It should be much safer to move around once your community achieves herd immunity — the point when the virus can’t easily spread because enough people have become vaccinated or have already had the illness. This also helps protect people who can’t get vaccinated for health reasons, and people who work in crowded or indoor places. Many scientists think at least 70 percent of people need to have acquired immunity for the whole community to be protected. That number is just an estimate, though, and might need to be revised once we know more about how vaccines affect the virus’s ability to spread.

When a majority of people are vaccinated, scientists said, it will be safer to do things in your community, like eat at indoor restaurants, attend a party or ride a bus. Next Christmas, families can probably gather in ways they should avoid this year, they said.

It’s too early to know exactly when we’ll hit that threshold. Although federal officials have said the United States should have the resources to vaccinate hundreds of millions of people by summer, many scientists say that timeline is optimistic. There could be logistical challenges to vaccinating everyone, and some people have expressed hesitancy about getting the vaccine.

It’s likely that some regions will have higher vaccination rates than others. Just as some communities have found themselves vulnerable to measles because of low childhood vaccination rates, areas with low Covid-19 vaccination rates may see outbreaks even if the country has reached a herd immunity level over all. Knowing that context will be crucial for decision making.

Also, experts stressed that even when herd immunity is reached, Covid-19 is not likely to disappear outright. Outbreaks could still be likely, probably in winter.

“Winter is going to start being flu and Covid season,” said Andrew Noymer, an epidemiologist studying Covid-19 at the University of California, Irvine. The last things he will return to, he said, are international travel and crowded events like concerts — but he expects to do those again at some point. He is waiting not just for the vaccine, but also for the virus’s spread to decrease greatly and for hospitals to have more capacity: “I intend to go back bit by bit.”

During the pandemic, experts have asked people to think of themselves as having a risk budget: If you spend some of that limited supply by engaging in riskier behaviors, you will need to cut back in other aspects of your life. Vaccines can expand an individual’s risk budget, Professor Lofgren said. But they do not make the budget infinite: If you travel to see friends, you might still want to offset that decision by avoiding indoor restaurants.