LA County now estimates 1 in 3 residents got COVID. What might that mean for immunity?

Written by Danielle Chiriguayo and Amy Ta, produced by Bennett Purser

New modeling from LA Public Health estimates that approximately one in three people here have been infected with COVID-19 since March — meaning three million people. However, the county’s official daily COVID-19 positivity rate is around 20%. 

Dr. Roger Lewis is the director of COVID-19 Demand Modeling for the LA County Department of Health Services. 

He says the official count is based on regional cases that have been confirmed via coronavirus testing, but these confirmations only represent a percentage of total cases in LA County. 

“Our modeling is based on a belief that for every person who is truly infected — many of whom never have symptoms — about one in 40 will ultimately become sick enough to require hospitalization,” Lewis says. “We use the number of hospitalizations across Los Angeles County to estimate the number who were truly infected.”

Lewis also notes that in the early days of the pandemic, only patients with the most severe symptoms were tested for COVID-19. In turn, he theorizes that many other cases went on unidentified. 

He points out, however, “It is good news in the sense ... that that means one in three persons in LA County ... are very unlikely to get reinfected in the near future or become seriously ill with COVID."

People usually have a robust immune response to COVID for at least eight months after being infected, according to a study co-authored by Daniela Weiskopf, a researcher at the La Jolla Institute for Immunology. 

Lewis continues, “Even with one third of the population presumably immune to COVID, we're still engaging in behaviors that lead to so much transmission that we have an out of control pandemic. … This does provide some degree of herd immunity and slows transmission. But what we see in case counts, is it simply not enough, given the amount of transmission that's occurring.” 



  • Roger J. Lewis - MD, PhD, Director of COVID-19 Demand Modeling, Los Angeles County Department of Health Services