It remains unclear exactly how many people in the United States have had Covid-19, but a new study suggests that most do not appear to have antibodies and are likely susceptible to infection.
Across the country, the prevalence of Covid-19 antibodies appears to range from fewer than 1% to 23%, according to the study published in the Journal of the American Medical Association’s JAMA Internal Medicine on Tuesday. When someone recovers from Covid-19, their blood plasma can contain antibodies that helped fight the coronavirus that caused their illness — and therefore antibodies serve as clues to a past infection.
The new study included data from blood serum samples taken from 178,000 people across all 50 states, the District of Columbia and Puerto Rico. Samples were obtained for routine screening or clinical care during four collection periods: July 27 to Aug. 13; Aug. 10 to Aug. 27; Aug. 24 to Sept. 10; and Sept. 7 to Sept. 24.
The researchers — from the US Centers for Disease Control and Prevention, Quest Diagnostics, BioReference Laborites and the company ICF Inc. — found that the prevalence of antibodies among those samples ranged from 0% in South Dakota in collection period 2 to 23.3% in New York in collection period 1.
“In nearly all jurisdictions, fewer than 10% of people in the US had evidence of previous SARS-CoV-2 infection,” the researchers wrote in the study. SARS-CoV-2 is the name of the coronavirus that causes Covid-19.
“Seroprevalence varied across regions and between metropolitan/nonmetropolitan areas, with estimates as high as 23% in the Northeast and 13% in the South, while estimates in the Midwest and West were less than 10%,” the researchers wrote.
“Seroprevalence was often lowest in older age groups,” they wrote. “Our results reinforce the need for continued public health preventive measures, including the use of face masks and social distancing, to limit the spread of SARS-CoV-2 in the US.”
For certain states — such as Iowa, Pennsylvania and Tennessee — more people living in metropolitan counties had antibodies. In other states — such as Alabama and Mississippi — people living outside metropolitan areas were more likely to have been infected, according to the study. Changes over time, from collection period 1 to 4, also varied across states. The researchers found that the largest drop in prevalence occurred in New York and North Dakota, while large increases occurred in Georgia and Minnesota.
The study was not designed to produce a nationwide estimate of prevalence. More research is needed to determine whether similar findings would emerge among a larger group of people representative of the general public.