Study: US exposure rate far from ‘herd immunity’
By the end of July, about 9% of American adults had been exposed to the coronavirus that causes COVID-19, according to a new study of dialysis patients, the largest yet looking for evidence of the disease in people’s blood.
The infection rates varied from essentially zero in some states that avoided infection by mid-summer, to more than one-third of residents in parts of New York in the spring.
The American public is a long way from achieving “herd immunity” – having enough infections to prevent further spread of the virus.
Infection rates are so patchy that even if some areas had high infection rates, people are unlikely to be protected because others will bring the virus from elsewhere, said William Hanage, an epidemiologist at the Harvard T.H. Chan School of Public Health.
Hanage is also concerned that communities not yet hit by COVID-19 have a false sense of security.
“We expect small-town America not to be in the first surge,” he said, but over time, as people move around more, more communities will have outbreaks.
Dr. George Rutherford, an epidemiologist and biostatistician at the University of California, San Francisco, is surprised the national infection rate appeared so high. He would have expected closer to 3-4%.
“The only way we’re going to get to herd immunity, unless you’re in a very closed community like a prison, is for everybody to get vaccinated,” Rutherford said.
Hanage said the study also confirmed what he has long thought: that the outbreak in the spring was enormously underestimated, “ridiculously so,” and started earlier and infected more young people than have been diagnosed.
The new study, published in The Lancet, is in line with previous, smaller studies, and also showed areas with high numbers of Black and non-white Latino residents had higher infection rates than mostly white communities.
Stanford University researchers, who led the new study, looked for antibodies to COVID-19 in the blood of people who receive dialysis in clinics or at home. When someone mounts an immune response to an infection, the body produces antibodies that can be read to determine their infection history.
Dialysis patients, whose kidneys are failing, get routine blood tests, so researchers were able to use existing blood samples. The patients were treated at 1,300 dialysis centers in 46 U.S. states, representing about one-third of counties.
Dialysis patients tend to be older than the general population, and more diverse.
Dr. Shuchi Anand, a kidney specialist at Stanford and the paper’s first author, said the dialysis patients provide a useful and interesting – though imperfect – snapshot of the COVID-19 outbreak.
The patients might have been more careful to avoid infection but also more likely to suffer severe consequences if they were infected, said Dr. Glenn Chertow, the paper’s senior author and a Stanford nephrologist.
“It should be a pretty good estimate of the population in the United States with a little bit of tweaking around the edges,” he said.
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