Hundreds of thousands of Los Angeles County residents may have been infected with the coronavirus at some point, even without showing any symptoms, according to preliminary test results from an antibody study released Monday.

A team from the University of Southern California and the county’s public health department tested a random sample of 863 county residents earlier this month, finding that around 2.8% to 5.6% of them already had the antibodies in their blood, the county’s public health director Dr. Barbara Ferrer announced.

This translates to an estimated 221,000 to 442,000 adults in the county who may have been infected with COVID-19 at some point before April 9 — a much higher number than the 13,816 cases confirmed by the health department as of Monday.

These results mean that the coronavirus has been much more widely spread throughout the county than previously thought.

“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.

Ferrer said the study suggests that the number of people in the county with past or current infections is 28 to 55 times higher than the number of confirmed cases found through testing for current infections.

Does having antibodies mean someone is immune?

More research is needed to tell if people with antibodies are immune and to what degree, scientists say.

USC’s study represents one of the first such in the country amidst the pandemic, and researchers hope the results will help with creating models for how the pandemic will play out in the future and inform decisions about when to relax social distancing rules.

Serology testing is different than testing that seeks to diagnose current infections. It can find people who had been infected in the past, including those who don’t show symptoms of COVID-19, by looking for the antibodies in their blood.

Antibodies are the proteins made in response to an infection and indicate “an immune response,” according to the Centers of Disease Control and Prevention.

But people who have the antibodies are not automatically considered immune to COVID-19, and it’s not clear they won’t get sick if they’re exposed again.

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

Though more research is needed, the results of the study can help show how many people are potentially immune, the USC scientist leading the study, Neeraj Sood, has previously said.

For now, Ferrer said that results only reinforce the need to continue practicing social distancing, especially because the study showed that about 95% of people in the county don’t have the antibodies.

Who’s likely to be infected?

Although the sample size of people tested in L.A. County was relatively small, Ferrer said the early results already show that men were more likely than women to be infected with the coronavirus.

Of those tested for antibodies in the county, 6% of men tested positive compared to 2% of women.

Seven percent of the African Americans in the sample group tested positive for antibodies. The same was true for 6% of white participants, 4.2% of Asian participants, and 2.5% of Latinx participants.

In terms of age, 2.4% of people between 18 and 34 tested positive for antibodies, along with 5.6% of participants between 35 to 54 years old, and 4.3% of participants 55 and older.

Sood said that the random sample of people tested for antibodies was picked through a market research firm. The group was representative of L.A. County’s population, taking into account the percentages of different age, race and gender groups.

What the results mean for L.A. County residents

With the results indicating that the virus may been much more widespread than previously known, Ferrer urged people to continue practicing physical distancing.

“Given the high rate of people that many have been infected at some point with COVID-19, we need to assume that at any point in time, we could be infected and that all of the other people that we come in contact with can also be infected,” Ferrer said.

There was a silver lining to the findings: Having up to 442,000 people who may have already had the illness means that the fatality rate from the virus is much lower than originally believed, Sood said Monday. In L.A. County, 617 patients have died from the virus.

But the results also show that if only 4% of the county’s population has been infected, the county is still at an early stage in the pandemic and “many more people in L.A. County could potentially be infected,” Sood said.

“As the number of infections rise, so will the number of deaths, the number of hospitalizations and the number of ICU admissions,” Sood said.

Though the university released its initial findings Monday, the testing will continue at six undisclosed sites throughout L.A. County for the next three to four months.

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, the university said.

USC and county health officials hope the data from the study will paint a clearer picture of how widespread coronavirus has been in the county, and help inform decisions on when to lift social distancing measures and let businesses open back up.

“It will give us an understanding of how many people out there maybe be infected and not even aware,” Dr. Paul Simon of L.A. County Public Health previously said. “If they are antibody positive, they will be able to more quickly resume their normal activities.”

Stanford University released results of antibody testing in Santa Clara County last week, similarly estimating that between 2.5% and 4.2% of Santa Clara County residents had antibodies to the coronavirus in their blood by early April.