CDC is collaborating with Vitalant Research Institute, American Red Cross, and Westat Inc. to conduct a nationwide COVID-19 seroprevalence survey of blood donors. De-identified blood samples are tested for antibodies to SARS-CoV-2 to better understand the percentage of people in the United States who have antibodies against SARS-CoV-2 (the virus that causes COVID-19) and to track how this percentage changes over time. Both SARS-CoV-2 infection and COVID-19 vaccines currently used in the United States result in production of anti-spike (anti-S) antibodies but only infection results in production of anti-nucleocapsid (anti-N) antibodies. Infection-induced seroprevalence estimates the proportion of the population with antibody evidence of previous SARS-CoV-2 infection and refers to the percent of the population with anti-nucleocapsid antibodies. Combined infection-Induced and Vaccination-Induced seroprevalence estimates the proportion of the population with antibody evidence of previous SARS-CoV-2 infection, COVID-19 vaccination, or both, and refers to the percent of the population that has anti-spike antibodies, anti-nucleocapsid antibodies, or both.