(CNN) Dr. Sergio Segarra says when Covid-19 patient numbers in the Miami hospital he works in dipped below 20 last month, he began feeling optimistic.
"I remember seeing articles in the news about hospitals in California with empty Covid units and I longed for that experience," Segarra, the chief medical officer with Baptist Health's Baptist Hospital, said. "It's an experience we were working our way towards that unfortunately has taken a rather sad turn."
In the weeks since, the hospital's Covid-19 patient numbers have more than tripled, and staff are now treating more than 70 people, an "exponential growth," he said, that they were not expecting. To accommodate the climbing patient numbers, the hospital recently reopened two units that were previously shut down.
The overwhelming majority of those coming in sick with Covid are unvaccinated, Segarra said. Many are young -- people in their 20s and 30s who are getting "extremely, extremely sick" and some of whom are dying.
Roughly 45% of people in Florida are fully vaccinated, according to state data. The low vaccination rates, along with a dangerous coronavirus variant that's now the dominant strain in the United States and the relaxed Covid-19 guidelines, are what Segarra said he thinks have led to the increase.
"It's very sad to see as a health care professional, to see that this is generally an avoidable and preventable disease and to see so many people dying from a preventable illness," he said. "It's heartbreaking."
With cases of the virus surging in most of the United States -- driven by unvaccinated Americans and fueled by the highly contagious Delta variant -- Covid-19 hospitalizations are climbing in other parts of the country, too, and hospitals are again bracing for another round of devastation.
And in Covid hot spots such as Florida and Missouri, where patients are quickly filling Covid units, experts warn a rise in deaths could soon follow.
In Texas County Memorial Hospital in Houston, Missouri, hospital leaders say half the number of Covid-19 deaths they've seen since the start of this year -- eight in total -- occurred over the past week.
A little more than 23% of the county's population has received at least one Covid-19 vaccine dose, according to hospital spokesperson Helania Wulff. And the county, now labeled as "very high" risk, saw its positivity test rate jump from 9.5% last week to more than 30% this week, Wulff said.
Lauren Toman, the hospital's director of respiratory care, said that while during previous surges patients tended to be older and have preexisting conditions, patients now are younger and healthier -- but are coming in sicker and getting worse more quickly.
"They rapidly decline, very fast, and then even after intubation we'll see them rapidly decline and unfortunately we are seeing people passing quicker than before," Toman told CNN.
All the patients she has worked with in recent weeks have been unvaccinated, Toman said.
An hour and a half away, Erik Frederick, the chief administrative officer at Mercy Hospital Springfield, told CNN they're seeing patient numbers accelerate "at a pace that's almost unbelievable."
The hospital went from 26 Covid-19 patients June 1 to more than 130 on Saturday -- higher than ever before, including their winter surge.
"Last year, it took us from September 1 to our peak, on December 28, to go from 24 to 113 (patients)," Frederick said. "We eclipsed that this year in 39 days."
There's a "direct line," he said, between the low vaccination rates the community has seen to the rise in cases and hospitalizations. Roughly 40% of residents in Missouri are fully vaccinated, according to state data, and some of the counties the hospital serves still haven't hit the 20% mark, Frederick said.
Patients there have also trended younger. Roughly 91% of patients in the intensive care unit are on ventilators, and these include young patients, in their 20s, 30s and 40s, he said.
While Missouri is quickly emerging as a new Covid hot spot, health leaders in other parts of the United States are reporting similar patterns. Dr. Jeffrey Chapman, the chief medical officer at Wyoming's Cheyenne Regional Medical Center, say the hospital's increase in Covid-19 patients includes a younger demographic that is overwhelmingly unvaccinated and "deteriorating" faster."
In Mississippi, where less than 34% of the population is fully vaccinated, the state's top health officer warned the Delta variant surge has led to seven children being treated for Covid in state ICUs -- including two on ventilators.
Virtually of all the cases in the state involved the Delta variant, Mississippi State Health Officer Dr. Thomas Dobbs said on Twitter, adding the "vast majority" of cases, hospitalizations and deaths were among unvaccinated Americans.
In Kansas, the University of Kansas Health System recorded several days with just two or three Covid-19 patients back in late May. Now, it's treating more than three dozen patients, according to Dr. Steven Stites, the chief medical officer.
More than 80% of new patients are unvaccinated, though others have been vaccinated, he said.
"When you dig into those patients, what you see is that they're all severely chronically ill patients," he said. "Vaccination is not an entirely 'Get out of Jail Free' card. It makes things a lot better, but it doesn't make it go away."
Experts say that while vaccines are very effective, they're not perfect -- and Americans who have been fully vaccinated should still consider the levels of transmission and the type of environment they'll be in when deciding whether to wear a mask. And the Centers for Disease Control and Prevention has said immunocompromised people or those who are chronically ill should consult with their physicians before shedding their masks.
"I'm a little nervous about the open society and people thinking everything is normal when it's not and especially when you're more chronically ill you still need to keep your mask on," Stites said.
In Mississippi, Dobbs said on Twitter that 7% of the state's Covid-19 deaths were in vaccinated people, calling that number "worrisome."
"We are allowing too much circulating Delta to reach our most vulnerable," he wrote.
Hospital leaders say the latest surge in patients comes as staff are already exhausted and traumatized from a battle with the virus that has now lasted more than a year.
In Texas County, Toman said she worries they're going to see a continued surge, fueled by July 4 festivities and the ongoing summer gatherings, that will "overwhelm every hospital in Missouri."
"What I fear is, can we survive this again? Can we stay positive, can we all keep our morale up?" she said. "We're trying hard to be there for each other but we're tired and we're scared."
She said that while she understands the hesitations some community members may have about the vaccine, getting the state's vaccination numbers higher will be "the only way we're going to stop this monster."
Segarra, in Miami, said he agrees.
"We haven't made the progress that we wish in terms of treating Covid, we don't have that magic bullet in terms of treating Covid, but boy, we've come pretty close to having that magic bullet to avoid it -- and to see it not being used is very frustrating," he said.
The country fell short of meeting President Joe Biden's goal of at least partially vaccinating 70% of American adults by Independence Day earlier this month. More than a week later, about 67.7% of US adults have had at least one Covid-19 vaccine dose, and nearly 59% are fully vaccinated, according to CDC data.
"We're dealing with a public health situation that's beyond our personal needs, so we need to be a little bit more selfless as opposed to selfish," Segarra said.
"So if you're on the fence on whether you get vaccinated or not ... think of the good that you'll bring to your society, that you'll bring to your community, because not only does the vaccine prevent you from getting sick, it prevents you from transmitting it to others."