No backyard barbecues, Fourth of July celebrations or Little League games - getting people back to work but keeping them away from social activities that bring them into contact with many others is the key to safely reopening America's economy, say experts.

"The 'no fun' economy, I call it," said Harvard University political economy professor James Stock. "It doesn't sound like a lot of fun, but is better than the alternative."

Since mid-March, many non-essential businesses and schools have been closed and individuals have stayed at home to curb the spread of the coronavirus.

The economic cost of doing so has since come into sharper focus, with more than one in five workers filing for out-of-work benefits since March and America's unemployment rate soaring to 14.7 per cent last month - a level not seen since the Great Depression.

Now, using an array of calculations and sometimes-conflicting models, economists and epidemiologists are urgently figuring out how to balance lives and livelihoods.

"The economy has completely fallen off a cliff," said Prof Stock at a Brookings Institution event on Tuesday on when and how to reopen the economy. "It's clear that we need to reopen the economy now. This is urgent."

The longer the economy stays shuttered, the harder it will be to reverse its effects as more people will be out of work for longer and more businesses will go bankrupt, said White House senior economic adviser Kevin Hassett at the event.

The White House's guidelines recommend that states reopen in phases, as long as there is a pattern of declining deaths and infections, while keeping social distancing in place for longer.

Low-contact, high-value jobs should be reopened quickly and returning workers must feel safe, said Prof Stock, who advocated for modifications to offices, shop floors and workplaces to facilitate worker distancing, or encouraging employees to work from home.

But high-contact activities like live fans at professional sports, auto shows and Independence Day celebrations may have to be suspended until a vaccine is ready, he said.

"If people go back to an even near-normal social life, the virus can rekindle and deaths will rise, no matter what protections there are at the workplace," he said, citing simulations his colleagues had run of reopenings that follow federal guidelines. His warning echoed that of National Institute of Allergy and Infectious Diseases head Anthony Fauci. At a Senate hearing on Tuesday, Dr Fauci said that reopening the economy too soon could lead to "little spikes that might turn into outbreaks".

But keeping social distancing in place for the longer term will likely prove difficult. Having spent most of spring indoors, Americans are getting increasingly stir-crazy, with photos and footage of crowded restaurants, busy beaches and even a packed airplane cabin sporadically going viral. And although Americans consistently say in surveys that they worry about reopening too quickly, cellphone data confirmed that more and more people were leaving their homes late last month, possibly flouting social distancing guidelines.

A National Public Radio analysis of 18 million phones' worth of mobile phone location data found that the number who stayed home peaked on April 12, and has since been on the decline.

The rise in people leaving their homes and certain states' relaxation of social distancing have caused the University of Washington's Institute for Health Metrics and Evaluation (IHME), whose coronavirus model the White House uses in its calculations, to revise its forecasted deaths upwards. It doubled its forecast to 135,000 deaths last week and, on Tuesday, further raised it to 147,000 by early August. More than 83,000 people in the US have died from the coronavirus.

"When we started off making projections, we had assumed that all the states were going to follow the New Zealand model, which is to keep social distancing in place until transmission gets to a really low level," IHME director Christopher Murray told CNN. But "we're speeding towards relaxing social distancing," he added. "I think we'll see the numbers go up unless we see the benefits of people wearing masks, and capacities to test, contact trace and isolate go up faster than we think they may."