Senior epidemiologist explains community testing by CDHD

Submitted by Veronica Farias, Chelan-Douglas Health District
East Wenatchee, WA – Since mid-August, the Chelan-Douglas Health District has been conducting community COVID-19 testing, with drive-thru testing having been administered in 15 communities.
 
As of Oct. 9, more than 4,500 tests have been administered. There have been 77 positive test results, or about 1 percent positive cases.
 
Dr. Peter Houck, senior epidemiologist with CDHD, explained that the health district’s epidemiologists pay close attention to the actual number of positive tests in a community to look for possible outbreaks. The trick is to figure out what to worry about and what is of no concern, Houck said.
 
“The numbers can sometimes be misleading, because if we increase the number of tests being performed we are also likely to find more positives,” Houck said. “The harder we look the more we find, even if there is no outbreak and the actual number of infections in the community is not changing.”
 
For this reason, epidemiologists pay close attention to the percent of tests that are positive. A large increase in this number can be a stronger indicator that something unusual is going on, explained Houck, whose background includes serving as a pandemic specialist for the U.S. Centers for Disease Control and Prevention.
 
For example, the percent of tests that have been positive in Chelan and Douglas counties has been stable at about 1 percent, give or take a small amount, he said. But in the second week of September the percent of tests done in Leavenworth that were positive increased suddenly to over 3 percent (see table below).
 
“Clearly, something unusual was happening,” Houck said. “Closer investigation found an outbreak with over 20 infections in workers at one agricultural workplace.”
 
Measures including isolation of infected people and quarantine of those who had been exposed to infection were put in place and the outbreak was quickly controlled, Houck said. The number of new infections being found in Leavenworth returned to its baseline, he added.
 
Sometimes, even an increase in the percent of tests that are positive can be a false alarm for an outbreak, Houck explained. For example, during one week the percent of tests that were collected in Peshastin that was positive increased suddenly to over 7 percent.
 
“This time, close investigation showed us that the increase in the positive percent was caused by only four positive tests among 53 total tests done,” Houck said. “The positive percent went up because so few tests were being performed. Epidemiologists refer to this as ‘rate instability.’ It can happen when there are random changes in the number of infections in a small group of people. There was no real outbreak and the number of cases returned to baseline on its own.”
 
There are ongoing planning efforts to set up more community testing sites throughout Chelan and Douglas Counties. The health district is looking for community partners who have significant outside open space available to set up drive-thru community testing sites.
 
If you are interested in helping us, please contact the Incident Management Team – Operations Section at imt.osc@cdhd.wa.gov; the team will evaluate the site to see if it fits the needs for a testing site.
 

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