(CNN) Less than half of the US population is fully vaccinated against Covid-19 -- and with cases on the rise, experts are urging a return to precautions reminiscent of the earlier days of the pandemic.
"What I would say bluntly is: If you are not vaccinated right now in the United States, you should not go into a bar, you should probably not eat at a restaurant. You are at great risk of becoming infected," CNN medical analyst Dr. Jonathan Reiner, professor of medicine and surgery at George Washington University, told CNN's Fredricka Whitfield.
In 48 states, the rate of new Covid-19 cases in the past week jumped by at least 10% compared to the previous week, according to data from Johns Hopkins University. In 34 of those states, the rate of new cases increased by more than 50%.
And with the highly transmissible Delta variant now the dominant strain, the US could see 200,000 new Covid-19 cases a day within the next six weeks, Dr. Tom Frieden, former director of the US Centers for Disease Control and Prevention, said Monday.
The last time there were more then 200,000 new US cases in one day was in January, according to Johns Hopkins University data.
And hospitals are filling up with Covid-19 patients again, except now, patients are younger than before, said doctors in Alabama, Mississippi, Florida and Missouri.
Despite the encouragement of many officials and experts' warnings that the best protection from the virus comes from vaccinations, only 49.1% of the US population is vaccinated, according to data from the CDC.
If a large swath of people remains resistant to vaccination, Reiner said, the US is left with two options to control the spread: shut down businesses -- which few people want to do -- or return to masks.
"The only way to get the unvaccinated to mask up is to mask everyone up," Reiner said.
While masking, like vaccination, could come from mandates by state leaders, Vanderbilt University Medical Center professor of infectious diseases Dr. William Schaffner said he anticipates more success if the push to return to precautions comes from local officials and trusted community leaders.
And for those who say vaccination is a personal choice, Schaffner said, they are half right.
"This is a little like driving on the red light. Yes, that's your decision, puts you in danger, but endangers others also," Schaffner said.
Part of the urgency to control spread and implement precautions is due to the prevalence of the Delta variant, which is believed to be more transmissible than other strains of the virus.
"At the beginning of the pandemic, the CDC said that a close contact was somebody that you're indoors with, unmasked for 15 minutes or more," Dr. Celine Gounder, who served on President Joe Biden's transition Covid-19 advisory board, said in an interview with STAT published Friday. "The equivalent of that with the Delta variant is not 15 minutes, it's one second."
That means people might need to do more than one thing to protect themselves from the more transmissible variant, said Gounder, an infectious diseases specialist at NYU's Grossman School of Medicine.
In addition to vaccination, "Some of the other layers that we should consider would be masking indoors when you're outside of your household bubble, optimizing ventilation in the home -- just opening your window works really well," she said.
People should think in terms of how much virus there is in the air, plus how long they might be in contact with that virus-laden air.
"So if you're indoors, there's not a lot of air dilution unless you're opening up windows and doing that sort of thing. When you're outdoors, it's almost infinitely diluted. And so outdoors, your risk is really low," Gounder said.
As the Delta variant spreads, rates of infections and hospitalizations are on the rise.
In California, San Diego and Los Angeles counties both reported their highest number of cases since February, and hospitalizations in LA County have more than doubled in two weeks. On Sunday, officials there said hospitalizations surpassed 700 for the first time since March.
Florida reported an average of 10,452 new cases each day over the past week -- more than triple the daily average from two weeks ago, according to data from Johns Hopkins University.
Over the past week, Florida accounted for nearly a quarter of all cases in the United States -- more than any other state.
The case rate in Florida over the past week -- about 49 new cases per 100,000 people each day -- is more than three times the US rate of about 16 new cases per 100,000 people each day. Only Arkansas and Louisiana had higher case rates over the past week.
Orange County, Florida, is now seeing 1,000 new cases a day in the region, the same amount as in the highest peak last year, according to Mayor Jerry Demings.
"We are now in crisis mode," Demings said at a news conference Monday.
A total of 870 hospitalized patients were reported Sunday in Alabama, according to the state's public health Covid-19 dashboard. Hospitalizations there have been steadily rising since early July; on July 4, there were just 213 hospitalized patients reported.
And Louisiana now has the highest increase in cases per capita in the US, state officials said Friday.
"We know that more than 80% of these are the Delta variant -- that is what's causing this surge," Gov. John Bel Edwards said.
Vaccination rates would likely go up, Reiner and Schaffner said, if employers and schools required vaccines.
Former US Surgeon General Jerome Adams said the same, but added that it likely won't happen until the vaccines gain full approval by the US Food and Drug Administration.
"If you want to get a bunch of people vaccinated really quickly, get these vaccines licensed," Adams told CBS Sunday. "And then you'll see the military make it mandatory. You'll see businesses make it mandatory."
Asked Sunday whether vaccine mandates should be on the table, White House coronavirus coordinator Jeffrey Zients told CNN that municipal governments and businesses should consider any strategy that would make it safe to enter their facilities, whether that's requiring vaccines or negative Covid-19 tests.
Those mandates are starting to happen. California Gov. Gavin Newsom announced Monday that all state employees and health care workers will be required to provide proof of vaccination status or get regular testing.
"As the state's largest employer, we are leading by example and requiring all state and health care workers to show proof of vaccination or be tested regularly, and we are encouraging local governments and businesses to do the same," Newsom said. "Vaccines are safe -- they protect our family, those who truly can't get vaccinated, our children and our economy. Vaccines are the way we end this pandemic."
And New York City Mayor Bill de Blasio announced the entire city workforce will be required to get vaccinated or be tested for Covid-19 once a week by September 13. Unvaccinated employees will be required to wear face masks indoors on the job. The city has about 340,000 employees.
Two cities reinstated face mask mandates on Monday to deal with rising infections.
In St. Louis, Mayor Tishaura Jones said anyone 5 years of age or older will be required to wear masks in indoor public spaces and on public transportation.
Jones said during a news conference that the city is still at a "dangerous point" with the virus and facing new case counts not seen since December.
Savannah, Georgia, Mayor Van Johnson said everyone regardless of vaccination status is required to wear facial coverings inside all city government facilities, as well as Savannah city schools and early childhood centers. The mandate does not apply to private businesses or institutions.
More than 50 health care groups -- including the American Medical Association, the American College of Physicians and the American Academy of Pediatrics -- issued a joint statement calling for all health care and long-term care employers to mandate employees be vaccinated against Covid-19.
"This is the logical fulfillment of the ethical commitment of all health care workers to put patients as well as residents of long-term care facilities first and take all steps necessary to ensure their health and well-being," they wrote in the joint statement.