Why a vaccine that stops coronavirus won't be enough to end the pandemic

A vaccine's ability to forestall a disease is also how vaccine developers typically design - and how regulators typically evaluate - Phase 3 clinical trials for vaccine candidates

Not long after the new coronavirus first surfaced last December, an ambitious prediction was made: A vaccine would be available within 12 to 18 months, and it would stop the pandemic.

Despite serious challenges — how to mass manufacture, supply and deliver a vaccine worldwide — the first prong of that wish could well be fulfilled. Eight vaccine candidates are undergoing large-scale efficacy tests, so-called Phase 3 trials, and results are expected by the end of this year or early 2021.

But even if one, or more, of those efforts succeeds, a vaccine might not end the pandemic. This is