CNN  — 

A new nationwide study of the blood of more than 28,000 dialysis patients may help answer one of the big questions surrounding the pandemic: how many people have antibodies to SARS-CoV-2, the novel coronavirus?

The answer could tell us a lot about how many people in the United States have been exposed to the virus and how much community spread there has been. Plus, the testing strategy used in the study points to a relatively easy way to track Covid-19 disease activity over the long-term, especially among vulnerable populations.

The short answer to how many people had antibodies, as of July, is approximately 9.3% – although numbers ranged from an average of 3.5% in the West to an average of 27% in the Northeast.

“This research clearly confirms that despite high rates of COVID-19 in the United States, the number of people with antibodies is still low and we haven’t come close to achieving herd immunity. Until an effective vaccine is approved, we need to make sure our more vulnerable populations are reached with prevention measures,” study author Dr. Julie Parsonnet, a professor of medicine at Stanford University, said in a statement.

For the study, which was published Friday in The Lancet, researchers led by Stanford University’s Dr. Shuchi Anand, analyzed samples of plasma – a component of blood – from more than 28,500 patients receiving dialysis in July at approximately 1,300 facilities in 46 states run by one lab.

The overall percent of people who were positive for antibodies among those sampled was 8%. Because dialysis patients aren’t representative of the US population, the researchers standardized the results with respect to age, sex, race and ethnicity and region, to get an estimate of 9.3% seropositivity for the US adult population.

They found that there was a wide variation by state, with seven states having 0% seropositivity to New York, an early pandemic hotspot, topping the list with 33%.

The researchers were also able to see who was more likely to have antibodies. They found that, compared to the White population, residents of predominantly Black and Hispanic neighborhoods were two to three times more likely to be seropositive, people living in poorer areas were two times more likely and those living in the most densely populated areas were ten times more likely.

They also compared rates from their study with case counts from Johns Hopkins University. From that, they were able to estimate that only 9.2% of seropositive patients were actually diagnosed with Covid-19 using standard testing methods.

Why dialysis patients? For starters, “Patients receiving dialysis in the USA undergo routine monthly laboratory studies,” the researchers wrote, so there was no need for “considerable infrastructure and expense” to gather samples, nor were there other pandemic-related challenges.

Additionally, the risk factors for becoming infected with the coronavirus and for developing severe disease – including advanced age, non-white race, poverty, and diabetes – “are the rule rather than the exception in the US dialysis population.”

For those reasons, the researchers said that dialysis patients might be considered “an ideal sentinel population” in which to study the evolution of the pandemic.

But, as the authors of an accompanying commentary point out, questions still remain about how long the antibodies last and how protective they are. Still, studies like this, especially if it can be repeated on an ongoing basis, can help find answers.

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“Anand and colleagues deserve credit for pioneering a scalable sampling strategy that offers a blueprint for standardised national serosurveillance in the USA and other countries with a large haemodialysing population,” wrote professors Barnaby Flower and Christina Atchison from Imperial College London. They were not involved in the study.

The study authors indicated the same. “A surveillance strategy relying on monthly testing of remainder plasma of patients receiving dialysis can produce unbiased estimates of SARS-CoV-2 spread inclusive of hard-to-reach, disadvantaged populations in the USA. Such surveillance can inform disease trends, resource allocation, and effectiveness of community interventions during the COVID-19 pandemic,” they wrote.