By Christina Maxouris, CNN

(CNN) -- As US leaders work to control the spread of coronavirus, researchers across the globe are working to answer the mysteries that remain around infections.

One of those mysteries: why the experience can be so different from person to person. One expert says the answer may involve looking at previous vaccines individuals have had.

"When we looked in the setting of Covid disease, we found that people who had prior vaccinations with a variety of vaccines -- for pneumococcus, influenza, hepatitis and others -- appeared to have a lower risk of getting Covid disease," Dr. Andrew Badley, an infectious disease specialist at Mayo Clinic, told CNN's Anderson Cooper on Monday night.

It's what immunologists call immune training: how your immune system creates an effective response to fight off infections, Badley says.

"A good analogy is to think of your immune system as being a muscle," he said. "The more you exercise that muscle, the stronger it will be when you need it."

There's been no definitive evidence of any other vaccines boosting immunity against Covid-19. But some researchers have suggested it's possible.

In June, a team of researchers in the US proposed giving a booster dose of the measles, mumps and rubella (MMR) vaccine to people to see if it helps prevent the most severe effects of coronavirus infections. And last month, researchers found that countries where many people have been given the tuberculosis vaccine Bacillus Calmette-Guerin (BCG) had less mortality from coronavirus, a finding that fits with other research suggesting the vaccine can boost people's immunity in general.

But once you're infected, how much of the virus made it into your body could also have an impact on what your experience is, another expert told CNN on Monday.

Dr. Monica Gandhi, an infectious disease specialist at University of California, San Francisco, has been working with a team of researchers to understand how more people could go through their infections with minimal or no symptoms. About 40% of people infected with the virus don't have symptoms, according to an estimate last month by the US Centers for Disease Control and Prevention.

Gandhi's team found masks make a difference.

"What the mask does is really reduce the amount of virus that you get in, if you do get infected," she said. "And by reducing that ... you have a lower dose, you're able to manage it, you're able to have a calm response and you have mild symptoms or no symptoms at all."

'We have nothing to celebrate' about current case levels, physician says

So far, more than 5 million Americans have tested positive for the virus and at least 164,000 have died, according to Johns Hopkins University data.

The average number of daily new cases in the US is more than 54,000, down from more than 65,000 per day in mid- to late-July.

Average daily Covid-19 deaths, however, have hovered above 1,000 for more than two weeks. The country had been below that level for seven consecutive weeks before that.

"We have nothing to celebrate (just) because we're going to 50,000 cases per day. We have a huge amount of morbidity and mortality at our feet right now and in the weeks ahead," Dr. Rochelle Walensky, chief of infectious diseases at Massachusetts General Hospital, said Tuesday.

"Even at 18,000 cases per day (as the US had) in mid-May, we were unable to really squelch this," Walensky said.

90% increase in child cases in a month

More research is also emerging around children and coronavirus amid back-to-school season.

The number of coronavirus cases among children in the US rose by 90% over four weeks this summer, according to a report published Monday by the American Academy of Pediatrics and the Children's Hospital Association.

The report said 179,990 new Covid-19 cases were recorded among US children from July 9 to August 6.

More than 380,000 children had been infected as of August 6, according to the report, making up just over 9% of the total cases in states that report cases by age.

The report comes as recent studies suggest children could play an important role in transmission of the virus. One study said older children can transmit coronavirus just as much as adults, and another found children younger than 5 carried a higher load of the virus, which also raised questions about their role in its spread.

And while some US officials, including the President, have said the virus doesn't pose a big risk to children, experts say that's not the case.

"It's not fair to say that this virus is completely benign in children," Dr. Sean O'Leary, vice-chair of the American Academy of Pediatrics Committee on Infectious Diseases, said Monday. "We've had 90 deaths in children in the US already, in just a few months. Every year we worry about influenza in children, and there are roughly around 100 deaths in children from influenza every year."

Still, many schools and universities across the country are moving forward with plans to begin in-person instruction, despite protests nationwide from teachers who say a return to class could be deadly.

In Georgia, multiple school districts have reported cases of the virus after reopening. In the Cherokee County School District, more than 800 students and more than 40 staff members have been quarantined since school began earlier this month, over concerns they may have been exposed to the virus.

The district, which has more than 42,000 students, reported at least seven positive cases among students and 10 cases among staff as of Friday.

In Mississippi, at least 22 schools across the state have reported positive cases, State Health Director Dr. Thomas Dobbs said this week. There have been 19 cases reported among students and 15 cases among staff.

These states require face masks

Cases spiked in nursing homes due to community spread, group says

Covid-19 cases rose rapidly in US nursing homes in July after a steady decline in June -- signaling a serious risk to an especially vulnerable group of people, a health organization said Tuesday.

The jump in nursing home cases was due to an overall jump in cases in the general population, and it should spur the US government to take steps to protect the homes, the American Health Care Association and National Center for Assisted Living said.

"We need public health officials to focus on reducing spread within the larger community and prioritizing long term care for resources," including personal protective equipment, testing and staff support, Mark Parkinson, CEO of the AHCA/NCAL, wrote to CNN on Tuesday.

New weekly Covid-19 cases in nursing homes dropped from 9,072 at the end of May to 5,468 by June 21 -- but then rose to 8,628 for the week of July 19, the last week for which data is available, the group said. The group cited data collected by the Centers for Medicare & Medicaid Services.

The report said this confirmed fears the group had: That cases would rise in nursing homes as they spiked in the wider community across the country from June to July.

Older adults and people with certain underlying medical conditions increase the risk of severe illness, the CDC says.