On June 25th, CDC Director Robert Redfield announced that the actual number of cases in the U.S. is ten times greater than the reported cases, by their best estimation. This information was based on results from antibody testing. I was skeptical. The number was much higher than I thought. Over the next few weeks, experts and officials continued to use this number, so I began to believe that the information was accurate. Here is a link to an article from June 25th: CDC estimates 10 times as many U.S. coronavirus cases than have been reported
The reason I was skeptical was because other antibody studies that have been touted by the media turned out to use heavily biased sampling. I read about two of them. The first was from Chelsea, Massachusetts, where they took samples from people who were out-and-about on the street. The second was from New York, where they sampled people in grocery stores. By the nature of infectious spread, those people who are out-and-about more often are more likely to contract this disease. And people who are out more often are much more likely to be included in these samples than those who are out less often. My opinion is that the sampling in these studies was so biased that the results are worthless.
Chelsea article: One third of participants in Massachusetts study tested positive for antibodies linked to coronavirus
New York article: New York antibody study estimates 13.9% of residents have had the coronavirus, Gov. Cuomo says
The New Study
A few days ago, someone posted a link to the new study that CDC Director Redfield was referencing. The study was published online on July 21st. I have had a chance to look over the study and I’ve come to the conclusion that Director Redfield was wrong in his June 25th statement. There were not 10x the number of actual cases than reported cases on June 25th and there certainly isn’t now.
Article: Coronavirus Infections Far Exceed Reported Cases, CDC Says
Study: Seroprevalence of Antibodies to SARS-CoV-2 in 10 Sites in the United States, March 23-May 12, 2020
CDC Dashboard with results: Interactive Serology Dashboard for Commercial Laboratory Surveys
“We obtained convenience samples of deidentified residual patient sera collected for routine screening (eg, cholesterol screening) or clinical management”. Did some people who were more cautious skip a routine visit to the clinic because of coronavirus? Many of these patients have chronic health conditions (thus the need to go to a clinic). Are these people more cautious overall because of underlying health conditions? The study also says, “It is possible that specimens were drawn from patients seeking care for suspected COVID-19 symptoms, potentially biasing results, particularly in settings such as NY where disease incidence was higher.” So it’s hard to say for sure how biased these samples are. I think there is a very slight bias towards positive test results, but nowhere near the bias in the other studies.
The Date Range
The date range is the biggest issue for me. The samples were taken between Mar 23 and May 12, depending on the location. For instance, NY metro samples were taken from Mar 23 to Apr 1. The average seems to be about mid April. The results were compared to the number of cases at the time the samples were retrieved. But the testing capacity in the U.S. has been increasing since then, thus more cases are being detected now than they would have found back in April. So, while the ratio of actual cases to reported cases may have been 10x back then, it’s surely not 10x now, or even at the end of June when Redfield made his statement.
According to 91-divoc.com, The U.S. was testing about 450 ppl per million in mid-April. By the end of June, about 1200 per million and now it’s 2500 per million. So five times as many tests are being given now than at the time of the study. That doesn’t mean 5 times as many cases are being detected because a lot more asymptomatic people are being tested, but there are certainly a lot more. Maybe double the amount of cases are being detected? That would mean the 10x number no longer applies.
For these reasons, I believe CDC Director Redfield was incorrect. In my non-expert opinion, the actual number of cases is only about 4x to 5x the current reported cases. What do you think? Are there flaws in my reasoning?