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 more is known, and vaccination coverage increases, some prevention measures will continue to be necessary for all people, regardless of vaccination or antibody status. People should continue to:

  • Wear a mask over your nose and mouth
  • Stay at least 6 feet away from others
  • Avoid crowds
  • Avoid poorly ventilated spaces
  • Wash your hands often

Seroprevalence by Region (02/28/21 - 03/27/21)

Region4-Week Average Seroprevalence*
Bay Area40.5
Central Coast41.5
Greater Sacramento42.7
Los Angeles51.7
Northern California48.9
San Joaquin Valley51.1
Southern Border

*Note: The data presented here are an estimate and may differ from seroprevalences measurements collected in different ways.  Data will be updated monthly. Estimates include all antibody tests conducted in the preceding four weeks. 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 02/28/21-03/27/21 was between 38.3% and 54.7%.

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