there aren't enough non-immune members to spread a disease - the
non-immune members could still get the disease, but there isn't anyone
around who's infected to give it to them)
>Peter Fairbrother

"Herd immunity" can also be obtained in part by vaccines, of course.  And these days, that is the usual way.

As for COVID-19:   There will come a point, I think, where more damage will be done by failing to use vaccines, than will likely be done using them.  Maybe that point is already in the past.  As long as there are people willing to volunteer to take the risk of getting vaccinated with these new COVID-19 vaccine candidates, I think society should take up their offer.  Test the vaccines to the point where they are not likely to cause major harm, and then give them to tens of thousands of volunteers.   Monitor their conditions.  Maybe even do "challenge trials", meaning deliberately expose some of them to the virus, and see if they get sick.  If no major negatives show up, start vaccinating hundreds of thousands, even millions.  Keep monitoring.

Those vaccines that 'work', we will soon be able to distinguish from those that don't.  

             Jim Bell