Washoe study: Bigger tally of past infections
Results of a first-of-its kind study in Nevada indicate that about
2.3 percent of adult residents in Washoe County have been infected with the coronavirus. That is roughly five times the number diagnosed through nasal-swab testing.
Unlike a nasal-swab diagnostic test, which tells whether a person is currently infected by COVID-19, an antibody test detects proteins in the blood created by the immune system
that indicate a previous infection.
By identifying past infection, including in people who had no symptoms, antibody testing can help create a clearer picture of how many people have been infected.
The study helps identify cases beyond the “tip of the iceberg” findings of diagnostic testing, said Heather Kerwin, epidemiology program manager for the Washoe County Health District.
The study, conducted by state and county public health authorities, included 1,270 randomly selected households from 128 unique census blocks in Washoe County. There were 234 adult participants who provided a blood sample for a COVID-19 antibody test on June 9 or 10.
By June 10, 1,832 adults and children in Washoe County had tested positive through diagnostic testing for the virus. The study estimates that about 8,230 adults may actually have been infected.
No ‘herd immunity’
The study also gives an indication of what percentage of the population might have some level of immunity to the disease.
With just 2.3 percent of the county’s adult population estimated to have antibodies, Washoe County is “very far from being anywhere near herd immunity, which we knew inherently, but it’s nice to be able to back that up with actual data,” Kerwin said during a Wednesday video briefing with reporters.
The Centers for Disease Control and Prevention previously has estimated that in communities across the country, less than 5 percent to as high as 25 percent of their populations have antibodies to the virus.
Death rate
Antibody studies also allow public health authorities to estimate more accurately a disease’s fatality rate.
As of June 10, there had been 67 reported Covid-19-related deaths in Washoe County. The fatality rate based on individuals testing positive for the disease is 3.66. But because many more people were actually infected, the fatality rate from the disease is about 0.81 percent, according to the study.
An antibody study conducted in
Los Angeles County suggested that 40 times as many people had been infected there than the number of confirmed cases, lowering the estimated fatality rate to between 0.1 and 0.2 percent.
Antibody testing results in New York City indicated that as many as one in five residents had been infected, suggesting a fatality rate of 0.5 percent or, if factoring in probable COVID-19 deaths, 0.8 percent.
The death rate from the flu is about 0.1 percent.
The study released Wednesday was conducted by the Washoe County Health District, University of Nevada, Reno’s School of Community Health Sciences and the Nevada State Public Health Laboratory.
Future studies
Another coronavirus antibody study is about to be launched at a community health center in Reno and yet another in August at the
UNR School of Medicine, said Mark Pandori, director of the Nevada State Public Health Laboratory in Reno.
The lab has not been asked to participate in any studies in Southern Nevada, he said.
The Southern Nevada Health District is planning to do antibody surveys when additional funding from the CDC is received.
Epidemiologist Brian Labus, a member of Gov. Steve Sisolak’s medical advisory team, said he believes that the percentage of people in Southern Nevada who have been infected also would be low.
“There’s no reason to think it would be any different in Southern Nevada” than in Northern Nevada, said Labus, an assistant professor in UNLV’S School of Public Health. “Southern Nevada was not on the leading edge of this outbreak, like New York City.”