On Aug. 31, Israel registered 11,000 new Covid-19 cases, the highest daily number since the pandemic began. The worrying thing was: That day’s case count beat a record set in mid-January, when only a small proportion of Israel’s population had been vaccinated.

By the end of August, at least 68% of Israelis had received at least one vaccine dose, but even the vaccinated were falling sick enough to need hospitalization. Alarmed, the Israeli government set about administering booster shots, trying to contain a surge in cases driven largely by the Delta variant of the coronavirus.

In its speed and thoroughness, Israel’s vaccination drive was a shining model. But to other governments, Israel’s late-summer spike now presents the frightening prospect that vaccine immunity may wane quicker than expected. Most countries have been slower than Israel in inoculating their populations, so the logical conclusion seems to be that their drop-off in vaccine immunity will follow Israel’s with a similar lag. Israel’s present looms in other countries’ futures. Indeed, said David O’Connor, a professor of pathology at the University of Wisconsin, “I have every reason to believe that the US is already seeing a version of that same spike in cases.”

But O’Connor and other scientists watching closely don’t necessarily believe that Israel’s patterns have to replicate themselves precisely in other countries. Israel’s Covid-19 escalation does offer signs of what will happen elsewhere in the coming months—but not always in ways that we expect.