COVID-19: Lessons from case investigations and contact tracing

While many cases have an unconfirmed origin, that doesn’t mean people aren’t at least somewhat sure where they caught the virus

CLARK COUNTY — Clark County set an unfortunate record over the weekend with 157 newly confirmed cases of COVID-19, reaching 3,956 cases since the start of the pandemic.

One other person also died after coming down with an infection, a woman in her 80s with underlying health conditions. She becomes the 63rd death blamed on the virus.

A computer model of the SARS-CoV-2 virus, which causes COVID-19. Image courtesy US Centers for Disease Control and Prevention
A computer model of the SARS-CoV-2 virus, which causes COVID-19. Image courtesy US Centers for Disease Control and Prevention

Clark County Public Health updates the rate of new cases per 100,000 people usually on Tuesdays, though the rate of new cases indicates that number is likely to surpass its current level of 95.60 cases.

As of Monday there were 24 people in Clark County hospitals with a confirmed case. Fourteen other people were hospitalized with a presumed case, but still awaiting test results.

According to Clark County Public Health, there are currently 134 active cases, defined as people who are still within a 10-day window of either symptom onset, or a positive test, if they do not exhibit symptoms.

It is worth noting that testing delays can lead to people learning they were infected shortly before they complete their isolation period, which is why active cases are less than recent test results.

In response to the growth in new cases, Clark County is working with The Public Health Institute of Oregon (PHI) to do contact notification. Case investigations are done by Clark County Public Health nurses, who then give close contact information to PHI teams.

Currently, Clark County has 40 PHI employees under contract in five teams of eight people each.

That work has resulted in data since early July on where people are most likely to become infected.

According to case investigation data released by CCPH covering July through September, a total of 2,294 cases were interviewed. Of those, data was unavailable for 538 cases, or 23 percent. That could mean the individual could not be contacted after multiple attempts, declined to be interviewed, or was simply not asked where they may have contracted the virus. 

“There could be a variety of different reasons why we don’t have data on a specific person,” said Marissa Armstrong, communications director for Clark County Public Health. “In some situations we may be speaking to a caregiver or a spouse, if the case is not well, or is hospitalized. So we may be speaking with someone on their behalf, who doesn’t have that information.”

In another 27 percent of cases, no exposure site could be confirmed, according to the county’s data.

That doesn’t mean the individual being interviewed has no idea of where they might have been exposed, says Armstrong, just that the county couldn’t link their case to another known infection.

“We need to know that our current case had contact with a known case, while they were contagious at that location,” Armstrong told Clark County Today. “And that’s a pretty high threshold. We’re not just saying, ‘Oh, they were at this restaurant, so that must have been where they got it.’”

In the simplest terms, that means the county only confirms a case as being linked to a restaurant, grocery store, church, or social gathering if another known case emerges from that location or event.

One example cited by Armstrong is the Orchards Tap outbreak in late June and early July.

“The customers there, they did not have any other known exposure,” she says. “But they were all in the Orchards Tap. And in the Orchards Tap we also knew that there was a confirmed case at that location as well.”

In other cases, there may not be a known single individual at a location who has tested positive, but if several other cases link back to that place, investigators could be reasonably certain someone there was spreading the virus.

“We need to know that they were in contact with someone who is contagious at a location,” says Armstrong, “or we need to have multiple cases kind of tied to the same location at the same time, with no other known exposures.”

Failing that, the origin of a case is listed as “unknown,” until or unless more infections emerge linked back to a location.

Clark County Public Health released data on COVID-19 case investigations from July through September. Click to view PDF.

Of the roughly 2,300 cases investigated between July and September, public health was able to determine a likely infection source in 1,141 of them, or just under half.

Out of those, 45 percent of cases were transmitted through the home.

During the measles outbreak in 2019, 87 percent of cases were spread between members of the same household.

To public health officials, that means community transmission of COVID-19 is much more prevalent.

As for where that community spread is happening, nearly 28 percent of cases were linked to social gatherings, with the vast majority of those coming from small, private gatherings of 10 people or less.

In September, six percent of cases were linked to private social gatherings of between 11 and 49 people, up from four percent in July, and just two percent in August.

It should be noted that the data does not break down far enough to know how many cases could be linked to a single gathering, or a handful of them.

Long-term care facilities and healthcare settings were the next most likely place for COVID-19 transmission, with an average of 12 percent of cases linked to those types of locations during the three months, including 15 percent in August.

While a handful of cases could be linked to restaurants or grocery stores, the requirement for people to wear masks seems to generally be limited exposure for most people. 

Restaurants accounted for less than two percent of known COVID exposures, while grocery stores averaged the same, topping out at 12 cases in September.

The contact notification contract with Public Health Institute currently runs through October, but Armstrong said CCPH is planning to seek an extension through Dec. 30 from the Board of Public Health and the Clark County Council.