is not perfect.  Hong Kong failed to keep the bug out and away from the cell apartments.  Trump sent agents untrained in infectious diseases to meet some early returnees from Asia and cruise ships, and allowed them to break quarantine then stay in the local communities.  If you are not extremely rigorous, border closure doesn't work, though some island communities who back it up with tests and quarantine can make it work as part of a larger strategy.  Still, it can provide some delay and cut down on the initial infection rate.  

I would not count on border control as a sole technique.