(CNN) -- With COVID-19 cases rising in all 50 states, health officials say it's clear that unvaccinated people are both driving the increase in cases and are most at risk.
"This is becoming a pandemic of the unvaccinated," US Centers for Disease Control and Prevention Dr. Rochelle Walensky said during Friday's White House COVID-19 briefing.
"We are seeing outbreaks of cases in parts of the country that have low vaccination coverage because unvaccinated people are at risk," Walensky said. Meantime, "communities that are fully vaccinated are generally faring well."
According to White House coronavirus coordinator Jeff Zients, just four states made up 40% of COVID-19 cases in the past week, "with one in five cases occurring in Florida alone."
But cases are rising in all 50 states and Washington, DC, with the average of new cases at least 10% higher than a week ago -- and 38 states are seeing at least a 50% increase, according to a CNN analysis of data from Johns Hopkins University.
The US recorded an average of 26,448 new cases per day over the last week -- up 67% from the week before -- and case rates are highest in states with lower vaccination rates: Among those states that have fully vaccinated less than half its residents, the average COVID-19 case rate was 11 new cases per 100,000 people last week, compared to 4 per 100,000 among states that have fully vaccinated more than half its residents.
Many experts have attributed the rise to slowing vaccination rates with just 48.4% of the US population fully vaccinated, per CDC data.
"Our biggest concern is that we are going to continue to see preventable cases, hospitalizations and, sadly, deaths among the unvaccinated," Walensky said.
The danger is fueled by the growing prevalence of the Delta variant, first identified in India. Pointing to an "extraordinary surge" of the variant worldwide, Dr. Anthony Fauci said the Delta variant now has more than 50% dominance in the US. In some areas, it's greater than 70%, he said, calling this "sobering news."
"The bottom line is we are dealing with a formidable opponent in the Delta variant," Fauci said, adding people who are not vaccinated face "extreme vulnerability."
In Arkansas, where only 35.1% of the population is fully vaccinated, the Delta variant has had a big impact, Chancellor of the University of Arkansas for Medical Sciences Cam Patterson said, adding hospitals are "full right now and cases are doubling every 10 days." And emergency response services in the state say there are receiving a record number of calls due to the rise in the virus, according to CNN affiliate KATV.
"The good news is that if you are fully vaccinated, you are protected against severe COVID, hospitalization and death," Walensky said Friday, "and are even protected against the known variants, including the Delta variant."
"If you are not vaccinated," she added, "you remain at risk."
Experts underscore importance of being fully vaccinated
In response to climbing case numbers, some jurisdictions are opting to reinstate mask guidelines.
In California, Los Angeles County -- the nation's largest county with a population of 10 million people -- has responded to a surge in cases and hospitalizations by reinstating a mask mandate beginning Saturday. Health officials in the San Francisco Bay Area are similarly recommending people wear face coverings in indoor public places, regardless of vaccination status.
The Southern Nevada Health District, which serves Las Vegas, is also recommending masks for both vaccinated and unvaccinated people, saying masks have been proven effective in preventing the spread of the coronavirus. But vaccinations remain the "most important and effective step people can take to protect themselves and others from Coivd-19," the health district said.
Echoing Walensky's comments, Dr. Peter Hotez, a vaccinologist and dean of the National School of Tropical Medicine at Baylor College of Medicine, said this was also a "pandemic of the partially vaccinated."
"If these trends continue ... anyone who is unvaccinated -- or possibly even just gotten a single dose of the Pfizer or Moderna vaccine -- there's a good likelihood they're going to get infected," Hotez said.
Health officials recommend that people who get their first dose of a vaccine get their second dose three or four weeks later, depending on whether they received the vaccine by Pfizer/BioNTech or Moderna. Johnson & Johnson's vaccine requires just a single dose.
But falling behind schedule shouldn't stop people from getting their second dose, Walensky said Friday.
"If you are beyond that window, I want to reiterate: There is no bad time to get your second shot," Walensky said Friday.
Both vaccines exceed 90% effectiveness against severe disease, hospitalization and deaths in real-world studies, she said. But those who are only partially vaccinated still face a risk of illness.
"Do it for yourself, your family and for your community," Walensky said. "And please, do it for your young children who right now can't get vaccinated themselves."
Vaccine misinformation costing lives
Meanwhile, key reasons for the hesitancy around COVID-19 vaccines are mistrust and misinformation, according to a CNN analysis of data from the US Census Bureau's Household Pulse Survey.
Nearly half of people who said they will "definitely" or "probably" not get a COVID-19 vaccine cited mistrust in the vaccines as a reason for not getting vaccinated, according to the latest data, published Wednesday and based on survey responses from June 23 to July 5. That's an increase from about a month ago, when 46% of people who said they did not plan to be vaccinated gave the same reason.
"Millions of people don't have access to accurate information right now, because on social media platforms and other tech platforms we're seeing the rampant spread of misinformation, and it's costing people their lives," US Surgeon General Dr. Vivek Murthy told CNN's Jake Tapper.
Much of that information frequently comes out of people with good intent, he added, saying that they think they are spreading helpful information, but that often misinformation spreads more quickly than accurate information.
US Health and Human Services Secretary Xavier Becerra agreed, telling CNN's Poppy Harlow, "People are being told things that aren't true, and they're becoming more hesitant."
"But fortunately, there are people who are seeing the facts," he said. "They're seeing a loved one, unfortunately, get hospitalized, maybe die. And they're changing their minds."
One of the best ways to combat the misinformation, Murthy said, is to have conversations with your friends and family.
"It's about peers talking to peers," Murthy said during a Stanford University panel event on Thursday. "Remember, all of these conversations first start with listening... so try to understand where somebody is coming from, why they may be worried. It may not always be what you think."
Colleges and universities requiring vaccinations
Some businesses and hospitals have already required their employees to be vaccinated, and now some universities are implementing requirements as well.
Rhode Island has become the first state where all public and private colleges and universities require their students to be fully vaccinated before returning to campus this fall, Governor Dan McKee announced this week.
Dr. Nicole Alexander-Scott, Rhode Island's director of health, said in a news release vaccinations are "key" to having a successful academic year, and the Delta variant was "circulating in parts of the country where many of our students live."
The University of California, the nation's largest public university system, plans to mandate all students, faculty and staff be fully vaccinated before returning to campuses in the fall. Those who are not exempt from receiving the vaccine will be barred from in-person classes, activities and housing, UC officials announced Thursday.
The Association of American Medical Colleges Friday also urged its member institutions to require vaccinations for employees to protect patients and health care personnel. President Dr. David Sorkin acknowledged the "sensitive nature" of the recommendation, saying AAMC understood such requirements would be subject to state laws.
Such mandates for employees could become easier for private companies as the vaccine approval process move further along. Each vaccine available in the US has been authorized for emergency use. But the companies are still working toward full US Food and Drug Administration approval.
Pfizer and BioNTech said Friday their application for full approval of their vaccine was granted priority review by the FDA, and an FDA official told CNN a decision on full approval is likely to come within two months.
Full approval will "clear up any legal questions that private employers may have," former US Health and Human Services Secretary Kathleen Sebelius said Tuesday. Employers, schools and universities, she said, should "get more serious" about telling people that choosing to not get vaccinated could mean losing access to places that could put others at risk.
"I think that it's time to say to those folks, 'It's fine if you don't choose to get vaccinated, (but) you may not come to work.'"