(CNN) The US surpassed 2 million confirmed coronavirus cases as experts predicted Thursday that tens of thousands more people will get infected and die in the months ahead.
More than 113,700 people have died from Covid-19 nationwide, according to Johns Hopkins University.
An influential model cited by the White House issued the dire prediction, saying the US death toll could reach 169,890 by October 1, with a possible range of about 133,000 to 290,000 deaths.
Daily deaths are expected to decrease through June and July, then remain relatively stable through August before rising sharply in September, the model forecasts.
"If the US is unable to check the growth in September, we could be facing worsening trends in October, November and the following months if the pandemic, as we expect, follows pneumonia seasonality," said Dr. Christopher Murray, director of the Institute for Health Metrics and Evaluation at the University of Washington School of Medicine.
"Seasonality will be a very big driver of the second wave, we believe," Murray said at a briefing Thursday.
Fears of a resurgence of the pandemic -- which already has caused soaring unemployment as parts of the economy shut down -- appeared to help rattle Wall Street on Thursday, sending indexes to their lowest levels since March 16.
The Dow plunged 1,862 points, or 6.9%. The S&P 500 dropped 5.9%, and Nasdaq closed 5.3% lower.
Despite shaky coronavirus trends, Treasury Secretary Steven Mnuchin said the federal government won't put the entire nation back into lockdown to fight the pandemic.
"We can't shut down the economy again," Mnuchin told CNBC on Thursday. "We've learned that if you shut down the economy, you're going to create more damage, not just economic damage. Medical problems and everything else that gets put on hold."
The "biggest and most difficult choice" states will face in the fall is managing a potential second shutdown, IHME's Murray said.
"Because of quarantine fatigue, because of the economic effects of quarantine, another round of shutdowns might have even larger effects on businesses that may be on the edge of not being able to stay solvent."
Many states have loosened restrictions that began in March to stop the spread of the virus. But with no vaccine, more people congregating in public places and recent protests for racial justice in major cities, one health expert predicted that an additional 100,000 people in the United States will die of coronavirus by September.
That prediction from Dr. Ashish Jha, director of the Harvard Global Health Institute, would put the US death toll well past 200,000, higher than what the University of Washington model forecasts by that time.
"And we won't be done," Jha told CNN's Wolf Blitzer Thursday, with many months of fall and winter ahead. "It's really stunning to me that we have this much suffering and death, and we're just not doing enough about it."
That projection is conservative, he said Thursday as at least 20 states were showing an upward trend in average daily cases -- a rise of at least 10% -- over the previous seven days, according to CNN's analysis of Johns Hopkins University data.
At least 22 states' average daily cases have dropped more than 10% over the past week.
But, if cases continue to trend upward in some parts of the country, "then I am being too optimistic, and (the next 100,000 deaths) will come sooner than that," Jha told CNN Thursday morning. "But I'm hopeful that we can get our act together and at least put back some of the social distancing stuff that we've let go."
He is not recommending lockdowns, he said. But getting more people to wear masks, even outside, and increasing testing and contact tracing, he said, would help.
Since Memorial Day, the number of coronavirus hospitalizations has gone up in at least a dozen states, according to data CNN aggregated from the Covid Tracking Project between May 25 to June 9.
In Arizona, officials are telling hospitals to activate emergency plans.
At its previous peak, the state's intensive care unit beds were 78% in use. As of this week, 79% were occupied. Arizona's Director of Health Services Dr. Cara Christ asked that hospitals "be judicious" in elective surgeries to ensure bed capacity.
"We know Covid-19 is still in our community, and we expect to see increased cases," the Arizona Department of Health Services tweeted.
Bed capacity and medical resources were among the top concerns in treating the coronavirus pandemic when the nation first reached a peak. Health experts say it is a matter of when -- not if -- the country sees another surge in cases that could overwhelm health care systems once more.
Fears of another spike are not limited to any particular region.
States need to be ready for an uptick in coronavirus cases, though, in areas where protests have taken place over weeks, warned Dr. Tom Inglesby, the director of the Johns Hopkins Center for Health Security.
"We need to be ready for the possible rising Covid cases from the peaceful demonstrations that have been occurring and we need to do what we can to lower the risks of those protests," Inglesby said during a media briefing Thursday on the impact of reopening amid the ongoing pandemic.
North Carolina recently broke its record with 780 coronavirus hospitalizations by early Thursday, according to the North Carolina Healthcare Association.
Though there is plenty of hospital capacity left, state officials are concerned about increased trends in hospitalizations when restrictions first eased, then again after Memorial Day weekend.
Alabama on Thursday reported its largest number of cases reported in one day -- 849 -- since the state began tracking Covid-19 cases.
The number of Covid-19 hospitalizations since Memorial Day has gone up in Alaska, Arkansas, Arizona, California, Kentucky, Mississippi, Montana, North Carolina, Oregon, South Carolina, Texas and Utah, according to data aggregated from the Covid Tracking Project.
Overall, the new data mark an increase in several states that began in the past couple weeks. The lag period between when people are exposed to the virus to the time that they may actually get tested and come back with a confirmed infection can be about two to three weeks.
Areas that were considered early hot spots remain cautious.
While New Jersey's numbers are improving, it's not yet out of the woods, Gov. Phil Murphy said. And Los Angeles is encouraging residents who have attended protests over the death of George Floyd to monitor for symptoms, to quarantine or get tested.
"You could have an exposure and it will not come through a contact tracing system. Nobody knows that you were there, nobody has your name," Los Angeles County Public Health Director Barbara Ferrer said.
Los Angeles County is seeing an average of 1,300 new coronavirus cases a day as it allows more businesses to reopen, including gyms and museums.
A medicine that may treat and prevent Covid-19 is now being tested in patients in multiple sites around the United States, according to an announcement Thursday from Regeneron Pharmaceuticals Inc.
It is the first trial of a Covid-19 antibody cocktail in the United States. If successful, Regeneron hopes it could be available by the fall.
The clinical trial started Wednesday.
Antibodies are proteins the body naturally makes to protect the body from a threat like Covid-19. To make what's called monoclonal antibodies for an antibody cocktail, scientists comb through thousands of antibodies to figure out which ones fight the novel coronavirus most effectively.
In this case, Regeneron's scientists picked two antibodies, scaled them up and put them into a medicine that it hopes can be used to treat symptoms and as protection for vulnerable communities such as the elderly or health care workers.
An antibody treatment is not a vaccine and does not provide permanent protection. But this kind of passive immunization works right away and could potentially be available before a Covid-19 vaccine.
Moderna confirmed Thursday that it expects to begin the final phase of its vaccine candidate next month.
The study will include 30,000 subjects in the United States and will be placebo-controlled, the company said in a press release.
Moderna is still on track to deliever about 500 million to 1 billion doses a year, starting next year, the company said.
Its primary endpoint will be prevention of symptomatic Covid-19, with secondary endpoints to include the prevention of severe Covid-19 that leads to hospitalization.