More than one in 10 Maricopa County residents have been infected with COVID-19, according to a countywide study.
The study, conducted in mid-September, found that about 10.7% of residents have detectable antibodies for COVID-19, meaning about 470,000 Maricopa County residents have likely been infected. Only about 177,000 cases have been reported countywide from test results.
This means that the number of actual COVID-19 infections is far greater than the number of people being tested for the virus. For every case reported to county public health through mid-September, three or four additional cases were not reported, according to the county.
Antibodies were found more often within households rather than across individuals, supporting evidence that infections occur at the household level.
Marcy Flanagan, the county’s public health director, said the study emphasizes that “sustained close contact” drives most infections, so household members should be separated when someone is diagnosed and isolating at home.
The 11-day “serosurvey” collected blood samples from 260 individuals in 169 households from randomly selected neighborhoods representative of the entire county. The blood tests searched for COVID-19 antibodies, which are disease-specific blood proteins built during an immune response to fight an infection. The study initially planned to survey 500 individuals, but Flanagan said enough households and neighborhoods were surveyed to get a 95% confidence interval.
The county public health department worked with Arizona State University to collect the samples and with Mayo Clinic to process the samples. The Centers for Disease Control and Prevention selected the neighborhood sample that would represent the county.
What do the results mean for herd immunity?
Having COVID-19 antibodies means an individual likely has had COVID-19 in the past, but it’s not clear how long and how much antibodies provide immunity against reinfection. Antibodies are likely to provide immunity, at least for a period of time, research suggests.
A University of Arizona study of nearly 6,000 people published in October in the journal Immunity found that immunity to COVID-19 persists for at least several months after being infected with SARS-CoV-2, the virus that causes COVID-19.
The UA antibody study found “high-quality” antibodies still being produced five to seven months after SARS-CoV-2 infection.
The nearly 11% infection rate in Maricopa County is still far below what’s needed to achieve herd immunity, according to Flanagan.
“When you look at us reaching herd immunity this way, it’s really long and kind of painful. It results in a lot of very sick people and hospitalizations,” she said.
“From the beginning of the pandemic at the end of January all the way to September when we did this, it feels like a lot, a lot of people in Maricopa County have been sick with COVID, when the results are showing us it’s about 10.7%. That means there’s still a lot of individuals that haven’t been infected — the vast majority of our population is still able to be infected with COVID-19 if they’re exposed.”
Flanagan said a vaccine will be the best way for the county to hit herd immunity. She said between 40% and 80% of the population would need to be vaccinated in order to reach herd immunity.
Flanagan said the serosurvey results indicate that continued mitigation efforts, as well as getting the vaccine once it’s available, are critical.
“Because there’s so many of us that are still at risk of infection, we need to have everyone continue to take all the preventative measures to protect themselves that we’ve been saying like a broken record for the past ten months or so.”
“Wash your hands frequently, use hand sanitizer if it’s not available, practice social distancing, wear a mask when you’re out in public or unable to obtain the six feet of distance, stay home if you are sick and really limit your social gatherings,” Flanagan said.
Will Humble, executive director of the Arizona Public Health Association, said the serosurvey results were about what he would have expected, and possibly on the low end.
“Actually, I probably would expect it to be slightly higher given the mess we were in over the summer and the lack of testing and then super slow turnaround times,” he said.
Humble said progress on vaccines shows “light at the end of the tunnel.” Humble anticipates that about 75% of the population will need to get vaccinated to hit herd immunity, even with a vaccine that’s 90% effective.
Nationwide study shows 9% infected
A Stanford School of Medicine study published Sept. 25 in The Lancet, a prominent medical journal, estimated about 9% of people nationwide had been infected with coronavirus during the first wave of the pandemic in the U.S.
The Stanford findings were based on antibody levels from a cross-section of more than 28,000 patients on dialysis across 46 states in July.
The study found variations depending on income, ethnicity, population density and state. It found that 33.6% of the population of New York had antibodies, whereas that figure was 17.5% in Illinois. In Pennsylvania, it was 6.4%, and in California, it was 3.8%
The study’s results showed disproportionately high antibody levels in Hispanic and Black populations — 16.3% — compared with 4.8% of the white population. It also found that living in densely populated areas puts people at 10 times greater risk for COVID-19 than living in low-density areas.
“With this survey, we were able to provide a very rich picture of the first wave of the COVID-19 outbreak in the U.S. that can hopefully help inform strategies to curb the epidemic moving forward by targeting vulnerable populations,” the study’s lead author, Dr. Shuchi Anand, said in a prepared statement when the study was released. “This study also showed a higher prevalence of undiagnosed cases consistent with other studies.”
Maricopa County did not immediately disclose Monday how its survey’s findings broke down along race and ethnicity.
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