China’s zero-covid policy initially saved lives, kept hospitals from being overwhelmed and gave the country time to disseminate vaccines. But it essentially pushed much of the pandemic’s impact into the future.

Now, that future has arrived.

When China ended the drastic lockdowns and restrictions that were in place for the past three years, it triggered a surge of cases and deaths in a population with little natural immunity and low levels of vaccine boosting. With data about the scale of this covid surge sparse and unreliable, scientists observing China’s crisis must piece together scraps of evidence to forecast the trajectory of this outbreak and what it might mean for the spread and evolution of the virus.