Today, we’re turning this section over to our colleague Apoorva Mandavilli, who has been covering the pandemic for The Times’s Science desk.

The pandemic will end only when enough people are protected against the coronavirus, whether by a vaccine or by already having been infected. Reaching this threshold, known as herd immunity, doesn’t mean the virus will disappear. But with fewer hosts to infect, it will make its way through a community much more slowly.

In the early days of the crisis, scientists estimated that perhaps 70 percent of the population would need to be immune in this way to be free from large outbreaks. But over the past few weeks, more than a dozen scientists told me they now felt comfortable saying that herd immunity probably lies from 45 percent to 50 percent.

If they’re right, then we may be a lot closer to turning back this virus than we initially thought.

It may also mean that pockets of New York City, London, Mumbai and other cities may already have reached the threshold, and may be spared a devastating second wave.

The initial calculations into herd immunity assumed that everyone in a community was equally susceptible to the virus and mixed randomly with everyone else.

The new estimates are the product of more sophisticated statistical modeling. When scientists factor in variations in density, demographics and socialization patterns, the estimated threshold for herd immunity falls.

In some clinics in hard-hit Brooklyn neighborhoods, up to 80 percent of people who were tested at the beginning of the summer had antibodies for the virus. Over the past eight weeks, fewer than 1 percent of people tested at those same neighborhood clinics have had the virus.

Likewise in Mumbai, a randomized household survey found that about 57 percent of people who live in the poorest areas and share toilets had antibodies, compared with just 11 percent elsewhere in the city.

It’s too early to say with certainty that those communities have reached herd immunity. We don’t know, for example, how long someone who was infected stays protected from the coronavirus. But the data suggests that the virus may move more slowly in those areas the next time around.