Preliminary results from antibody tests in Los Angeles County indicate that the true number of COVID-19 infections is much higher than the number of confirmed cases there, which implies that the fatality rate is much lower than the official tallies suggest. "The mortality rate now has dropped a lot," Barbara Ferrer, director of the Los Angeles County Department of Public Health, said at a press briefing today. In contrast with the current crude case fatality rate of about 4.5 percent, she said, the study suggests that 0.1 percent to 0.2 percent of people infected by the virus will die, which would make COVID-19 only somewhat more deadly than the seasonal flu.

Based on a representative sample of 863 adults tested early this month, researchers at the University of Southern California (USC), working in collaboration with the public health department, found that "approximately 4.1% of the county's adult population has antibody to the virus." Taking into account the statistical margin of error, the results indicate that "2.8% to 5.6% of the county's adult population has antibody to the virus—which translates to approximately 221,000 to 442,000 adults in the county who have had the infection." That is 28 to 55 times higher than the tally of confirmed cases at the time of the study.

As of noon today, Los Angeles County had reported 617 deaths out of 13,816 confirmed cases, which implies a fatality rate of 4.5 percent. Based on that death toll, the new study suggests the true fatality rate among everyone infected by the virus is somewhere between 0.1 percent and 0.3 percent (without taking into account people infected since the study was conducted). The lower end of that range is about the same as the estimated fatality rate for the seasonal flu.

"These results indicate that many persons may have been unknowingly infected and at risk of transmitting the virus to others," Ferrer said in a press release. "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."

Since the number of infections in Los Angeles County is much higher than the official numbers indicate, Ferrer said at the press briefing, the risk of infection is correspondingly higher, which reinforces the case for social distancing measures. At the same time, she said, the fact that 95 percent or so of the county's adult population remains uninfected shows those measures, including the statewide lockdown, are working. She also acknowledged that the revised estimate of the fatality rate, which is dramatically lower than many people feared, is good news for residents who are infected despite those precautions.

"The fatality rate is lower than we thought it would be," said Neeraj Sood, the USC public policy professor who oversaw the study. But he also emphasized that "we are very early in the epidemic," meaning the number of infections and the death toll are bound to rise.

Sood addressed two of the methodological concerns that were raised by a recent study of Santa Clara County residents, which likewise estimated that the COVID-19 fatality rate is not far from the rate for the flu. Critics of that study suggested it may have been undermined by biased sampling and false-positive antibody test results.

The sample for the Los Angeles County study, Sood said, was randomly drawn from a database maintained by the LRW Group, a market research firm. The researchers capped subjects representing specific demographic groups so the sample would reflect the county's adult population.

As for the accuracy of the antibody tests, Sood said validation by the distributor of the test kits, Premier Biotech, found a false positive rate of 0.5 percent in 371 samples. In subsequent tests by a Stanford laboratory, there were no false positives. "We think that the false positive rate of the tests is really low," Sood said.

While Ferrer portrayed the study as proof of the need for aggressive control measures, a fatality rate as low as the Los Angeles County and Santa Clara County tests suggest also changes the calculus of those policies' costs and benefits. If COVID-19 really is only a bit more lethal than the seasonal flu, the benefits that can be expected from continued lockdowns, in terms of deaths prevented, are much lower than most projections assumed. If these results are confirmed, they should play an important role in discussions about when and how to reopen the economy.