Informative discussion with Dr. Paul Offit (Penn), a vaccine expert.  He thinks there will be a vaccine by the "first quarter" of 2021; he explains why we are able to produce this vaccine so quickly (basically, government is assuming all the risks); and other topics.   One interesting data point:  if a vaccine is 75% effective, then you would need to vaccinate about two-thirds of the population to stop spread of the virus; but if the vaccine is only 50% effective (like the flu vaccine), then to stop spread  you'd need to vaccinate the entire population (which you can't do, since some people can't take vaccines for health reasons).  As Dr. Offit emphasizes, the arrival of a vaccine will not eliminate the need for hygienic measures to control the virus.

(Thanks to Michael Weisberg for the pointer.)