Antibody Testing Reveals Covid-19 Cases Could Top 440,000 in LA County

Health workers in Los Angeles County collect residents’ blood samples at a drive-up testing site in early April as part of a study to detect antibodies in people who contracted Covid-19 but later recovered after infection. Officials said Monday early results show as many 442,000 county residents may had the virus. (Courtesy of LA County Department of Public Health)

LOS ANGELES (CN) — Preliminary results of an ambitious effort to test people for novel coronavirus antibodies indicate the virus may have spread to hundreds of thousands of Los Angeles County residents by early April, vastly outpacing positive cases confirmed by health officials in that period.

LA County Department of Public Health officials and University of Southern California researchers collected blood samples of over 860 residents earlier this month to better understand how the virus spread and how patients overcame infection. The serological test looks for immunoglobulin M (IgM) and immunoglobulin G (IgG) antibodies, proteins that develop after infection and help people fight off severe illness.

Health officials said Monday an analysis of test results shows up to 5.6% of the county’s adult population — as many as 442,000 residents — has antibodies to the virus in their blood. The figure, which is at the high end of the statistical margin of error, could be as low as 2.8% of the county’s adult population or 221,000 residents, according to the study.

The estimates by researchers are 55 and 28 times higher, respectively, than the 7,994 confirmed cases of Covid-19 reported by the county through early April. But the first round of testing also reveals the fatality rate is lower given the much greater number of positive cases in the county.

Officially, more than 13,800 people have contracted the virus and 617 have died in LA County as of Monday.

Neeraj Sood, lead scientist on the study for the University of Southern California, said in a statement Monday the results paint a clearer image of the outbreak and could impact measures to protect public health.

“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,” Sood said in the statement. “The estimates also suggest that we might have to recalibrate disease prediction models and rethink public health strategies.” 

The county’s 10 million residents have been under strict social distancing measures since March, when schools and many of the region’s businesses were also ordered closed. 

LA County chief science officer Paul Simon said in the statement measures to prevent further spread of the virus must be kept in place.

“Though the results indicate a lower risk of death among those with infection than was previously thought, the number of Covid-related deaths each day continues to mount, highlighting the need for continued vigorous prevention and control efforts,” Simon said.

Positive and borderline cases signal recent exposure and that someone is in the early stage of developing antibodies, health experts told Courthouse News. And negative results could also mean someone was recently exposed but has not yet developed antibodies.

Results of the study have not been peer-reviewed by other scientists, LA County health officials said Monday. 

Researchers plan to conduct a serological survey every few weeks in coming months with different sample groups in LA County to gauge the impact of the pandemic on the region.

LA County Department of Public Health director Barbara Ferrer said in the statement test results reveal the importance of ramped-up testing for Covid-19 infection in the county.

“These results indicate that many persons may have been unknowingly infected and at risk of transmitting the virus to others,” Ferrer said. “These findings underscore the importance of expanded polymerase chain reaction (PCR) testing to diagnose those with infection so they can be isolated and quarantined, while also maintaining the broad social distancing interventions.” 

The county has ramped up free, same-day Covid-19 testing for residents who have symptoms of infection and who register online for testing.

While an antibody test detects whether someone was previously infected with Covid-19, the PCR is needed to diagnose current infection, health officials said. Experts say antibody testing can help officials determine how to reopen the U.S. economy, redeploy the workforce and aid development of a vaccine.

On Saturday, the county reported 81 people had died after contracting Covid-19, its highest one-day death total during the outbreak.

Sood said in a press conference Monday that researchers will increase the sample size of the study — which invited 1,000 participants in the first phase — and that officials will continue recruiting individuals who are socioeconomically and ethnically diverse.

“This will give better data to [virus outbreak] models so they can make better predictions and lead to better decisions about public policy,” Sood said. “We want to increase participation of the Hispanic population. We didn’t hit that goal. We also want to offer more in-home testing for people that don’t own cars or can’t access child care.”

Sood said the county has enough serological kits for another wave of antibody testing and that researchers will analyze data from across the U.S. to determine if another kit is more accurate.

Simon told reporters the kits are for public health surveillance, not for individual patient treatment, and that officials should not rely on serological survey results to make determinations about population immunity. 

“We don’t want the public to interpret [positive antibody test results] as, ‘oh, I have immunity,’” Simon said. “It’s still too early to say how much protection an antibody gives you with regards to reinfection.”

After Ferrer said Monday that study results put the county’s fatality rate from Covid-19 closer to 0.1% or 0.2%, researchers said any decisions to relax restrictions should come after health officials review testing data in months to come.

“That mortality rate is specific to LA County,” Sood told reporters. “You can’t export that rate to other jurisdictions. The mortality changes as the epidemic changes.”

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