A Multifaceted Evaluation of a COVID-19 Contact Tracing Program in King County, Washington

Abstract

Context: Despite the massive scale of COVID-19 case investigation and contact tracing (CI/CT) programs operating worldwide, the evidence supporting the intervention's public health impact is limited.

Objective: To evaluate the Public Health-Seattle & King County (PHSKC) CI/CT program, including its reach, timeliness, effect on isolation and quarantine (I&Q) adherence, and potential to mitigate pandemic-related hardships.

Design: This program evaluation used descriptive statistics to analyze surveillance records, case and contact interviews, referral records, and survey data provided by a sample of cases who had recently ended isolation.

Setting: The PHSKC is one of the largest governmental local health departments in the United States. It serves more than 2.2 million people who reside in Seattle and 38 other municipalities.

Participants: King County residents who were diagnosed with COVID-19 between July 2020 and June 2021.

Intervention: The PHSKC integrated COVID-19 CI/CT with prevention education and service provision.

Results: The PHSKC CI/CT team interviewed 42 900 cases (82% of cases eligible for CI/CT), a mean of 6.1 days after symptom onset and 3.4 days after SARS-CoV-2 testing. Cases disclosed the names and addresses of 10 817 unique worksites (mean = 0.8/interview) and 11 432 other recently visited locations (mean = 0.5/interview) and provided contact information for 62 987 household members (mean = 2.7/interview) and 14 398 nonhousehold contacts (mean = 0.3/interview). The CI/CT team helped arrange COVID-19 testing for 5650 contacts, facilitated grocery delivery for 7253 households, and referred 9127 households for financial assistance. End of I&Q Survey participants (n = 304, 54% of sampled) reported self-notifying an average of 4 nonhousehold contacts and 69% agreed that the information and referrals provided by the CI/CT team helped them stay in isolation.

Conclusions: In the 12-month evaluation period, CI/CT reached 42 611 households and identified thousands of exposure venues. The timing of CI/CT relative to infectiousness and difficulty eliciting nonhousehold contacts may have attenuated the intervention's effect. Through promotion of I&Q guidance and services, CI/CT can help mitigate pandemic-related hardships.

FIGURE 1

Depiction of COVID-19 Case Surveillance, Contact Tracing, and Response in King County, Washington, July 2020 to June 2021^{a a}After a patient underwent SARS-CoV-2 testing, his or her specimen was sent to a laboratory for analysis. As a notifiable condition in Washington State, laboratories and health providers are legally required to report COVID-19 suspected or confirmed cases to local or state public health authorities along with the following information: dates corresponding to test order, specimen collection, and availability of test results; test result; ordering provider details; laboratory details; and patient date of birth, sex, and address. Laboratories typically submit this information through the Washington Electronic Laboratory Reporting System (WELRS). The WADOH manages WELRS, processes data received through it and other sources, and creates case reports in the Washington Disease Reporting System (WDRS) that are subsequently assigned to CI/CT teams across the state for investigation. During the case interview, PHSKC contact tracers collected the following information: language preference, race, ethnicity, and affiliation with an immigrant community; symptom status and onset; reason for testing; vaccination status; suspected source of infection; participation in WA Notify exposure notification; and detailed information about places recently visited, including worksites, schools, daycares, health care and behavioral health facilities, long-term care facilities, correctional facilities, airports and other modes of transportation, bars/restaurants, gyms, places of worship, salons and spas, and community and social events. This information was subsequently analyzed and summarized for the public, public health decision makers, and outbreak response teams. Cases were also asked to provide the names, dates of birth, and COVID-19 symptom and testing status of each person in their household, as well as other persons they had been within 6 ft for at least 15 minutes during their infectious period. Finally, PHSKC contact tracers discussed the I&Q guidance tailored to the household's circumstance, assessed need for clinical and support services, and referred to support services team accordingly. This figure is available in color online (www.JPHMP.com).

FIGURE 2

Timing of Case Interview Relative to Inferred Duration of Infectiousness Period of SARS-CoV-2^{a a}The area under the dotted line represents the distribution of when case interview was conducted relative to onset of symptoms. The underlying pastel curves are reproduced from Byrne et al and represent SARS-CoV-2 viral shedding relative to symptom onset. Three curves are depicted as there remains uncertainty as to whether peak viral shedding occurs prior to, at, or after symptom onset, and how the temporality of infectiousness varies by SARS-CoV-2 variant. This figure is available in color online (www.JPHMP.com).

FIGURE 3

Number (Bars) and Percentage of Case-Households That Received Groceries or Were Referred for Financial Assistance by Social and Economic Risk Index of Residential (SERI) Census Tract Abbreviations: HH, households; SERI, Social and Economic Risk Index; SES, socioeconomic status. ^aThe COVID-19 relief funded one-time cash stipend and bill assistance programs operated in December 2020 and March-present, respectively. ^bThe bars and percentages are limited to HH with one or more COVID-19 cases, whereas the total number of HH includes all households received or were referred to service, including those in which only contacts resided. This figure is available in color online (www.JPHMP.com).