California antibody testing suggests coronavirus may be far more widespread than official count

The first large-scale coronavirus antibody study of 3,300 people in Santa Clara County, Calif., found that 2.5 to 4.2 percent of those tested were positive for antibodies, suggesting more people have been infected with the disease than public health officials have counted.

The study published Friday estimates that 48,000 to 81,000 people in the county of 2 million could be infected. In early April when the samples were taken, approximately 1,000 confirmed cases were recorded by the county.

Researchers estimate there are likely 50 to 80 times more infections in the state’s counties than what is being reported.

“The most important implication of these findings is that the number of infections is much greater than the reported number of cases,” the researchers wrote.

The study, which has yet to be peer-reviewed, required a drop of blood to reveal whether the volunteer test subjects, who were recruited from online ads, had antibodies for the virus.

Widespread antibody tests have been touted by public health officials as a tool that could help governments resume business as usual. Though more research needs to be done on whether the presence of antibodies guarantees immunity from COVID-19, it could help determine who is less at risk and able to return to work and daily life. 

However, Eran Bendavid, the associate professor of medicine at Stanford University who led the study, told ABC News’s Diane Sawyer on Friday that his research also suggests about 95 percent of the population is still without antibodies which would make a system based on that data point difficult to execute.  

“Knowing that well upwards of 90% of the population doesn’t have antibodies is going to make that a very difficult choice,” he said. Bendavid also urged those who have tested positive for antibodies to “keep following public health guidelines.”