Sero-prevalence-COVID-19-Data COVID-19 Seroprevalence Data

COVID-19 Seroprevalence Data

Seroprevalence measures the percentage of people in a population who have antibodies to an infectious agent (such as a virus) in their blood. Understanding seroprevalence for the virus that causes COVID-19 (called SARS-CoV-2) can help us understand how many people may have been exposed to this virus. People who have antibodies to COVID-19 have either had COVID-19 at some point in the past or may have received a COVID-19 vaccine, or both.

Antibodies are proteins made by a person's immune system that attach to a virus once it enters the body. Antibodies help the body's immune system find and kill the virus. Serology tests use a blood sample to measure the antibodies that a person has. In most cases, antibodies can be detected in blood for at least several months after infection.

Serology tests usually measure antibodies against two proteins from SARS-CoV-2—spike protein and nucleocapsid protein. People who have had COVID-19 will develop antibodies against both spike and nucleocapsid proteins. People who have received the COVID-19 vaccine will develop antibodies against spike protein only, because COVID-19 vaccines focus on the spike protein.

The percentage of people with antibodies changes over time as people are infected or vaccinated. Also, antibody levels can decrease over time after infection or vaccination. If serology tests do not detect antibodies, that does not necessarily mean that people are not protected. Antibodies are just one component of the body's response to infection or vaccination. Although people have antibodies to COVID-19 due to infection or vaccination, prevention measures are still necessary to keep people safe:

  • Get vaccinated and boosted if you are eligible
  • Stay home if you are feeling sick, and get tested for COVID-19
  • Wear a mask
  • Follow all local public health guidance


CalScope is a population-based seroprevalence study that will help the California Department of Public Health learn how many Californians have antibodies to the virus that causes COVID-19 and how this may change over time. Through CalScope, households in selected counties are randomly invited to take a brief, anonymous survey and a free, at-home COVID-19 antibody test (using a finger-prick blood sample). Counties participating in CalScope (Alameda, El Dorado, Kern, Los Angeles, Monterey, San Diego, and Shasta) were selected to represent different regions of California so that data from this study will better estimate how seroprevalence varies throughout California.

The CalScope seroprevalence study is taking place in three separate waves during 2021-2022. Wave 1 was conducted from May-June 2021. Wave 2 was conducted from October 2021-January 2022, and Wave 3 will occur in Spring 2022.

Additional results will be added as they become available.


*In the past, CDPH has collected information in other ways about antibodies in California from people who have had their blood tested for COVID-19. From March-June 2021, CDPH posted statewide seroprevalence estimates using antibody test data reported to CDPH by clinical laboratories and blood banks. As vaccination rates increased in California during this time period, fewer people were tested for antibodies. Additionally, blood banks stopped routine antibody testing of donors. As a result, CDPH no longer had sufficient data to produce monthly seroprevalence estimates using these data sources alone, so seroprevalence estimates were no longer posted.