Imagine today’s date is Sept. 1, 2021. You’ve received both of your vaccine doses. Your neighbors have been fully vaccinated too, so you’re having them over for dinner tonight. COVID-19 cases have become rare in your town. You’ll wear a mask when you go out to pick up groceries, just to be safe, and there are still signs up at the pharmacy counter advertising COVID-19 vaccination. For the most part, though, life feels pretty normal.

Your brother, who lives a few states away, is living in a different reality. Several clusters of cases related to a new viral variant have emerged in his area, prompting schools to delay their start dates. Masks are required in public, and restaurants are asking patrons to leave their information in case they need to start contact tracing. The health department is setting up public testing and vaccination sites, and health officials are on the news each night encouraging unvaccinated people to get their shots. You were planning to visit your brother for Thanksgiving, but you may scrap those plans if things get much worse in his area.

That’s a hypothetical scenario, of course. COVID-19 is a new disease, and there’s no road map for predicting its future. No one knows how long it will take the U.S. to reach herd immunity or whether we’ll get there at all–if the virus mutates faster than vaccines can be administered, or if a significant share of the population opts not to get vaccinated, the window may slam shut. Scientists don’t know how many people need to get vaccinated to reach that threshold even if everything goes well, though recent estimates put the figure at well above 70% of the population. That’s a daunting goal, since only about 8% of people in the U.S. have been vaccinated so far.