The Maricopa County Department of Public Health announced Nov. 16 that an estimated 10.7% of county residents have detectable antibodies for COVID-19—translating to roughly 470,000 people in Maricopa County who have likely been infected with the virus since the pandemic began.

The results stem from an 11-day study conducted in mid-September by the Maricopa County Department of Public Health in partnership with Arizona State University and Mayo Clinic thtat collected specimens from 260 participants in 169 households randomly selected from across the county to test for antibodies for the virus that causes COVID-19, according to a news release from the county.

"MCDPH and ASU worked with the Centers for Disease Control and Prevention (CDC) to implement a method that samples a small number of households in randomly selected communities that, when combined, represent all of Maricopa County," read the release. "The CDC gave MCDPH and ASU a list of 29 communities that are representative of the entire county."

Key findings from the serosurvey, according to the news release, include:

  • For every case reported to the public health department, there were three to four cases that were not reported up to mid-September, when the serosurvey was conducted.

  • The number of true infections is likely far higher than the number of people who are being tested for COVID-19.

  • There was a higher seroprevalence within households than across individuals, which supports evidence that infections cluster within households.


"We know that sustained close contact drives the majority of infections," said Marcy Flanagan, the executive director of the public health department, in a news release. "The serosurvey findings emphasize the importance of separating household members once someone is diagnosed with COVID-19 and in isolation at home."

Flanagan said in the news release that the percentage of the Maricopa County population with antibodies is "far less than is needed to reach herd immunity," which is the point at which the virus cannot spread effectively.


“We estimate that somewhere between 40% and 80% of the population would need to be vaccinated with the COVID-19 vaccine to reach herd immunity,” Flanagan said. “It’s critical that we all wear face masks correctly and consistently, physically distance from others, wash our hands and avoid large groups of people to help slow the spread of COVID-19.”