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A study finds 50 to 85 times as many infections as known cases—meaning a far lower fatality rate.
By Andrew Bogan
April 17, 2020 4:28 pm ET
Los Altos Hills, Calif.
The Covid-19 shutdowns have been based on the premise that the disease would kill more than two million Americans absent drastic actions to slow its spread. That model assumed case fatality rates—the share of infected people who die from the disease—of 1% to 3%. The World Health Organization’s estimated case-fatality rate was 3.4%.
Some experts—including in these pages—have questioned this assumption. They argue that known cases are likely only a small portion of the true number of infections, and thus high case-fatality rates could be off by orders of magnitude. We don’t know what portion of infections have gone undetected for a lack of tests, restrictive qualifications to get tested, and a potentially large incidence of mild illness or asymptomatic infection.
New data support the skeptics’ view—a preliminary study by a Stanford team, released Friday. They conducted a seroprevalence study of Santa Clara County, Calif., on April 3 and 4. They studied a representative sample of 3,300 residents to test for the presence of antibodies in their blood that would show if they had previously been infected with the novel coronavirus.
The county, where I live, is home to about two million people in the heart of Silicon Valley, including San Jose, the state’s third-largest city. It has the largest known number of cases in Northern California and saw some of the earliest cases of community spread in the U.S.
The researchers found that the percentage of infections was indeed vastly larger than the roughly 1,000 known positive cases in the county at the time of the study. The preliminary results—the research will now undergo peer review—show that between 2.5% and 4.2% of county residents are estimated to have antibodies against the virus. That translates into 48,000 to 81,000 infections, 50 to 85 times as high as the number of known cases.
That may sound scary, but it’s great news. It suggests that the large majority of people who contract Covid-19 recover without ever knowing they were infected, and that the U.S. infection fatality rate may be more than an order of magnitude lower than authorities had assumed.
Based on this seroprevalence data, the authors estimate that in Santa Clara County the true infection fatality rate is somewhere in the range of 0.12% to 0.2%—far closer to seasonal influenza than to the original, case-based estimates.
