"Serosurvey” estimates that approximately one in 10 Maricopa County residents have been infected with COVID-19
PHOENIX (November 16, 2020) — Results from a serosurvey conducted across Maricopa County show that an estimated 10.7% of residents have detectable antibodies for COVID-19. This means that approximately 470,000 people in Maricopa County likely have been infected with the virus since the pandemic began.
The 11-day study, conducted in mid-September by Maricopa County Department of Public Health (MCDPH) in partnership with Arizona State University (ASU) and Mayo Clinic, 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.
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. The CDC gave MCDPH and ASU a list of 29 communities that are representative of the entire county.
Key findings from the serosurvey include:
- For every case reported to Public Health, 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.
Marcy Flanagan, executive director of MCDPH, said there are several key takeaways from the survey.
"We know that sustained close contact drives the majority of infections. The serosurvey findings emphasize the importance of separating household members once someone is diagnosed with COVID-19 and in isolation at home."
She said the percentage of Maricopa County population with antibodies—11%—is far less than is needed to reach herd immunity, which is the point at which the virus cannot spread effectively. This emphasizes the need for mitigation efforts as well as getting the COVID-19 vaccine when it becomes available.
“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,” said Flanagan. “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.”
As testing protocols and accessibility have developed greatly during the course of the pandemic, Flanagan also urges anyone who experiences COVID-like symptoms to get tested and, if positive, to take necessary precautions to stop the spread. Testing sites can be found at https://www.maricopa.gov/COVID19testing.