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Study shows COVID-19 often undetected in Maricopa County

A group of people confer around a map on a large screen at Sun Devil Stadium during the Maricopa County Serosurvey.
November 17, 2020

COVID-19 cases in Maricopa County are three to four times higher than testing efforts indicate, according to a recent antibody study.

The seroprevalence study, or Serosurvey, was a joint effort by the Maricopa County Department of Public Health, Mayo Clinic and Arizona State University to estimate the prevalence of COVID-19 antibodies in the community. Based on their results, they estimate that 10.7% of residents — approximately 470,000 people — have had a past infection with the SARS-CoV-2 virus. 

From Sept. 12 to Oct. 1, volunteers collected blood samples from 169 households throughout Maricopa County. The households were carefully selected to provide a sample that represents the larger population, using a Centers for Disease Control and Prevention technique called Community Assessment for Public Health Emergency Response (CASPER). 

Mayo Clinic tested the samples for antibodies, which are special proteins created by our bodies in response to different germs. If a participant tested positive for antibodies, it was highly likely they were infected with the coronavirus in the past. Antibodies can be detected one to three weeks after a person becomes infected. 

Where we are and the way forward

Maricopa County’s seroprevalence rate is second only to New York’s reported 14%. However, comparing results to serosurveys from other counties and cities is difficult.  

“There aren't many true comparisons,” said Megan Jehn, an infectious disease epidemiologist and associate professor in ASU’s School of Human Evolution and Social Change, who helped analyze the data. “Most of the published serosurveys were conducted early in the pandemic, around April. Also, we took great care in selecting a representative sample of population in Maricopa County, while most other serosurveys relied on convenience samples.”

Convenience samples include people sampled because they are easy to reach or “convenient,” such as someone who goes to a specific hospital or clinic. They are not randomly selected and therefore don’t accurately represent the whole population. For example, they don’t include people without access to health care. The Maricopa County Serosurvey requested samples from randomly selected households in the county to provide a more accurate estimate of infections throughout the community. 

“That fact that our seroprevalence is similar to what New York saw shows a lot of community spread,” said Marcy Flanagan, executive director of Maricopa County Department of Public Health. “This reinforces that we need to employ these tried and true mitigation methods — mask wearing, social distancing and hand sanitation — to limit spread.”

A woman in full personal protective gear indicates the site of blood draw on a man's arm.

A volunteer with the Maricopa County Serosurvey prepares a resident for a blood draw. Free antibody testing was offered to residents as part of the serosurvey in order to gain a more complete picture of COVID-19 infections in the county. Photo by Andy DeLisle

Another method for combatting COVID-19 will be vaccination. Maricopa County Department of Public Health estimates that 40%–80% of the population will need to be vaccinated to reach herd immunity. Herd immunity occurs when a large enough portion of the population becomes immune to a disease — either through natural infection or a vaccine — to prevent it from spreading widely. Usually, between 50% and 90% of a population needs to be immune to provide herd immunity, depending on how contagious the disease is. While Maricopa County has had more infections than expected, the population is still a long way from herd immunity.

“The take-home point is that the majority of the population is still at risk,” Jehn said.

Vaccinating will be especially important because we don’t know how long COVID-19 antibodies last or how much immunity they confer against the virus, according to Erin Kaleta, director of Infectious Disease Serology and co-director of Clinical Chemistry at Mayo Clinic. Kaleta led the sample testing.

In addition to blood samples, the survey used a questionnaire to collect household-level information. It covered experiences with COVID-19 testing and quarantine, chronic illnesses, employment and access to health care as well as knowledge, attitudes and household practices related to COVID-19.

While the survey also captured demographic information, Jehn said that the study was not designed to report subgroup-specific seroprevalence estimates.

A dedicated group of volunteers

Jehn, who co-led the Maricopa County Serosurvey, said she’d never undertaken a project of this scale before. With only about 10 weeks of preparation, its success rested on an army of volunteers willing to knock on doors at all hours of the day to offer free antibody tests to an often-skeptical public. Volunteers included nurses, doctors and public health workers, as well as a multitude of ASU students in nursing, global health and social work programs.

Operating out of Sun Devil Stadium in Tempe, volunteers fanned out to the 29 clusters throughout the Valley, visiting neighborhoods at different times of day and days of the week to create the most representative population sample possible. Volunteers braved 110-degree temperatures, lugging equipment from house to house in full personal protective gear.

“I was really impressed with our volunteers,” Jehn said. “They never quit. I had students return to Sun Devil Stadium late at night after a 12-hour shift and two samples short — and they would head right back out to try and hit our target.”

“They totally stepped up,” added Heather Ross, a clinical assistant professor in ASU’s Edson College of Nursing and Health Innovation and School for the Future of Innovation in Society. “Most of them had not done field epidemiology work before, and nothing phased them. They dealt with rejection, but they were incredibly successful at getting people to participate.”

Ultimately, the project attained 80% of the targeted household enrollment, enough to achieve a valid sample.

Volunteers with the Maricopa County Serosurvey greet a resident.

From Sept. 12 to Oct. 1, volunteers visited 29 randomly-selected clusters throughout Maricopa County, braving triple-digit temperatures at all times of the day to gain a representative sample of 169 households. The volunteer staff consisted of public health and health care professionals, as well as ASU students from nursing, social work and public health programs. Photo by Andy DeLisle

Valuable for education, public health and the community

The Maricopa County Serosurvey also provided a one-of-a-kind learning experience for students volunteering, particularly those in the nursing program. 

Often, health care practitioners interact with patients solely in a clinical setting. These patients have access to health care and have chosen to seek it out. Field work such as the serosurvey allows students to engage with the community on different terms.

“Nursing, like our mission at ASU, is a fundamental responsibility to care for the communities that we inhabit,” Ross said. “It's critically important to understand what it means to go out to the community rather than to require the community to come into you.”

Ross volunteered as field faculty during the survey, overseeing nursing student volunteers as they went door-to-door and providing support where needed. She ensured that samples were labeled correctly, aided with blood draws and spent time centrifuging samples in the mobile lab.

Amanda Smith, a registered nurse studying for her doctorate of nursing practice in the Edson College, was one of the student volunteers. She said it was important to take advantage of every opportunity to gain experience and round out her practice. She also noted how important experiences like the serosurvey are for students just beginning their nursing careers.

“For the same reason we have different rotations in nursing school — community health, psych or home health — these experiences show several different avenues that you can pursue in nursing. The more exposure you get to different kinds of nursing, the better nurse you will be. It enables you to find your calling, and to make an impact,” she said.

Smith was working in a hospital treating COVID-19 patients before leaving to focus on school full time. Even now, she’s working with recovering COVID-19 patients as part of her curriculum. Though the serosurvey was not her first COVID-19 assignment, she said the work was gratifying, as she was contributing to a large-scale effort that could affect public health. 

“It pushes the needle in the right direction,” she said. “We need more information to understand how COVID-19 is affecting the community. And when we understand how it affects the community, we're better able to handle the pandemic.”

Jehn thinks that projects such as the Maricopa County Serosurvey shine a light on the importance of public health work overall.

“So much of the work we do in public health is invisible until a disaster occurs,” Jehn said. “Projects like this allow the community to have their voices heard and to see hard-working public health teams in action.”

Top photo: Maricopa County Serosurvey staff at Sun Devil Stadium review strategies for recruiting households for the study, which ran from Sept. 12 to Oct. 1. The study ultimately achieved 80% of the targeted household enrollment, enough for a valid sample of the entire county. Photo by Sandra Leander

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