A large, nationwide study of people receiving dialysis reveals that less than 10% of the adult population in the United States had formed antibodies against SARS-CoV-2 by July 2020.

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Also, less than 10% of those with antibodies received a diagnosis of COVID-19, suggesting that most of the nation remains at risk. The results appear in The Lancet.

Many experts consider seroprevalence to be the most comprehensive method of tracking the spread of COVID-19 in communities. Seroprevalence refers to the percentage of individuals in a population who have antibodies against an infectious agent.

Seroprevalence surveys use antibody tests to check blood samples for the presence of antibodies. In this case, the researchers checked the presence of antibodies against SARS-CoV-2, which is the virus that causes COVID-19.

A person who is seropositive usually has measurable levels of antibodies in the blood around 1–3 weeks after contracting the infection.

However, seroprevalence studies are challenging, as they require outreach into communities to perform random blood draws, and some people — such as those from marginalized racial and ethnic groups, people with fewer language skills, and individuals with underlying health conditions — are harder to reach with these types of surveys.

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Researchers from Stanford University in California decided to investigate data from people who required dialysis. These people had already provided plasma for evaluation, which provides a potentially more representative assessment of COVID-19 seroprevalence than community-based surveys.

People who require dialysis are more likely to be from groups who are more at risk of COVID-19. For instance, they tend to be older than the general population and are more likely to be a member of a marginalized racial or ethnic group. These people are also more likely to come from disadvantaged backgrounds.

“Not only is this patient population representative ethnically and socioeconomically, but they are one of the few groups of people who can be repeatedly tested,” explains lead study author Dr. Shuchi Anand.

“Because renal disease is a Medicare-qualifying condition, they don’t face many of the access to care barriers that limit testing among the general population.”

The researchers joined up with scientists from Ascend Clinical in Redwood City, CA. This is a company that processes laboratory tests for people undergoing kidney dialysis.

The team examined blood plasma samples from 28,503 randomly selected people receiving dialysis care across 1,300 dialysis facilities in 46 states during July 2020.

Using anonymized electronic health records, they collected information about each person’s age, sex, race, and ethnicity, as well as information about where they lived and the healthcare facility they used.

The scientists observed that their sampled dialysis population had more older adults, males, and people living in majority Black and Hispanic neighborhoods than the general U.S. adult population.

The analysis revealed that relatively few of the seropositive people had actually received a diagnosis of COVID-19. When the researchers adjusted the results for the general population, they found that an estimated 9.3% of the U.S. population had antibodies against SARS-CoV-2 and that, among those who tested positive, 9.2% had received a diagnosis of COVID-19.

The study authors note that seroprevalence ranged greatly from region to region. For example, seroprevalence was substantially higher in the northeastern U.S., the nation’s hardest hit region, where an estimated 27.2% of the national dialysis population was seropositive. In western states, meanwhile, an estimated 3.5% were seropositive.

The investigators also found that marginalized groups more often had coronavirus antibodies. Compared with the majority non-Hispanic white population, people living in mainly Black and Hispanic neighborhoods had a two-to-four times higher chance of COVID-19 seroprevalence.

The most densely populated areas also had a 10 times higher chance of COVID-19 seroprevalence.

These findings also suggest that the U.S. is far from achieving herd immunity without the creation of a vaccine.

Herd immunity occurs when enough of the population has developed protection against the virus so that it cannot spread efficiently.

Experts suggest that approximately 70% of the population needs to receive a vaccination or develop antibodies through natural infection to achieve herd immunity.

“This research clearly confirms that despite high rates of COVID-19 in the [U.S.], the number of people with antibodies is still low, and we haven’t come close to achieving herd immunity,” says study co-author Dr. Julie Parsonnet.

“Until an effective vaccine is approved, we need to make sure our more vulnerable populations are reached with prevention measures,” she adds.

The authors note several limitations to their study.

For instance, it is possible that seroprevalence estimates in the U.S. dialysis population overestimate seroprevalence in the U.S. adult population at large. People undergoing dialysis attend health facilities more often, meaning that their risk of SARS-CoV-2 exposure is likely higher.

On the other hand, because these people are “less likely to be employed and more likely to have restricted mobility, the data might underestimate overall seroprevalence in the general population.”

Furthermore, dialysis units are more commonly located in urban areas, so there may also be an underrepresentation of rural areas.

Nevertheless, this study shows that a surveillance strategy relying on the monthly testing of the blood plasma of people undergoing dialysis can provide helpful estimates of COVID-19 spread.

The study authors conclude that their scalable sampling strategy offers a blueprint to understanding seroprevalence and will be key to studying the effects of any prevention strategies and interventions — even for understanding vaccine effectiveness for the next phases of the COVID-19 pandemic.

The team also intends to continue looking at samples from people undergoing dialysis to follow COVID-19 over time. “It will give us a good sense of the epidemic as it evolves,” concludes Dr. Anand.