(CNN) -- The US has now tallied more than 40 million Covid-19 cases across the pandemic, with more than 4 million of them reported in the last four weeks alone, according to data from Johns Hopkins University.
The count comes with a caveat: 40 million represents just officially reported positive test results, and many experts believe the actual number of infections is much larger.
Covid-19 cases have nevertheless generally been on the rise in much of the country since early summer. And the country's seven-day average of new cases Monday (137,270 daily) was more than four times higher than Labor Day of last year (39,355 daily), according to Johns Hopkins data.
The jump in cases has translated into overcrowded hospitals and a rise in infections among children -- of particular concern as many students return to their classrooms. And experts fear that a holiday weekend could make matters worse.
Last week, the US Centers for Disease Control and Prevention Director Dr. Rochelle Walensky advised unvaccinated Americans not to travel for the holiday and reminded vaccinated people that the high rate of virus transmission meant that it could be risky for them to travel as well.
The risk played out last year, when cases surged in 31 states and the positivity rate went up in 25 of them only two weeks after the Labor Day holiday.
One big difference between this year and last is the more transmissible Delta variant. Another is that Americans over 12 years old can get highly effective Covid-19 vaccines, which experts say is the best defense against the virus.
But only 53% of the total US population is fully vaccinated, and just 62% of eligible Americans are, leaving tens of millions very vulnerable.
"Here's the important thing: Everyone that I'm hospitalizing is not vaccinated. We are, by and large across the country, not needing to hospitalize people that have gotten both doses of the vaccine," Dr. Megan Ranney, professor of emergency medicine and associate dean at Brown University's school of public health, said. "This is a disease of the unvaccinated right now."
Alabama, Wyoming, Idaho, Mississippi and West Virginia all have less than 40% of their populations vaccinated, according to the CDC. Two of those states, Alabama and Mississippi, are also contending with their more than 90% ICU utilization.
Georgia, Arkansas, Texas and Florida join those states in less than 10% ICU capacity, according to data from the US Department of Health and Human Services.
"The takeaway for everyone is get your shots and certainly wear a mask for that added layer of protection if you're in public indoor spaces right now," Ranney said.
Many students head back to school without school nurses
Experts have encouraged adults to get vaccinated in order to protect young children returning to school.
"The way you protect children who, because of their age, cannot get vaccinated yet is to surround the children -- be it friends, family, school teachers, personnel in the school -- surround the children with vaccinated people," director of the National Institute of Allergy and Infectious Diseases Dr. Anthony Fauci told CNN on Sunday.
The situation is made even more concerning as an estimated quarter of schools to which students are returning don't have a school nurse at all. The CDC recommends that schools have one full-time nurse for every 750 students.
Based on the latest data from a national school nurse workforce study, published in the Journal of School Nursing in 2018, approximately 39% of schools employ full-time nurses and about 35% employ part-time school nurses, while 25% do not employ school nurses. Experts emphasize that the nation has had a shortage of school nurses for years, but the pandemic now sheds light on just how dire the shortage has become.
Schools in the rural regions appear to be "significantly more likely" than schools in urban areas to report having no nurse at all, according to the study published in the Journal of School Nursing in 2018. In that study, 23.5% of rural schools report having no nurse compared with 10.3% of urban schools.
"Funding is a key issue. There is an inconsistent mishmash of state and local funding that puts small rural school districts with inadequate tax bases at a disadvantage," Laura Searcy, a pediatric nurse practitioner who is a past president of the National Association of Pediatric Nurse Practitioners and a fellow of the American Association of Nurse Practitioners, told CNN.
"And those areas also are likely to have a shortage of primary care pediatric health care providers as well."
West Virginia governor frustrated at booster pace
Meanwhile, the US Food and Drug Administration is set to meet September 17 to discuss Covid-19 booster shots. Last month, the White House said people who got the two mRNA vaccines -- the two-shot vaccines made by Pfizer/BioNTech and Moderna -- may receive boosters starting September 20.
In West Virginia, Gov. Jim Justice said Monday that he is eager for the boosters to begin.
"If we could just get the go-ahead from our government to absolutely start administering these booster shots, we would be all over that and we'll be doing that immediately," Justice said. "We're ready to go."
Justice accused the federal government of holding up West Virginia from leading in its Covid response.
"We've got people that are well beyond six months that are 60 and older that need the booster shot. And we can't give it to them because we're being held up by, you know, the nation and on the federal level right now," Justice said.
On Sunday, Fauci predicted that Moderna may be rolling out its booster dose later than Pfizer.
With the potential timing discrepancy, researchers are looking into whether different types of Covid-19 vaccines can be mixed and matched.
"We're lining up Pfizer against Pfizer, Pfizer for Moderna, and vice versa," Fauci told CBS' "Face the Nation" on Sunday. "Hopefully, within a reasonable period of time, measured in a couple of weeks, we will have that data."
At this point, it is OK for healthy vaccinated Americans to wait and see what the CDC and FDA have to say before going and getting a booster dose, CNN medical analyst Dr. Leana Wen said. The most important thing at this time is to get the unvaccinated their first two doses, she added.
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