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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 lookingat 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 the 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
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.
Study: Many Americans Weren't Infected Because of Distancing Orders
Up to 80% of Americans would have been infected with COVID-19 had states not implemented physical distancing policies, according to a recently published modeling study.
Researchers from Harvard University and University College London found that every state in the US passed at least one physical distancing measure in March to slow the spread of the pandemic.
The policies resulted in the reduction of more than 600,000 cases within just three weeks, according to the study published Tuesday in the journal PLOS.
"The results show the timing of government-issued orders correlated strongly with reductions in both cases and deaths. In short, these measures work, and policymakers should use them as an arrow in their quivers to get on top of local epidemics where they are not responding to containment measures," said Dr. Mark J. Siedner, a co-author of the study from Harvard Medical School and an infectious diseases doctor at Massachusetts General Hospital.
The death rate decreased by 2% per day beginning a week after a physical distancing policy started, the study says.
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.