CAPE TOWN — A few months from now, a factory in South Africa is expected to begin churning out a million doses of Covid-19 vaccine each day in the African country hardest-hit by the pandemic.

But those vials will probably be shipped to a distribution center in Europe and then rushed to Western countries that have pre-ordered them by the hundreds of millions. None have been set aside for South Africa.

The country, which will help manufacture the vaccine and whose citizens have enrolled in clinical trials, does not expect to see the first trickle of doses until around the middle of next year. By then, the United States, Britain and Canada may already have vaccinated more than 100 million people.

At what point does a country achieve herd immunity? What portion of the population must acquire resistance to the coronavirus, either through infection or vaccination, in order for the disease to fade away and life to return to normal?

Since the start of the pandemic, the figure that many epidemiologists have offered has been 60 to 70 percent. That range is still cited by the World Health Organization and is often repeated during discussions of the future course of the disease.

Although it is impossible to know with certainty what the limit will be until we reach it and transmission stops, having a good estimate is important: It gives Americans a sense of when we can hope to breathe freely again.

Recently, a figure to whom millions of Americans look for guidance — Dr. Anthony S. Fauci, an adviser to both the Trump administration and the incoming Biden administration — has begun incrementally raising his herd-immunity estimate.