Fig 1.
Chronology of SARS-CoV-2 cases, tests and test positivity rate for King County and Vashon.
A) Case rates are presented as a rolling 7-day average normalized per 100K population. The Vashon case rates approached King County’s during the Beta surge in winter 2020 and briefy exceeded it during the Delta surge in winter 2021. B) Tests are presented as a 14-day rolling average to smooth the curves. Vashon MRC performed 41% of all Vashon tests but identified 65% of all Vashon cases. * Indicates a 2-week gap in WADOH test reporting that affected King County and All Vashon data, but not Vashon MRC. C) Test positivity rates are presented as 14-day rolling averages. The Vashon MRC positivity rate often exceeded King County’s during surges due to contact tracing activities.
Fig 2.
Models of cumulative incidence of COVID-19 cases, hospitalizations and deaths in King County from February, 2020 through November, 2021.
Zip code-aggregated metrics of age, race/ethnicity, education and wealth were used to construct multiple linear regression models of cumulative incidence of COVID-19 cases (A), hospitalizations (B) and deaths (C). Open circles show predicted incidence in rank order and solid circles show observed incidence. Vashon data points are shown in gold. Beta-terms, confidence intervals and p-values for independent variables are shown in (D). 95% confidence intervals (CI) are calculated from robust HC3 standard errors. PI: Prediction interval; RMSE: Root mean squared error.
Fig 3.
Mobility of Vashon and Whidbey Island populations.
A) Daily total ferry traffic for Vashon, and road plus ferry traffic for Whidbey Island, are shown by quarter for 2020 and 2021, normalized for population. Averages for the study period are shown as horizontal lines in gold for Vashon and blue for Whidbey. While traffic varied significantly by quarter, Vashon and Whidbey’s rates were similar and highly correlated throughout the study period. B) Daily passenger roundtrips to/from Vashon and Whidbey, normalized for population, is shown by quarter. An estimate of South Whidbey passenger traffic, created as daily roundtrips on the Mulkiteo-Clinton ferry, normalized to the South Whidbey population, is also shown. Averages for the study period are shown in gold for Vashon, blue for Whidbey and gray for South Whidbey.
Table 1.
Demographics of King County, Vashon Island, and Island County communities.
Fig 4.
Models of cumulative incidence of COVID-19 cases, hospitalizations and deaths in the Puget Sound region from February, 2020 through November, 2021 including remote geography.
Multiple linear regression models for cumulative incidence of cases (A), hospitalizations (B) and deaths (C) were expanded to include a categorical variable for remote geography. Open circles show predicted incidence in rank order and solid circles show observed incidence. Vashon data are shown in gold and Island County data are shown in green. Beta-coefficients, confidence intervals and p-values for independent variables are shown in panel D. PI: Prediction interval; RMSE: Root mean squared error.
Fig 5.
Cumulative cases and vaccinations for Vashon, South Whidbey, King County and all Whidbey.
A) Cumulative cases normalized for population over time. B) Cumulative vaccination doses normalized for population. Vashon reached 1500 doses/K mnearly 4 months before King County or South Whidbey. §, Vashon vs King County and South Whidbey both p<0.001. *, South Whidbey vs all Whidbey p<0.001.
Table 2.
Case investigation and contact tracing on Vashon, South Whidbey and in King County.
Fig 6.
Structure of Vashon COVID-19 transmission.
Data for all cases known to VMRC is shown. Arrows show transmission chain- contacts testing positive lead to the next generation of interviews tests and cases. The 1st 55 index cases were asked to provide all contact information (Complete Tracing). In a 2nd cohort, the focus shifted to identifying and testing household contacts (Household Tracing), although index patients were asked to notify contacts and have them call VMRC. *Sixty-eight percent of index cases in the Household Tracing cohort identified contacts.