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New serosurvey results show 1 in 10 Maricopa County residents have had COVID-19

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Posted at 2:18 PM, Nov 16, 2020

PHOENIX — A newly released serosurvey by the Maricopa County Department of Public Health (MCDPH) shows that approximately 470,000 county residents have been infected with COVID-19.

The serosurvey tests people's bloodstream for COVID-19 antibodies to give an idea of how widespread the virus has become in a community.

The testing offered several helpful takeaways, according to county officials. For every positive COVID-19 case reported to the county, officials estimate there are three to four times as many cases that were not reported, and the number of true infections is likely much higher than the number of people who are being tested.

Health officials say the percentage of Maricopa County population with antibodies, approximately 11%, is "far less than is needed to reach herd immunity." According to MCDPH Executive Director Marcy Flannagan, that concern highlights the need for slowing the spread, as well as people getting the vaccine as soon as it becomes available to the public.

“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 Flannagan.

Additionally, evidence suggests to officials that infections tend to cluster within households. "Findings emphasize the importance of separating household members once someone is diagnosed with COVID-19 and in isolation at home."

The 11-day serosurvey was conducted by the Maricopa County Department of Public Health, working with Arizona State University and the CDC, in mid-September.