(CN) — Following assumptions by health officials that the number of Covid-19 infections are being underreported, preliminary study results released Friday from Santa Clara County in California suggests significantly more cases are not being reported.
The study, conducted in early April by scientists from Stanford University and has yet to be peer reviewed, conducted antibody tests on 3,300 adults and children using a sample meant to represent the county’s population based on factors such as age, gender and race.
The authors largely relied on Facebook ads to recruit participants to drive-through test sites where blood sample were drawn.
From the test results, researchers estimate 2.49% to 4.16% of the county had been infected, representing between 48,000 and 81,000 people. At the time the study was conducted, the county only reported less than 1,000 residents as having been infected with the novel coronavirus.
Antibodies develop in the body to fight off viruses. Scientists examined the blood samples for the presence of specific antibodies used to combat the coronavirus, a sign that a person had already been infected.
“While this prevalence may be far smaller than the theoretical final size of the epidemic, it suggests that the number of infections is 50-85-fold larger than the number of cases currently detected in Santa Clara County,” the scientists wrote in the preprint study.
The first reported cases of infection in Santa Clara County happened on Jan. 31 and Feb. 1 by two returning travelers. By the end of March, more than an additional 1,000 cases had been reported, the most out of any county in Northern California.
The researchers cautioned that the results of the testing in Santa Clara County may not be representative of other areas, but noted that similar testing elsewhere could help health officials better estimate infections and need for hospital beds in their own jurisdictions.
“We found that there are many, many unidentified cases of people having Covid infection that were never identified with it with a virus test,” said Dr. Jay Bhattacharya, a professor at Stanford University and an author of the paper in an interview with CNN. “It’s consistent with findings from around the world that this disease, this epidemic is further along than we thought.”