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Is San Diego's contact tracing working? Why it's hard to tell from the data

County's metrics may have blind spots
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Posted at 4:43 PM, Jun 30, 2020
and last updated 2020-06-30 20:58:01-04

SAN DIEGO (KGTV) — San Diego County health officials say the county’s contact tracing program has not been hampered by the same issues plaguing efforts in other areas like New York City, but there may be blind spots in the county’s data.

One of the keys to successful contact tracing is eliciting a full list of close contacts from an individual who tests positive. A “close contact” is defined as someone who was within 6 feet of an infected person for at least 15 minutes, beginning 48 hours before illness appeared and lasting until the patient was isolated.

It’s sensitive work, but the breadth of that list is critical to suppression efforts. “By identifying or ‘tracing’ the contacts of people infected with COVID-19, we can identify other individuals who might be infected, test those individuals for infection, treat those who are also infected and trace their contacts as well,” the county says on its website.

However, in New York City, only 35% of the residents who tested positive actually provided information about their close contacts to tracers during the first two weeks of June, the New York Times reported.

That means that even if tracers were able to reach virtually all of the individuals on their list, they would still likely have a vastly incomplete picture of those who may be at risk.

That’s where San Diego’s first potential blind spot comes in.

Unlike New York City, San Diego County is not actively monitoring the percentage of individuals who provide information on close contacts to tracers, County Medical Director Dr. Eric McDonald said Monday.

“We don’t specifically follow that metric,” he said. “We think we’re probably doing better than the numbers you hear from New York. It’s difficult though for us to drill down on that specifically.”

Instead, the county bases one of its triggers on attempts to reach the close contacts it has on file, regardless of how complete or incomplete that list may be, and regardless of whether tracers actually got in touch with those individuals -- a second potential blind spot.

McDonald acknowledged that tracers are often given out-of-date or inaccurate contact information, and must rely on public records and other sources to try to reach out to close contacts.

As of Monday, the county had attempted to contact 87% of known close contacts within 24 hours, well above the county’s goal of 70%.

“I think we’re doing pretty well from the contact tracing perspective,” McDonald said of that metric.

“I would say the general gestalt from talking to our contact tracers is that most individuals are actually quite cooperative and do give as best information as they can about close contacts. And we really do appreciate that,” he added.

McDonald said that staff would reexamine the county’s database in light of the inquiry by ABC 10News, and on Tuesday a spokeswoman provided an update. “Approximately 60 percent of our cases identify one or more close contacts,” said communications officer Sarah Sweeney.

Assuming that every person who tests positive has at least one close contact, it means San Diego County disease detectives are placing calls to -- at most -- 52% of close contacts within the first 24 hours.

McDonald said while that tracers do not get close contact information from everyone, there are circumstances when a close contact is simply a household member and has no other unique close contacts to follow up on.

“Some close contacts don’t need to be identifying any other close contacts,” he said. “Overall, I would say that our cooperation is good to excellent.”

From May 4 to May 28, people who tested positive in the county identified an average of 2.2 close contacts, County Public Health Officer Dr. Wilma Wooten said at a Board of Supervisors meeting earlier this month.

Whether tracers actually reach those individuals is another story.

“I’m a person who thinks it would be great to have perfection: every single person telling us every single thing that they know. But I think that we are doing very well from the information we are getting from our close contact investigations,” McDonald said Tuesday.