(CNN) Fully vaccinated people who get a Covid-19 breakthrough infection can transmit the virus, US Centers for Disease Control and Prevention Director Dr. Rochelle Walensky said Thursday.
"Our vaccines are working exceptionally well," Walensky told CNN's Wolf Blitzer. "They continue to work well for Delta, with regard to severe illness and death -- they prevent it. But what they can't do anymore is prevent transmission."
That's why the CDC changed its guidance last week and is now recommending even vaccinated people wear masks indoors again, Walensky said.
Last week, the agency released a study that showed the Delta variant produced similar amounts of virus in vaccinated and unvaccinated people if they got infected -- data that suggests vaccinated people who get a breakthrough infection could have a similar tendency to spread the virus as the unvaccinated.
"If you're going home to somebody who has not been vaccinated, to somebody who can't get vaccinated, somebody who might be immunosuppressed or a little bit frail, somebody who has comorbidities that put them at high risk, I would suggest you wear a mask in public indoor settings," Walensky said.
The dangerous Delta variant has fueled the country's latest surge of Covid-19 cases and if more Americans don't get vaccinated and mask up, the country could soon be seeing "several hundred thousand cases a day," similar to the winter surge, Walensky said.
And while states across the South -- including Florida and Louisiana -- have seen exponential rises in cases, Walensky said, they have not reached their peak just yet.
Getting more people vaccinated won't just help crush this surge, experts say. It will help prevent other -- potentially even more aggressive -- variants from arising in the future.
"The next variant is just around the corner, if we do not all get vaccinated," Adm. Brett Giroir, the former coronavirus testing czar under President Donald Trump, told CNN's Chris Cuomo.
"I just beg the American people to understand that to defeat this virus, we have to get everybody's level of immunity up, and that's just the way it is," he added.
Roughly 58.2% of the US population has received at least one Covid-19 vaccine dose, CDC data shows, and about 49.9% is fully vaccinated.
There was some encouraging news Thursday, as White House data director Dr. Cyrus Shahpar tweeted there was the most number of doses reported administered in a single day in more than a month. He said that more than 864,000 doses had been reported administered over the previous day's total, including about 585,000 people who got their first shot.
In the coming weeks, surges will likely reach all across the US, not just areas with low vaccination rates, former CDC Director Dr. Tom Frieden said Wednesday. The outbreaks, however, will not be as explosive in areas with higher vaccination coverage, Frieden added.
As cases increase, hospitalizations and deaths will likely rise as well, according to ensemble forecasts published Wednesday by the CDC. The forecast predicts a total of 624,000 to 642,000 deaths will be reported by August 28. As of Wednesday, there have been 614,342 Covid-19 deaths in the US, according to data compiled by Johns Hopkins University.
If you're not protected against Covid-19, the virus will likely infect you, Michael Osterholm, director for the Center for Infectious Disease Research and Policy at the University of Minnesota, told CNN's Pamela Brown on Wednesday.
"This virus is highly infectious. If you decide to try to run the game clock out, don't try to do it. This virus will find you, it will infect you eventually," Osterholm, said
Fortunately, the available vaccines appear to offer a strong defense against the Delta variant, especially when it comes to severe illness and deaths, Frieden said.
"We are at war with this virus that has already killed more than 610,000 Americans. We now have the tools with vaccines and masks to stop further death and suffering and destruction," CNN medical analyst Leana Wen said Wednesday.
Meanwhile, as more questions arise over whether fully vaccinated Americans will need booster shots, a Biden administration official told CNN that internal discussions at the US Food and Drug Administration have centered around an early September timeline for laying out a strategy.
That strategy would apply for all vaccinated people. A decision for those who are immunocompromised and face greater risk from Covid-19 is expected sooner, the official said.
Earlier this week, Dr. Anthony Fauci said that people with compromised immune systems may need additional protection after receiving a Covid-19 vaccine -- and there is an effort to make vaccine booster shots available to those people "very soon."
"There are those individuals who are immune compromised -- transplant patients, patients on cancer chemotherapy, patients on immunosuppressive regimen, for example, for autoimmune diseases," Fauci said during a virtual event hosted by Virginia Gov. Ralph Northam on Tuesday. "Those individuals we know almost invariably do not have an adequate response, so the need to give them an additional boost is much more emergent than the general population."
Vaccine advisers to the CDC have met to discuss whether immunocompromised people may need additional protection from a vaccine booster but have not presented a recommendation or voted on guidance.
On Wednesday, US Surgeon General Vivek Murthy said a recommendation from the federal government on vaccine boosters will come "if and when" there is evidence that rising infections are due to decreasing vaccine immunity.
"I recognize that individual doctors and their patients may make a decision ... around ... getting an extra dose and that may be as it is but formally, we cannot make that recommendation yet until we feel that the data is clear and indicates boosters are required," Murthy said.
Low Covid-19 vaccination rates in the US place children -- many of whom cannot get vaccinated -- at risk, Dr. Paul Offit, a vaccine adviser to the US Food and Drug Administration, said Wednesday.
"I think we've let our children down," Offit told CNN's Wolf Blitzer.
There is not currently a vaccine authorized for children under the age of 12 in the US, so young children rely on the vaccination of those around them to protect them, Offit explained. And many children who are 12 and older have not yet gotten the vaccine, he added.
Offit noted that many children are about to go back to school, during a time of year when the virus is transmitted more easily and the Delta variant is circulating.
"We need to get vaccination rates up, so that these children can be protected," Offit said.
Highlighting that point, the president of Our Lady of the Lake Children's Hospital in Baton Rouge, Louisiana, told CNN his staff is seeing babies in the neonatal intensive care unit sick with Covid-19.
Dr. Trey Dunbar told CNN children are being victimized by a pandemic that has a simple solution: adult vaccination.
"Covid is a preventable disease," he said. "It's hard for us as pediatricians to see kids affected by a preventable disease. Children aren't like adults. They don't have the choice to get vaccinated.
"So, yes, it makes a big difference when adults make decisions for kids and adults make decisions that could maybe prevent diseases that we see in children," Dunbar said.
Concerns over America's youngest come as schools across the country gear up for reopening -- and as district leaders try to navigate the safest way to return to class. Having children wear masks remains a point of contention in communities throughout the US -- with some states requiring masks in schools while others have prohibited mask mandates.
On Thursday, Metro Nashville Public Schools sent a letter to families saying students, staff and visitors will be required to wear masks in school buildings when the school year kicks off next week. Masks will also be required on school buses, but not outdoors, the district said, adding the rule will stay in place "until further notice."
Even after the development and release of Covid-19 vaccines that so many health care professionals had hoped for, rising cases have led to overwhelmed hospitals.
Arkansas health officials reported a "record low number of available ICU beds," Wednesday. According to Arkansas Department of Health public information officer Danyelle McNeill, the state had just 25 ICU beds available.
According to Arkansas' Covid-19 dashboard, there was a total of 1,232 Covid-19 positive admissions. The total number of Covid-19 positive admissions in the ICU is 466, and the total Covid-19 positive admissions on ventilators is 260.
As of Wednesday morning, Mississippi had only six open ICU beds available in the entire state, Dr. Jonathan Wilson, chief administrative officer and Covid-19 incident manager, said during a Covid-19 briefing with University of Mississippi Medical Center leaders.
"A very simple number, six. That's how many open ICU beds we had in the state of Mississippi, this morning. Six. So, the situation is getting dire, not just here at the Medical Center, here in Metro Jackson, but the entire state. Our neighboring states are having similar situations," Wilson said.
"We're doing the best we can, from a state standpoint, to try to distribute patients to ensure healthcare as we know it is delivered. But we aren't on the cusp of this, we know that we aren't at the crest of this wave and it's bad, but it's probably going to get a little worse," Wilson added.
Clarification: This story has been updated to specify that Dr. Walensky was referencing fully vaccinated people who get a breakthrough infection when saying that vaccines no longer prevent transmission of Covid-19.