While Los Angeles County has reported a total of 13,816 coronavirus cases, early results from an antibody study conducted with the University of Southern California shows that hundreds of thousands more could have had COVID-19 in the past, officials announced Monday.

So far, 863 L.A. County residents have been tested between April 10 and 14 as part of the study.

The study estimates a prevalence of COVID-19 antibodies in the county to be 4.1%, with a range that could be as low as 2.8% and as high as 5.6%, when you factor in the reliability of the tests.

An estimated 221,000 adults to 442,000 adults at the high end may have been infected at some point before April 9 with COVID-19, suggesting that the number of total people in the county with a past or current infection is 28 to 55 times higher than the number of reported positive cases, Dr. Barbara Ferrer, L.A. County’s public health director said Monday.

“Although I report every day that we have thousands of thousands of people that have tested positive, the serology testing lets us know that we have hundreds of thousands of people that have already developed antibodies to the virus because at some point in time over the last couple of months, they have in fact been infected with COVID-19,” Ferrer said.

But having up to that many people who may have already had the illness means that the fatality rate from the virus is much lower than originally believed, Neeraj Sood, the USC scientist leading the study said Monday.

Residents should assume that they could be infected and that all of the other people they came in contact with can also be infected, and she again stressed the importance of staying home and observing physical distancing requirements, Ferrer said.

Although the sample size was relatively small, Ferrer shared some early estimates about who was most likely to be infected:

Men were more likely than women to be infected. The estimated prevalence is 6% among men and 2% among women

7% of African Americans, 6% of whites, 4.2% of Asians and 2.5% of people who were Latinx who were tested were found to be positive for COVID-19

2.4% of people who were between the ages of 18-34 were positive

5.6% were between 35 and 54

4.3% who were 55 and older tested positive

“Being positive for COVID-19 antibodies does not mean that a person is immune or that a person is not able to be reinfected. More research is really needed to understand what protection do people have who have already been infected with COVID-19,” Ferrer said.

The USC study will continue, and a representative sample of around 1,000 randomly pre-selected people will be tested every two weeks to track the trajectory of the virus in the county, officials said.

She also reported an additional 17 coronavirus-related deaths Monday, bringing the death toll to 617 countywide.

Over the weekend, officials received a total of 1,198 backlog test results dating back to April 7, which represents a “tremendous lag in data reporting,” Ferrer said.

There are 265 institutional settings with at least one confirmed COVID-19 case in L.A. County — that’s 37 more since Friday. A total of 2,733 people have tested positive in those settings, including 1,580 cases among residents and 1,153 among staff, Ferrer said. She noted that 241 residents have died from COVID-19, most of whom were in skilled nursing homes, representing 39% of all deaths in the county.

There are now 47 confirmed cases among people experiencing homelessness, most of whom were unsheltered. Twelve of them were sheltered and now they’re being isolated and their close contacts are quarantined.

The health department is investigating eight different shelters and identifying staff and guests that need testing, isolation and quarantine.

The county on Monday announced the death of a second inmate at a federal prison and noted that there are now 10 cases of coronavirus among staff at local juvenile facilities.

Over 80,000 people have been tested in Los Angeles and 13% have come back positive, Ferrer said.

Last week, officials announced that testing has significantly expanded across the county, but statewide, Gov. Gavin Newsom has asked for federal assistance to expand testing and get the raw materials needed to conduct the tests.

Newsom has previously outlined what needs to happen before California can reopen, and even then he expects to keep many physical distancing measures in place.

Also on Monday, Supervisor Kathryn Barger announced that the Department of Parks and Recreation is launching a virtual recreation center experience this week. The online feature includes fitness classes for children who have missed out on sports classes. Residents can also virtually explore county trails.