More than 35,000 lives could have been saved in the U.S. if social distancing measures had begun just a week earlier than they actually did in mid-March, according to a new estimate by researchers at Columbia University.

They said simulations based on several models showed that 61% of the U.S. cases of infection as of May 3, more than 700,000 and 55% of the more than 65,000 recorded deaths could have been averted if social distancing and other safety measures had been in place a week earlier.

These researchers said the simulations illustrate the danger of easing lockdown measures too early, as many experts have noted. All 50 U.S. states have begun to reopen, to one extent or another and with encouragement from President Donald Trump, to try to resurrect economies devastated by business closures and layoffs in the pandemic.

"Efforts to further raise public awareness of the ongoing high transmissibility and explosive growth potential of COVID-19 are still needed at this critical time," the researchers wrote.

"Our results also indicate that without sufficient broader testing and contact tracing capacity, the long lag between infection acquisition and case confirmation masks the rebound and exponential growth of COVID-19 until it is well underway," they said.

Ten days ago, a New York documentary filmmaker, Eugene Jarecki, set up a billboard in Times Square with what he calls the "Trump Death Clock." It gives the number of deaths that Jarecki says could have been avoided if Trump had encouraged social distancing practices on March 9, rather than on March 16.

His figures are higher than those of the Columbia researchers because Jarecki argues that 60% of U.S. deaths could have been averted by acting a week earlier.

The U.S. is the country hardest hit by the pandemic, with more than 1.5 million cases of infection and more than 93,000 deaths, according to a tally by Johns Hopkins University.

Top Democrats on Thursday urged President Donald Trump to order flags flown at half-staff when the U.S. coronavirus death toll reaches 100,000, saying the salute would reflect a "national expression of grief."

"Our hearts are broken over this great loss" from the pandemic, House Speaker Nancy Pelosi and top Senate Democrat Chuck Schumer wrote in a letter to the president.

This weekend's observation of Memorial Day, which honors those who died serving in the U.S. military, coincides with the country likely reaching the grim milestone of 100,000 COVID-19 fatalities.

As of Thursday, the nation had 93,558 confirmed virus-related deaths, by far the highest toll in a world severely impacted by the pandemic.