SEOUL, South Korea — The deaths were mounting, and so were the public’s fears.

South Korea had vastly expanded its flu vaccine program to cover millions more people, to prevent a one-two punch to its health system as the coronavirus spread globally. But as the injections got underway, reports of deaths started popping up.

South Korean scientists quickly determined that the deaths were unrelated to the flu shots. But they worried that if they didn’t stop the panic, the public might shun the vaccines altogether.

So health officials doubled down — and, in the process, gave the world a game plan for when coronavirus vaccines become widely available.