One in 10 residents in Maricopa County, Arizona, has had coronavirus, according to antibody testing done by the the county health department.
The results of the 11-day serosurvey conducted by the Maricopa County Department of Public Health, Arizona State University and the Centers for Disease Control and Prevention found that 470,000 residents had been infected with COVID-19, the health department reported this week.
Serology tests, which analyze patients' blood for antibodies, give health officials a better understanding of how the virus is spreading in a community over time. The tests provide information about previous infections in individuals who may have had few symptoms or been asymptomatic.
For every COVID-19 case that was reported in Maricopa County, officials estimate that there were up to four times as many cases that went unreported, meaning the county's true infection rate is likely higher than previously thought, ABC News affiliate KNXV-TV in Phoenix reported.
However, 10.7% of the county having coronavirus antibodies is not a high enough threshold to offer what's known as herd immunity, which is when the virus can't find new people to infect because enough people are immune to it.
"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,” Marcy Flanagan, executive director of the county health department said in a statement.
"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,” she said.
The testing also found that close contacts, not strangers, drove the majority of infections. The findings emphasize how important it is to separate household members once someone receives a COVID-19 diagnosis and is in isolation at home, Flanagan added.
Maricopa County, which encompasses the city of Phoenix, has reported the most COVID-19 cases of any county in the state. As of Wednesday, Arizona had reported 283,102 infections and 6,365 deaths, according to the Arizona Department of Health Services.
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