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Covid-19 infections of vaccinated people are expected. But the unvaccinated are 'the big highway of transmission,' expert says

(CNN) Tens of thousands of vaccinated people may catch Covid-19, but the majority will not fall severely ill -- a testament to the efficacy of inoculations even against the Delta variant that has been fueling case surges across the United States, a top health official said.

The severity of the illness -- not the number of people who contract the virus -- is a crucial concept for people to understand at this point in the pandemic, said Dr. Rochelle Walensky, who heads the US Centers for Disease Control and Prevention.

"I think we all have to recognize that with 164 million people who are vaccinated, we should expect tens of thousands, perhaps, of breakthrough infections," Walensky told CNN's Wolf Blitzer on Thursday.

"Those breakthrough infections have mild illness. They are staying out of the hospital. They are not dying, and I think that that's the most important thing to understand," Walensky added.

Breakthrough cases occur when the virus infects fully vaccinated people.

As the Delta variant of the coronavirus rips through the US, it is especially devastating regions with low vaccination rates as experts and government officials nationwide urge people to get their shots before a dire situation gets even worse.

Already, states are breaking grim records: Louisiana reported an all-time high of 2,421 people hospitalized with Covid-19 statewide, breaking a record that was set just a day earlier. Florida on Friday reported 134,506 new Covid-19 cases over the last week, more than any other 7-day period during the pandemic.

Roughly 58.4% of the US population has received at least one Covid-19 vaccine dose, according to CDC data. About 50% of the population is fully vaccinated.

Full vaccination is necessary for optimal protection against Covid-19 -- especially the highly contagious Delta variant, which accounted for an estimated 93.4% of all cases in the US in the two weeks ending July 31.

Dr. William Schaffner, a professor at Vanderbilt University Medical Center, echoed Walensky's take on how vaccination lowers the intensity of Covid-19 symptoms when breakthrough cases happen.

"This is largely a problem when it comes to severe disease, the disease that requires hospitalization, it's among the unvaccinated. It really is pretty unusual to have a vaccinated person be hospitalized. Most of those people tend to be older and very, very frail. They never were able to respond to the vaccine, and we have the occasional immunocompromised person whose immune system also couldn't respond optimally to the vaccine," Schaffner told CNN on Thursday.

"So the unvaccinated continue to be the big highway of transmission. The vaccinated, they're little side streets. Let's not get preoccupied with that. We need to get more people vaccinated."

The good news is that vaccinations have picked up recently.

An average of 694,138 doses have been administered each day over the past seven days, up from a spring/summer low average of 506,770 daily reached on July 11, according to CDC data published Friday.

And an average of 465,039 people initiated vaccination each day over the past seven days, the highest average daily pace in nearly seven weeks, since June 19, per the CDC. This is also an 11% increase over last week's pace -- about 47,000 additional doses each day.

If more people do get their shots, case surges can be controlled in a matter of weeks, Walensky said Thursday.

"However, our models show that if we don't (vaccinate people), we could be up to several hundred thousand cases a day, similar to our surge in early January," she said.

Back-to-school season is looking grim amid Delta surge

But even as vaccinations rise, some children who have gone back to school have tested positive for Covid-19.

Georgia's largest school district, Gwinnett County Public Schools, confirmed 253 cases on Friday, three days into the new school year. However, a spokesperson said many of the cases "are from communal spread," because the people reporting cases "haven't been in our buildings yet to have contracted" the virus.

"Now that we're back in school, we know we're going to get cases," said Bernard Watson, director of community and media relations for the school system.

Arizona's second-largest school district is dealing with 103 active cases of the virus, according to the district's online Covid-19 dashboard. Since the school year began on July 21 in Chandler Unified School District, there has been a total of more than 140 cases.

"We will continue to monitor confirmed cases and make adjustments to our mitigation plan as necessary," Chandler Unified School District spokesperson Terry Locke told CNN Thursday.

Only students who exhibit symptoms are required to quarantine in Chandler, and it is optional for all students with known exposure to Covid-19, regardless of their vaccination status.

Meanwhile, Indianapolis Public Schools officials on Tuesday notified the parents of 61 fourth-grade students that their child must "quarantine for 14 days after coming in close contact with a school staff member who tested positive for Covid-19," district spokesperson Alpha Garrett said in a statement.

The students will continue learning remotely during their quarantine, Garrett added, and said the district requires students to wear masks regardless of vaccination status.

Masking students in schools is exactly what Walensky is urging districts to do as children under 12 years old remain ineligible for a vaccine.

Speaking Friday at a town hall event in Boston, the CDC director said schools and summer camps where Covid-19 has spread are the ones that haven't followed mitigation measures like mask-wearing.

"We follow every jurisdiction, and we look for outbreaks that are happening in camps and schools," Walensky said. "The places that are having a problem, the places that are having disease that is transmitted in the schools, are the places that are not taking prevention strategies. ...

"If you're masked and you're doing all of the prevention mitigation strategies, it won't be transmitted in the school. It will be contained," Walensky said.

Some jurisdictions have mandated masks in schools, including New Jersey. Gov. Phil Murphy announced the mandate for the 2021-2022 school year on Friday, citing the spread of the Delta variant and the fact that many children are too young to be eligible for vaccination.

The University of Virginia also updated its Covid-19 policy Friday to include a temporary mask mandate for students, faculty, staff and visitors. The university's policy statement says officials will monitor case counts and hospitalizations over the next few weeks "with a goal of modifying or lifting this masking policy for fully vaccinated people by September 6th."

Vaccination more than halves chance of Covid-19 reinfection, study suggests

For adults previously infected with Covid-19, vaccines do a better job at protecting them from reinfection than natural immunity on its own, a new study shows.

The study, published Friday by the CDC, suggests that people who got Covid-19 in 2020 and didn't get a Covid-19 vaccine were more than twice as likely to be reinfected in May or June 2021, compared with people who also had Covid-19 but were later fully vaccinated.

"If you have had Covid-19 before, please still get vaccinated," Walensky said Friday. "Getting the vaccine is the best way to protect yourself and others around you, especially as the more contagious Delta variant spreads around the country."

The CDC has long recommended that people who have been previously infected also get vaccinated. The agency noted that reinfection risk "is low in the months after initial infection but may increase with time due to waning immunity." There is no minimum time to wait between recovering from Covid-19 and getting vaccinated.

Scientists are still unraveling how long and how robust natural immunity to Covid-19 is, the authors noted. A number of earlier studies have shown evidence of lasting immunity in some people with previous Covid-19 -- but scientific consensus has maintained that vaccines do an even better job.

With the Delta variant driving a surge in cases, the study also noted that "the emergence of new variants might affect the duration of infection-acquired immunity, and laboratory studies have shown that" those antibodies from natural infection might be weaker against certain variants of concern.

Meanwhile, studies of vaccinated people have continued to show high levels of protection, particularly when it comes to severe outcomes including hospitalization and death.

The new study of hundreds of people in Kentucky did not assess severity of reinfections, and it noted that vaccinated people may be less likely to get tested, thus potentially skewing the numbers.

CNN's Dakin Andone, Lauren Mascarenhas, Michael Nedelman, Melissa Alonso, Elizabeth Stuart, Deidre McPhillips, Rebekah Riess, Cheri Mossburg, Raja Razek and Andy Rose contributed to this report.