By Madeline Holcombe, Holly Yan and Theresa Waldrop, CNN

The Delta variant, a more transmissible and possibly more dangerous strain of coronavirus, now makes up more than half of all new Covid-19 infections in the US, according to estimates from the US Centers for Disease Control and Prevention (CDC).

Delta accounts for 51.7% of new infections in the US, according to the CDC. The B.1.1.7, or Alpha variant, which has dominated for months, now accounts for 28.7% of cases, the CDC said.

"If ever there was a reason to get vaccinated, this is it," Dr. Anthony Fauci told CNN's Anderson Cooper on Tuesday.

The variant poses a "significant threat," to unvaccinated people Fauci, director

of the National Institute of Allergy and Infectious Diseases, said.

The variant is not only more transmissible, it can also cause more severe disease, Fauci said.

And people in areas where the vaccination rate is low are especially at risk, health officials say.

"We're already starting to see places with low vaccination rates starting to have relatively big spikes from the Delta variant. We've seen this in Arkansas, Missouri, Wyoming ... those are the places where we're going to see more hospitalizations and deaths as well, unfortunately," said Dr. Ashish Jha, dean of the Brown University School of Public Health.

"Any time you have large outbreaks, it does become a breeding ground for potentially more variants," Jha said.

The US is already grappling with variants that are more contagious than the original strain of novel coronavirus. They include the "stickier" Alpha (B.1.1.7) variant, which is currently the dominant strain in the US, and the even more contagious Delta (B.1.617.2) variant, which is on track to become the dominant strain in the US, the director of the US Centers for Disease Control and Prevention said.

"The more unvaccinated people there are, the more opportunities for the virus to multiply," said Dr. William Schaffner, a professor in the Division of Infectious Diseases at Vanderbilt University Medical Center.

"When it does, it mutates, and it could throw off a variant mutation that is even more serious down the road."

Parts of the South, Southwest and Midwest are starting to see spikes in cases, and many of those states -- such as Alabama, Arkansas, Louisiana and Mississippi -- are among those with the lowest rates of vaccination, according to the CDC.

States with below-average vaccination rates have almost triple the rate of new Covid-19 cases compared to states with above-average vaccination rates, according to recent data from Johns Hopkins University.

And since vaccines are highly effective but not perfect, some health experts say they will wear masks in certain places despite being fully vaccinated.

"If you're in a low-infection, high-vaccination area, you don't need to be wearing a mask indoors if you're fully vaccinated," Jha said.

But "if I were in southwest Missouri right now, I'm fully vaccinated, but I would be wearing a mask indoors."

Israeli analysis vaccine may not be as effective against Delta as thought

While many health experts have said available vaccines offer protection from variants now circulating, an analysis in Israel suggests the Pfizer/BioNTech vaccine -- the one that Israel chose to inoculate its citizens with -- may not be as effective as was previously found there, possibly because of the Delta variant.

The Israeli government said in a brief statement on Monday that as of June 6, the vaccine provided 64% protection against infection. In May -- when the Alpha variant dominated in Israel and the Delta strain had not yet spread widely -- it found that the shot was 95.3% effective against all infections.

The government added that the vaccine was now 93% effective in preventing severe disease and hospitalizations, compared to 97% reported in the medical journal The Lancet in May.

Israel didn't release the data used in its analysis.

Israel is one of the most vaccinated countries in the world, with more than 60% of the population fully inoculated and two-thirds having received at least one dose.

A team at Hebrew University said in a separate statement that it was too soon to tell how much the Delta variant was affecting vaccine efficacy.

Dr. Ashish Jha, dean of Brown University's School of Public Health, was also cautious about drawing conclusions.

"Best data still suggest mRNA vaccines offer high degree of protection against infection and superb protection against severe illness. Lets await more data but as of now ... If you're vaccinated, I wouldn't worry," he tweeted.

Unvaccinated young adults help fuel the spread

If there is another Covid-19 surge, unvaccinated young adults could be a big part of the problem, said Dr. Megan Ranney, associate professor of emergency medicine at Brown University.

"We've already seen that the highest number of infections over the past few months have been in those younger adults," said Ranney, who's also an emergency physician and director of the Brown-Lifespan Center for Digital Health.

"These are the people that thought they were invincible."

West Virginia Gov. Jim Justice said he's encouraging parents and grandparents of those ages 12-35 to push them to get vaccinated.

"The young people out there are the ones that are dragging their feet. The sun is shining, they're out of school, all is good in the world and everything, yet they don't really realize that they could be the transmitters that could be passing this on to someone that's going to die," he said during a Tuesday briefing.

As concerns of the delta variant increase, Justice said, "if you're not vaccinated, you're part of the problem rather than part of the solution, that's all there is to it."

As of Tuesday, only 47.5% of Americans have been fully vaccinated, according to CDC data.

More than 11,000 new Covid-19 cases have been reported every day over the past week, according to data from Johns Hopkins University. About 200 Americans are dying from Covid-19 every day.

Much of the suffering is unnecessary and preventable with vaccines, emergency physician Dr. Leana Wen said.

"We have the ability to stop Covid in its tracks," she said.

On the other hand, all 50 states have fully vaccinated more than 60% of seniors,

CDC data shows, from about 62% in Utah to 94% in Vermont.

Missouri hospital transfers Covid-19 patients to other facilities

In Missouri, one hospital has been under such strain that Covid-19 patients are being transferred to hospitals in different health systems.

CoxHealth had 94 patients hospitalized for Covid-19 as of Monday, said Kaitlyn McConnell, a CoxHealth public relations director.

At Cox South, a hospital in Springfield, 12 Covid-19 patients were transferred to other facilities in the region between Friday and Monday morning.

"While we have ample supplies and space to care for additional patients, staffing remains a challenge," McConnell said. "Divert is not a permanent status; we only use it temporarily, it is tied to capacity at a particular moment in time and based on what is best for staff and patients."

The health system had about 170 Covid-19 patients last winter and 280 traveling staff members to help with the response, McConnell said.

Now the system is struggling to meet the current demand -- without the extra help.

"We are aggressively working to have more travelers return -- and hire more staff, as we have throughout the pandemic -- but it is currently a challenge due to times of year that travelers tend to be available," said McConnell.

In recent weeks, CoxHealth has transferred patients to facilities in Kansas City, St. Louis and out of state to Arkansas.

But now Arkansas is starting to feel the effects of low vaccination rates -- and if worsening trends continue, it could spell another surge, said Dr. Cam Patterson, chancellor of University of Arkansas for Medical Sciences.

"Arkansas, which has a very low vaccination rate, has seen over a hundred percent rise in daily cases over the last two weeks," CNN medical analyst Dr. Jonathan Reiner said.

As of Sunday, Arkansas' average rate or new Covid-19 cases was about five times higher than the national average, according to Johns Hopkins data. It's also one of the least vaccinated states in the country.

What could this mean for a return to school?

Children who are unvaccinated are still at high risk for infection, Wen said.

She said she disagreed with governors such as Texas Gov. Greg Abbott, who has banned public schools from requiring face masks.

"These governors that have said that schools no longer need masks indoors, that's a big mistake," Wen said.

Considering mental health concerns and educational outcomes, Wen said she thinks it's best to have kids back to school -- full time and in-person this fall.

But even mild cases of the Covid-19 can have long-term impacts for children, so safety precautions shouls be taken, she said.

Even vaccinated parents of unvaccinated children may want to wear masks indoors, said Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases.

Because vaccines are highly effective but not perfect, he said it's possible adults could get breakthrough infections and pass them to their children.

"We haven't formally proven yet how much diminution there is in the likelihood of transmitting it to someone else -- including children -- and that's one of the reasons why you've got to be careful when you're dealing with something like the Delta variant," he said.

The best protection against variants, health experts say, is to get vaccinated as soon as possible.