Interplay of demographics, geography and COVID-19 pandemic responses in the Puget Sound region: The Vashon, Washington Medical Reserve Corps experience

Abstract

Background: Rural U.S. communities are at risk from COVID-19 due to advanced age and limited access to acute care. Recognizing this, the Vashon Medical Reserve Corps (VMRC) in King County, Washington, implemented an all-volunteer, community-based COVID-19 response program. This program integrated public engagement, SARS-CoV-2 testing, contact tracing, vaccination, and material community support, and was associated with the lowest cumulative COVID-19 case rate in King County. This study aimed to investigate the contributions of demographics, geography and public health interventions to Vashon's low COVID-19 rates.

Methods: This observational cross-sectional study compares cumulative COVID-19 rates and success of public health interventions from February 2020 through November 2021 for Vashon Island with King County (including metropolitan Seattle) and Whidbey Island, located ~50 km north of Vashon. To evaluate the role of demography, we developed multiple linear regression models of COVID-19 rates using metrics of age, race/ethnicity, wealth and educational attainment across 77 King County zip codes. To investigate the role of remote geography we expanded the regression models to include North, Central and South Whidbey, similarly remote island communities with varying demographic features. To evaluate the effectiveness of VMRC's community-based public health measures, we directly compared Vashon's success of vaccination and contact tracing with that of King County and South Whidbey, the Whidbey community most similar to Vashon.

Results: Vashon's cumulative COVID-19 case rate was 29% that of King County overall (22.2 vs 76.8 cases/K). A multiple linear regression model based on King County demographics found educational attainment to be a major correlate of COVID-19 rates, and Vashon's cumulative case rate was just 38% of predicted (p < .05), so demographics alone do not explain Vashon's low COVID-19 case rate. Inclusion of Whidbey communities in the model identified a major effect of remote geography (-49 cases/K, p < .001), such that observed COVID-19 rates for all remote communities fell within the model's 95% prediction interval. VMRC's vaccination effort was highly effective, reaching a vaccination rate of 1500 doses/K four months before South Whidbey and King County and maintaining a cumulative vaccination rate 200 doses/K higher throughout the latter half of 2021 (p < .001). Including vaccination rates in the model reduced the effect of remote geography to -41 cases/K (p < .001). VMRC case investigation was also highly effective, interviewing 96% of referred cases in an average of 1.7 days compared with 69% in 3.7 days for Washington Department of Health investigating South Whidbey cases and 80% in 3.4 days for Public Health-Seattle & King County (both p<0.001). VMRC's public health interventions were associated with a 30% lower case rate (p<0.001) and 55% lower hospitalization rate (p = 0.056) than South Whidbey.

Conclusions: While the overall magnitude of the pre-Omicron COVID-19 pandemic in rural and urban U.S. communities was similar, we show that island communities in the Puget Sound region were substantially protected from COVID-19 by their geography. We further show that a volunteer community-based COVID-19 response program was highly effective in the Vashon community, augmenting the protective effect of geography. We suggest that Medical Reserve Corps should be an important element of future pandemic planning.

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.

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.

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 1^st 55 index cases were asked to provide all contact information (Complete Tracing). In a 2^nd 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.