Data from the California Department of Public Health, released earlier this month, show that while only 8.7% of the state’s population has ever tested positive for Covid-19, at least 38.5% of the population has antibodies against the novel coronavirus. Those numbers are from Jan. 30 to Feb. 20. Adjusting for cases between now and then, and accounting for the amount of time it takes for the body to make antibodies, we can estimate that as many as half of Californians have natural immunity today.

The same report found that 45% of people in Los Angeles had Covid-19 antibodies. Again, the number can only be higher today. Between “half and two-thirds of our population has antibodies in it now,” due to Covid exposure or vaccination, Mayor Eric Garcetti said Sunday on “Face the Nation.” That would explain why cases in Los Angeles are down 95% in the past 11 weeks and the positivity rate among those tested is now 1.7%.

Undercounting or removing the many Americans with natural immunity from any tally of herd immunity is a scientific error of omission. When people wonder why President Biden talks about limiting Fourth of July gatherings, it’s because his most prominent medical adviser has dismissed the contribution of natural immunity, artificially extending the timeline.