And if the country had begun locking down cities and limiting social contact on March 1, two weeks earlier than most people started staying home, the vast majority of the nation’s deaths — about 83 percent — would have been avoided, the researchers estimated.

Under that scenario, about 54,000 fewer people would have died by early May…

The findings are based on infectious disease modeling that gauges how reduced contact between people starting in mid-March slowed transmission of the virus. Dr. Shaman’s team modeled what would have happened if those same changes had taken place one or two weeks earlier and estimated the spread of infections and deaths until May 3.

The results show that as states reopen, outbreaks can easily get out of control unless officials closely monitor infections and immediately clamp down on new flare-ups. And they show that each day that officials waited to impose restrictions in early March came at a great cost.