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This is the latest explanation. We revised it to account for updates made to the study by the researchers. See revisions.

This is a summary of "COVID-19 Antibody Seroprevalence in Santa Clara County, California"


What percent of Santa Clara County, California has been infected by SARS-CoV-2, the virus that causes COVID-19?

In this updated version, more modest claims from the researchers put the estimated rate at between 1.3-4.7%, which seems in line with other studies, though there are still ongoing concerns about the methods.

Key takeaways

  1. This updated version of the study makes more reasonable claims than the first, and their estimate of 1.3-4.7% seems in line with other studies that have come out. These findings should not be extrapolated to other parts of the country like New York.
  2. There are still concerns about how participants were selected and the accuracy of the test used, and the researchers acknowledge this uncertainty.
  3. Figuring out what percentage of the population has had COVID-19 remains vitally important, and there are many ongoing studies both in California and across the world that will help answer this question.

Why is this question important?

The question is important for two reasons:

  1. We don’t really know how many people have been exposed to the coronavirus. Due to a shortage of tests, many people with symptoms or suspected cases of coronavirus have not been tested, so the number of positive coronavirus tests probably underestimates the number of people who have had the virus. This can help decision makers understand how many people have the virus, which helps them make decisions about controlling its spread.
  2. How dangerous is coronavirus? This reason is related to the first. Since we don’t know how many people are infected, we can’t accurately calculate how common it is to suffer serious symptoms or die from the virus. This is really important for us to understand so that we can understand the risks of the virus and also try and prepare our hospitals for the number of people who might need medical help.

Getting more information about these areas would be helpful for public health experts, epidemiologists, hospitals, and political leaders to implement the correct actions to control the pandemic.

Our take

The first version of the study had come under heavy criticism from other researchers based on the way they tested for antibodies and the statistical techniques they used to arrive at their final estimate, so let’s take a look at the strengths and weaknesses of the updated version.


This was an ambitious study that was among the first of its kind in the United States. It provides a template for future researchers to improve upon. In the updated version of the study, the researchers acknowledged more of the flaws and limitations to their findings. Their estimated percentage (1.3-4.7%) of the population that has had COVID-19 seems more in-line with their data and the results of other studies in LA, New York, and other locations.


A full discussion of the study and its analysis could fill a book chapter. We discuss three main areas of concern below.

What did the study do?

How was it reported?

The original paper is a preprint study. It has not been certified by peer review from other researchers, and information presented may be erroneous. Do not use it to guide clinical practice! Learn more →

Original Paper DOI10.1101/2020.04.14.20062463

COVID-19 Antibody Seroprevalence in Santa Clara County, California [PDF]

Additional Reading

For more details and technical discussion of the study, we recommend reading the following resources:


  1. Source: SF Chronicle
  2. Source: Wired
  3. Source: Santa Clara County


The study authors revised their paper after posting it. We have explanations for the following revisions:

Updates and Corrections

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