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 for the virus that causes COVID-19, called SARS-CoV-2. People who have antibodies were either infected with COVID-19 at some point in the past or may have received a COVID-19 vaccine.

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 are designed to measure antibodies. In most cases, antibodies can be detected for at least several months after infection.

Antibody testing data that are reported to CDPH are collected from individuals for a variety of reasons and may not represent the general population of California. Seroprevalence increases over time as more people are infected or vaccinated.

Until vaccination coverage increases, some prevention measures will continue to be necessary:

  • Get vaccinated
  • Stay home if you are feeling sick and get tested for COVID-19
  • Follow all local health guidance

Seroprevalence by Region (05/02/21 - 05/29/21)

Region4-Week Average Seroprevalence*
Bay Area80.5
Central Coast70.3
Greater Sacramento73.7
Los Angeles73.3
Northern California54.4
San Joaquin Valley76.3
Southern Border

*Note: The data presented here are estimates and may differ from seroprevalence measurements collected in different ways.  Data will be updated every other Friday. Estimates include all antibody tests conducted during the four-week time frame in adults 18 and over. Data are presented as an average over the last month to provide an estimate of the current situation. The 95% confidence interval for statewide seroprevalence data between 05/02/21-05/29/21 was between 68.5% and 84.6%.

Counties by Region in California

California Counties by Region Map


  • Del Norte
  • Glenn
  • Humboldt
  • Lake
  • Lassen
  • Mendocino
  • Modoc
  • Shasta
  • Siskiyou
  • Tehama
  • Trinity

Bay Area

  • Alameda
  • Contra Costa
  • Marin
  • Monterey
  • Napa
  • San Francisco
  • San Mateo
  • Santa Clara
  • Santa Cruz
  • Solano
  • Sonoma

Greater Sacramento

  • Alpine
  • Amador
  • Butte
  • Colusa
  • El Dorado
  • Nevada
  • Placer
  • Plumas
  • Sacramento
  • Sierra
  • Sutter
  • Yolo
  • Yuba

​San Joaquin Valley

  • Calaveras
  • Fresno
  • Kern
  • Kings
  • Madera
  • Mariposa
  • Merced
  • San Benito
  • San Joaquin
  • Stanislaus
  • Tulare
  • Tuolumne


  • Inyo
  • Mono
  • Orange
  • Riverside
  • San Bernardino

Southern Border

  • Imperial
  • San Diego

Central Coast

  • San Luis Obispo
  • Santa Barbara
  • Ventura

Los Angeles

  • Los Angeles