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 partly because we seem to be focused at the moment on developing the kind of vaccine that may well prevent Covid-19, the disease, but that wouldn’t do enough to stop the transmission of SARS-CoV-2, the virus that causes Covid-19.

Doctors usually explain vaccines to patients and the parents of young children by describing how those protect us from a particular disease: An attenuated form of a pathogen, or just a bit of it, is inoculated into the human body in order to trigger its immune response; having learned to fight off that pathogen once, the body will remember how to fend off the disease should it be exposed to the same pathogen later.

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.

Yet the best vaccines also serve another, critical, function: They block a pathogen’s transmission from one person to another. And this result, often called an “indirect” effect of vaccination, is no less important than the direct effect of preventing the disease caused by that pathogen. In fact, during a pandemic, it probably is even more important.