(CNN) The US on Thursday surpassed 4 million officially recorded Covid-19 cases -- and a quarter of that count came in just the last 15 days.
The country's rising daily rate of confirmed coronavirus cases, along with a near-record number of hospitalizations, signals the US is far from containing a virus that is straining hospitals and labs, health experts say.
"We've rolled back essentially two months' worth of progress with what we're seeing in number of cases ... in the United States," Dr. Ali Khan, dean of the University of Nebraska Medical Center's College of Public Health, told CNN on Thursday.
About 59,600 people were hospitalized with Covid-19 in the US on Wednesday -- roughly 300 short of the country's peak recorded in mid-April, according to the Covid Tracking Project.
The US has officially recorded 4,032,430 cases, according to Johns Hopkins University. At least 144,167 people have died.
The reported count is picking up speed: The national seven-day average of new daily cases was 67,429 on Wednesday, a record.
It took the country nearly 100 days to count its first 1 million cases, from January 21 to April 28. It took only 15 days to rise from 3 million on July 8 to more than 4 million, according to JHU figures.
Many Covid-19 illnesses went undiagnosed, especially early in the pandemic when testing was less available. The Centers for Disease Control and Prevention has said true case totals are probably more than 10 times greater than official figures in most places. One study suggested the US might have had more than 8 million cases in March alone.
But physicians are sounding alarms about rising hospitalizations nationwide, and especially in the nation's hot spots like parts of Florida.
More than 50 hospitals there have reached capacity in their intensive care units, and only 15% of the state's ICU beds are available, the Agency for Health Care Administration said.
"Any spike in cases or increase in hospitalizations is going to put our ER system and hospital systems in peril," Dr. Damian Caraballo, an emergency room physician in Tampa, told CNN.
The spread has promised a bleak outlook for the months ahead, according to both health officials and President Trump.
What comes next is unclear: With now at least 41 states requiring face coverings, some have said strict measures like limiting gatherings and enforcing social distancing and masks can be as impactful as another lockdown. But others aren't as hopeful.
"Masks will help, but I think we need a lot more than masks to contain this epidemic that's running through our country like a freight train," said William Haseltine, the chair and president of global health think tank ACCESS Health International.
"Until we see major changes of behavior and until we see the public health services here stepping forward with many more resources, we aren't sure of containing this."
Many are calling for a reset: shut everything down again and start over. More than 150 prominent US medical experts, scientists, teachers, nurses and others have signed a letter to political leaders urging them do just that.
"The best thing for the nation is not to reopen as quickly as possible, it's to save as many lives as possible," they wrote in the document, which was sent to the Trump administration, leading members of Congress and state governors.
"Right now we are on a path to lose more than 200,000 American lives by November 1st. Yet, in many states people can drink in bars, get a haircut, eat inside a restaurant, get a tattoo, get a massage, and do myriad other normal, pleasant, but nonessential activities."
Still, the CDC spoke out strongly about the need to reopen schools, albeit in communities without substantial, uncontrolled transmission of the virus.
"It is critically important for our public health to open schools this fall," director Dr. Robert Redfield said in a statement announcing the updates.
In new guidelines issued Thursday, the CDC said children suffer from not being in school, and they don't suffer much from the virus and are less likely to spread it than adults.
"(The) United States reported that children and adolescents under 18 years old account for under 7 percent of COVID-19 cases and less than 0.1 percent of COVID-19-related deaths," as of July 17, the statement said.
The CDC has said for more than a week that it would issue new guidance for schools. President Donald Trump had complained its guidelines were "very tough" and "expensive."
With summer in full swing, it's becoming clear that warm weather isn't stopping the virus. A study published Thursday in the journal JAMA Network Open says social distancing, not weather, is better at stopping its spread.
The researchers did show some reduction of Covid-19 spread in temperate weather, but the benefit declined when residents didn't practice significant social distancing, especially in areas with high population densities.
"Our data reveal that if the United States had collectively waited longer, opened more slowly, and then kept our gathering sizes small, we might have reduced case counts like Europe or Canada and experienced a relatively normal summer, free of extreme disease burden from COVID-19," Dr. David Rubin, a pediatrics professor at the University of Pennsylvania's Perelman School of Medicine and the study's lead author, said in a press release.
What is becoming clear for statisticians at the CDC is that Covid-19 will end up among the top 10 causes of death in the country.
The CDC can't start ranking leading causes of deaths until the end of the year. But in 2018, the last year for which there is complete data, stroke was the fifth-leading cause of death for the whole year. In 2018, 147,810 people died of a stroke; Covid-19 has claimed almost 144,000 lives in just a matter of months.
"We know that based on the # of COVID-19 deaths so far in 2020, it will end up as a Top 10 leading cause of death but won't know exactly how high it will rank until next year," CDC mortality statisticians told CNN in an email.
The disease is already expected to be the second-leading cause of death in Los Angeles County, health officials there said.
Louisiana, where the governor said earlier this month that progress made in June against the virus was wiped out in weeks, on Thursday joined 11 other states that have reported a total of more than 100,000 infections.
Louisiana reached 101,650 cases, the state's health department said. The other states are California (with the most cases in the nation), New York, Florida, Texas, New Jersey, Illinois, Georgia, Arizona, Massachusetts, Pennsylvania and North Carolina.
Texas broke its record for hospitalizations two days in a row this week, with 10,848 patients reported Tuesday and 10,893 reported Wednesday. It also reported its highest single-day number of deaths: 197.
Florida reported its highest one-day death tally on Thursday: 173.
Florida has been sending nurses to hospitals that need more staff. More than 50 hospitals have asked for a total of 2,400 extra nurses, and more than 1,000 have so far been sent, the state has said.
Miami has announced heightened punishments for people who don't follow its mandate to wear masks in public. Warnings have been eliminated, and the first and second offenses are now punishable by $100 fines. The third could lead to an arrest, the city said Wednesday.
On Wednesday, Gov. Ron DeSantis said parents should have the option of sending their children back to the classroom or having them learn digitally from home, adding the "costs of keeping schools closed are enormous."
A 9-year-old is the youngest to die of Covid-19 in Florida, officials say
California surpassed New York for the most cases in the nation this week. With more than 420,000 cases, the state has seen a recent surge whereas New York's reported infections have slowed significantly. California reached another peak in new cases, reporting 12,807 positive tests in a day, Gov. Gavin Newsom said Wednesday.
Los Angeles County health officials announced that the virus is on track to be the second leading cause of death in the county -- with at least 3,400 fatalities in the first six months of the year.
That would mean the disease will claim more lives than Alzheimer's disease and strokes, health officials said. Coronary heart disease, the leading cause of death, claimed 6,000 lives in the first six months of 2019.
The news comes after the county reported 2,232 hospitalized patients Monday, breaking its own record of daily hospitalizations at least four times in a week. There were 2,207 confirmed cases hospitalized Wednesday, 27% of whom are in the ICU, health officials said.
Meanwhile, San Francisco is on "high alert" after averaging 79 new cases every day this week and seeing a 23% increase in hospitalizations, Public Health Director Grant Colfax said Wednesday.
Those two numbers play key roles in helping officials determine whether to pause or roll back reopening, Colfax added.