COVID-19 Test Results Undercount Nevada Case Numbers

July 9, 2020

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Source: Las Vegas Sun, by Scott Sonner

The first coronavirus antibody study conducted in Nevada suggests COVID-19 test results dramatically underestimate the number of people who actually have been infected by the disease, health officials said Wednesday.

The joint study by the Washoe County Health District and researchers at the University of Nevada, Reno concludes that four to five times more people likely have been infected with the virus that causes COVID-19 than have been formally confirmed as positive cases to date in the Reno-Sparks area.

Based on blood tests for antibodies, the study conducted last month indicates 2.3% of adults in Washoe County had an infection or exposure resulting in an immune response to the virus.

That means that while only about 1,800 cases of COVID-19 had been confirmed as positive in Washoe County as of June 10 based on nasal swab testing, the real number of adult infections in the county was probably closer to 8,200.

“We have more asymptomatic residents than we thought,” said Dr. Wei Yang, a professor of epidemiology, biostatistics and environmental health at the university. “Lots of people who have the disease are not symptomatic, but they still can spread the disease to high-risk people.”

Heather Kerwin, the epidemiology program manager at the county health district, said the number of positive cases the county and state report on a daily basis are still an accurate reflection of the number of people who formally have been diagnosed with COVID-19.

“It’s not our numbers are wrong,” she said. “But by looking at antibody prevalence in a population, you can extrapolate that number back to the entire population and say this is how many estimated number of cases we think have actually happened to date.”

While the study suggests the number of cases is up to five times higher than previously assumed, the coronavirus fatality rate is considerably lower than traditional testing suggests. Based on the new estimates, the fatality rate is about 0.8% — not 3.7%.

The study with a 95% confidence rate using a weighted formula began with the random selection of 1,270 households from within 128 unique census blocks in Washoe County. From those, 234 adult participants agreed to respond and provide a blood sample for the COVID-19 antibody tests conducted June 9-10.

Six of those 234 participants had markers for the antibody in their blood, which means that at some time they had contracted the virus. Only one of the six recalled feeling ill or showing any signs of symptoms of COVID-19.

“It definitely tells us there are more infections out there that either asymptomatic or so mild that people are not seeking testing. A lot of the disease goes unreported,” Kerwin said.

Washoe County Health District Officer Kevin Dick said Wednesday the study should serve as a sobering reminder that “we are nowhere near out of this.” He said that while the county currently has 1,073 confirmed active cases, the study suggests the true number of active cases is probably 5,000 or more.


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