More than a third of people who have been infected with the new coronavirus in the last 60 days likely caught it within their households, according to a new review of coronavirus exposures by Public Health – Seattle & King County.

The proportion of cases linked to households, social gatherings and workplaces other than health care has risen in recent months.

The review, which includes data from 37,482 cases, from Feb. 28 to Nov. 20, provides the most detailed view to date of how the virus is spreading in King County and how the patterns of transmission likely have changed over the pandemic’s course. The report also highlights data that describe the disease’s disproportionate impact on communities of color. 

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The report relies on data derived from outbreak investigations and contact-tracing interviews with people infected with the coronavirus. The health department was able to determine the most likely source of exposure or link to an outbreak in 27,172 cases, or 72%.

Patterns of transmission appear to have shifted.

In the early months of the pandemic, the most common source of exposure was in health care settings, mostly among those who lived or worked in long-term care facilities.

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But household transmission, social activities and workplaces other than in health care are now the more common sources of the spread, the data suggest. And by the time the virus is within households, it’s often too late, said Dr. Jeff Duchin, health officer with Public Health – Seattle & King County.

“This disease spreads so rapidly. The generation time is so quick. In most cases, by the time we reach a household, the household members might already be symptomatic and infected,” Duchin said.

Duchin said the county each day is counting nine times as many cases as it did in late September, with hospitalizations for COVID-19 increasing rapidly.

King County reported 17,162 positive tests, 649 hospitalizations and 77 deaths from Sept. 22 through Nov. 20, according to the data that underlies the county’s COVID-19 dashboard.

“We need to get real, with a serious all-of-the-community effort right now to prevent personal health and health-care-system catastrophe,” Duchin said, warning that the area might be weeks away from hospitals not being able to provide everyone in their care the lifesaving treatment they need.

The report provides more evidence of the disproportionate impact of the virus on communities of color. Matias Valenzuela, Public Health’s equity director, said systemic inequities and structural racism in housing and access to health care and wealth make these communities more vulnerable. 

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The new data show that Hispanic / Latino residents had the highest proportion of cases linked to workplaces not related to health care. Black residents had the highest proportion of cases within health care settings, and more than half of those cases were among health care workers.

Overall, 29% of cases are among essential workers, Duchin said.

Valenzuela said communities of color have a higher proportion of essential workers and often live in larger households.

“Where and how people live and work — workplaces and households — are important factors,” Valenzuela said. “Not everybody has the same options to limit exposure.”

During the past 60 days, white people were more often exposed during social gatherings — events that are likely optional.

Many cases can’t be tied to specific events.

Since February, about two-thirds of COVID-19 patients who were interviewed by Public Health had close contact with an infected person or could be linked to an outbreak. The others did not know they had been exposed and were not linked to an outbreak.

Since late February, Public Health has identified 836 outbreaks of coronavirus, including 27 outbreaks at local hospitals and 83 in the food-service industry. About one-quarter of outbreaks took place in long-term care facilities, which were hard hit early in the pandemic.

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Weekly testing of staffers in those facilities helped quell outbreaks over the summer, but Duchin said he is concerned that widespread transmission will drive up cases among the most vulnerable.

“It is inevitable workers will introduce COVID into their workplaces,” Duchin said, adding that screening processes are not perfect. “With the sheer degree of transmission happening in our community, we should expect COVID to be introduced anywhere and everywhere we go.”

Outbreak and exposure data are inherently messy, particularly when community transmission is so widespread. Some people don’t remember or divulge contacts or possible exposures, some choose not to be interviewed, some offer information on multiple possible exposures. Some workplaces might not report outbreaks as readily as others. As King County’s caseloads rise to between 800 and 900 each day, Duchin said November data remains incomplete.

“We’re overwhelmed,” Duchin said. “We can ask more thoughtful questions when we have fewer cases and investigate more thoroughly.”

Moreover, broad changes over the course of the pandemic — such as testing access and restrictions on allowed activities — could color the story the data tell.

Statewide, COVID-19 cases and hospitalizations continue to climb rapidly, and modelers estimate that every person passes the disease to about 1.5 others, on average, said Dr. Kathy Lofy, the state’s health officer. Cases are increasing in every age range.

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Hospitals in every region of the state were caring for more COVID-19 patients than they have in months, said Lofy during a Wednesday briefing. Hospitals reported 932 COVID-19 patients on Nov. 23, the most recent day data were available on the state’s COVID-19 risk assessment dashboard. A month ago, that number was 405.

Lofy said she had been looking for early signs that restrictions implemented by Gov. Jay Inslee a week ago were having an effect but said she was “not seeing a lot of positive signals in our data.”

Meantime, the state announced plans to add another 350 case investigators and contact tracers over the next two weeks to handle the increase in case volume statewide. These new employees will work remotely.

The Centers for Disease Control and Prevention has asked states to focus contact tracing efforts on family connections, congregate settings like long-term care facilities and large, super-spreader events, said Dr. Scott Lindquist, state epidemiologist for communicable diseases.

An earlier version of this story misstated Dr. Jeff Duchin’s description of the virus’ spread. Duchin said we are seeing in King County nine times as many cases each day now than in late September.

(Jennifer Luxton / The Seattle Times)

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