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

​​​​Note: ​CDPH is no longer updating seroprevalence data. As vaccination rates increase in California, antibody test volume has decreased. Additionally, blood banks have stopped routine testing of donors. As a result, CDPH no longer has data to make accurate weekly seroprevalence estimates. This page is for historical purposes only. 

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 (PDF) 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 — California's COVID-19 Antibody Study (2021-2022)

CalScope is a population-based serosurvey that helps the California Department of Public Health (CDPH) learn how many Californians have antibodies to the virus that causes COVID-19, either through infection, vaccination, or both, and how this may change over time. This information helps CDPH understand COVID-19 disease burden and will help design more effective and efficient mitigation strategies.

CalScope Ad: CalScope will help us learn more about COVID-19 in CaliforniaThrough CalScope, households in selected counties were 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 can estimate how seroprevalence varies throughout California.

​​The CalScope serosurvey took place in three separate waves during 2021-2022. Wave 1 was conducted from May-June 2021. Wave 2 was conducted from October 2021-February 2022, and Wave 3 was conducted from April-August 2022. As of July 29, 2022, participant enrollment in CalScope has ended.

Have questions about CalScope? Contact

Reports and Publications


U.S. Centers for Disease Control and Prevention (CDC)

California Department of Public Health (CDPH)

The following CalScope resources were created by CDPH for local partners in participating counties (Alameda, El Dorado, Kern, Los Angeles, Monterey, San Diego, and Shasta). 

CalScope Fact Sheet for Local Partners - overview of study

CalScope FAQ - for agencies to respond to questions from community members

CalScope Flyers

Outreach Flyer

Why Calscope Is Important Flyer

CalScope Infographic - with steps for participation

*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.

Originally published on February 11, 2022