Changes in SARS-CoV-2 Positivity Rate in Outpatients in Seattle and Washington State, March 1–April 16, 2020

The first reported case of coronavirus disease 2019 (COVID-19) in the US occurred on January 20, 2020, in Snohomish County, Washington.¹ The University of Washington (UW) Virology Division was among the first US laboratories to test for severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), and since March 1, 2020, has tested samples from more than 73 000 patients. More than 90% of the samples are from the UW health system and outpatient clinics in Washington State.

The positivity rates for SARS-CoV-2 in outpatient settings in Washington State and in emergency departments (EDs) in Seattle were analyzed to identify temporal trends that may reflect the local dynamics of the pandemic and the effect of mitigation strategies such as physical distancing.

Samples were collected from 17 232 patients at 127 outpatient clinics across 10 counties and from 1932 patients at 3 Seattle EDs. Specimens (>95% were nasopharyngeal swabs) were analyzed by a laboratory-developed reverse transcriptase–polymerase chain reaction test using the E/RdRp primer set from the World Health Organization,² N1/N2 primer sets from the US Centers for Disease Control and Prevention,³ or tests from Hologic (Panther Fusion), DiaSorin (Simplexa), and Roche (cobas).

The SARS-CoV-2 positivity rate was analyzed by fitting penalized cubic regression splines to binomial testing data, and accounting for variation in the daily testing totals.⁴ The results were aligned to the sample collection date for the study period (March 1–April 16, 2020). When a patient had multiple results, only the first positive test was included. Limited demographic data were available from the referring facilities. The 95% CIs for the estimated SARS-CoV-2 positivity rates in the demographic subgroups were calculated by the Wilson score method.⁵

Comparisons between groups were made using a 2-sided χ² test with a significance level of .05. The statistical analyses were performed using R version 3.6.1 (R Foundation for Statistical Computing), with smoothed models fit using the mgcv package. This study was approved and granted a waiver of consent by the institutional review boards at UW and the Fred Hutchinson Cancer Research Center.

The positivity rates for SARS-CoV-2 were 8.2% in Washington State outpatient clinics, 8.4% in Seattle-area outpatient clinics, and 14.4% in Seattle EDs. The SARS-CoV-2 positivity rates were higher in males than in females (P < .001 for Washington State and Seattle-area outpatients; P = .13 for Seattle ED patients), and higher in Seattle EDs than in Seattle-area outpatient clinics (P < .001) (Table and Figure).

Nonlinear trends were observed in the estimated smooth curves for outpatients in Washington State (part A of the Figure) and outpatients in the Seattle area (part B of the Figure), with SARS-CoV-2 positivity rates peaking around March 28-29, 2020, and then declining for both populations. The trajectory in Seattle ED patients showed a similar time to peak, followed by a more gradual decline (part C of the Figure). The SARS-CoV-2 positivity rate was 17.6% in the outpatient clinics and 14.3% in EDs at the peak period and 3.8% and 9.8%, respectively, at the end of the analysis period. Testing volumes steadily increased for the first half of March and, for outpatients, peaked around March 12-13, 2020 (parts D-E of the Figure), with fewer samples collected on the weekends.

SARS-CoV-2 infections in patients seen in Washington outpatient clinics and Seattle ED settings peaked in late March and have been declining. This trajectory is aligned with local physical distancing guidelines (statewide shutdown of bars and restaurants; expanded social gathering limits enacted on March 16, 2020) and the “Stay Home, Stay Healthy” order announced by Governor Inslee on March 23, 2020.⁶

Testing criteria in Washington have not changed significantly during the study period, and testing remains largely limited to symptomatic persons. Testing volume in the study populations did not increase, likely due to shortages of sample collection materials, and the UW Virology Division has not reached maximum testing capacity since mid-March, so declining SARS-CoV-2 positivity is not attributed to expanded testing. This is not a population-based sample, and the mix of cases seen over time is not constant. The Seattle-area outpatient population analyzed is a subset of the Washington State outpatient population, and most of the samples were collected in the Northwest and South Puget Sound regions of Washington and therefore are not representative of the entire state.

These results suggest that the early and aggressive physical distancing measures enacted in Washington State have influenced the course of the COVID-19 pandemic. Whether adherence to physical distancing will continue and how that affects acquisition trends remain to be determined.

Corresponding Author: Keith R. Jerome, MD, PhD, Division of Vaccine and Infectious Disease, Fred Hutchinson Cancer Research Center, 1100 Fairview Ave N, E5-110, Seattle, WA 98109 (kjerome@fredhutch.org).

Accepted for Publication: April 29, 2020.

Published Online: May 8, 2020. doi:10.1001/jama.2020.8097

Author Contributions: Drs Corey and Jerome had full access to all the data in the study and take responsibility for the integrity of the data and the accuracy of the data analysis.

Concept and design: Randhawa, Greninger, Jerome.

Acquisition, analysis, or interpretation of data: All authors.

Drafting of the manuscript: Randhawa, Fisher, Greninger, Andriesen, Corey, Jerome.

Critical revision of the manuscript for important intellectual content: Randhawa, Greninger, Li, Andriesen, Corey, Jerome.

Statistical analysis: Fisher, Li.

Administrative, technical, or material support: Randhawa, Greninger, Andriesen, Corey, Jerome.

Supervision: Randhawa, Greninger, Andriesen, Corey, Jerome.

Conflict of Interest Disclosures: Dr Greninger reported receiving personal fees from Abbott Molecular. No other disclosures were reported.

Funding/Support: The study was funded by the University of Washington Department of Laboratory Medicine and the Fred Hutchinson Cancer Research Center.

Role of the Funder/Sponsor: The funders/sponsors had no role in the design and conduct of the study; collection, management, analysis, and interpretation of the data; preparation, review, or approval of the manuscript; and decision to submit the manuscript for publication.

Additional Contributions: We thank the following persons affiliated with the University of Washington (UW) Department of Laboratory Medicine: Meei-Li Huang, PhD, for polymerase chain reaction technical guidance and Patrick Mathias, MD, PhD, Tuan Nguyen, Nathan Breit, and Rick Clayton for data acquisition and informatics support. We thank the following persons affiliated with the Fred Hutchinson Cancer Research Center: Lisa Bunts, BA, Sara Thiebaud, MS, and Emily Silgard, MS, for assistance with data management, Nicole Espy, PhD, and Raphael Gottardo, PhD, for input on the analysis strategy, and Mindy Miner, PhD, for assistance with editing the manuscript. We give special thanks to Geoff Baird, MD, PhD, chair of the UW Department of Laboratory Medicine, and Fred Hutchinson Cancer Research Center President Tom Lynch, MD, for institutional support of the laboratories and personnel associated with this project.