Some 4.1percent of adults tested positive for coronavirus antibodies in a study of Los Angeles County residents, health officials said on Monday, suggesting the rate of infection may be 40 times higher than the number of confirmed cases.
The serology tests, conducted by University of Southern California researchers on 863 people indicate the death rate from the pandemic could be lower than previously thought but also that the respiratory illness may be being spread more widely by people who show no symptoms.
"We haven't known the true extent of Covid-19 infections in our community because we have only tested people with symptoms and the availability of tests has been limited," Neeraj Sood, a professor of public policy at USC and lead researcher on the study.
"The estimates also suggest that we might have to recalibrate disease prediction models and rethink public health strategies," Sood said.
At least 17 additional fatalities were recorded in Los Angeles County on Monday, bringing the total to 600, with more than 12,300 positive cases, according to a Reuters tally. The county is home to roughly 8 million people.
The Los Angeles County results were announced as antibody tests come under increasing scrutiny over a high number of false positives reported in the kits.
A similar study conducted in Santa Clara County last week by a Stanford University researcher has been criticized over its methodology and sample size.
New York Governor Andrew Cuomo on Sunday said health officials there would begin conducting statewide antibody testing of 3,000 people on Monday.
The antibody tests, using decades-old ELISA technology, do not always pick up early-stage infections but show whether a person had the virus in the past, even if the person was asymptomatic.
In comparison, the so called RT-PCR-technology swab tests used at drive-through stations and clinics across the country determine whether a person has the virus at that moment by looking for it in nose or throat secretions.
Both tests are seen as critical in the coronavirus fight, but antibody tests are seen as a relatively cheap, fast means to sort populations into risk groups and measure virus spread.
Questions remain about how long coronavirus immunity levels last and whether people who have antibodies could still be contagious, according to some infectious disease specialists.
Nationwide 41,790 deaths have been reported from Covid-19, with more than 772,000 confirmed cases, according to the Reuters tally.