COVID-19 Vaccination Dynamics in the US: Coverage Velocity and Carrying Capacity Based on Socio-demographic Vulnerability Indices in California

Abstract

Coronavirus disease 2019 (COVID-19) disparities among vulnerable populations are of paramount concern that extend to vaccine administration. With recent uptick in infection rates, dominance of the delta variant, and authorization of a third booster shot, understanding the population-level vaccine coverage dynamics and underlying sociodemographic factors is critical for achieving equity in public health outcomes. This study aimed to characterize the scope of vaccine inequity in California counties through modeling the trends of vaccination using the Social Vulnerability Index (SVI). Overall SVI, its four themes, and 9228 data points of daily vaccination numbers from December 15, 2020, to May 23, 2021, across all 58 California counties were used to model the growth velocity and anticipated maximum proportion of population vaccinated, defined as having received at least one dose of vaccine. Based on the overall SVI, the vaccination coverage velocity was lower in counties in the high vulnerability category (v = 0.0346, 95% CI 0.0334, 0.0358) compared to moderate (v = 0.0396, 95% CI 0.0385, 0.0408) and low (v = 0.0414, 95% CI 0.0403, 0.0425) vulnerability categories. SVI Theme 3 (minority status and language) yielded the largest disparity in coverage velocity between low and high-vulnerable counties (v = 0.0423 versus v = 0.035, P < 0.001). Based on the current trajectory, while counties in low-vulnerability category of overall SVI are estimated to achieve a higher proportion of vaccinated individuals, our models yielded a higher asymptotic maximum for highly vulnerable counties of Theme 3 (K = 0.544, 95% CI 0.527, 0.561) compared to low-vulnerability counterparts (K = 0.441, 95% CI 0.432, 0.450). The largest disparity in asymptotic proportion vaccinated between the low and high-vulnerability categories was observed in Theme 2 describing the household composition and disability (K = 0.602, 95% CI 0.592, 0.612; versus K = 0.425, 95% CI 0.413, 0.436). Overall, the large initial disparities in vaccination rates by SVI status attenuated over time, particularly based on Theme 3 status which yielded a large decrease in cumulative vaccination rate ratio of low to high-vulnerability categories from 1.42 to 0.95 (P = 0.002). This study provides insight into the problem of COVID-19 vaccine disparity across California which can help promote equity during the current pandemic and guide the allocation of future vaccines such as COVID-19 booster shots.

Keywords: COVID-19; Health equity; Minorities; Resource allocation; Vaccination.

Fig. 1

Social Vulnerability Index (SVI), underlying themes, and components

Fig. 2

Chronological overview of major national and state-wide (California) vaccination events and policies. Prior to the arrival of the first shipment of Pfizer-BioNTech COVID-19 vaccines in California, the California Department of Public Health (CDPH) issued a statement on December 5, 2020, recommending an initial Phase 1a vaccine allocation for healthcare workers and older or at-risk residents of long-term care facilities. Vaccine supply was limited in the early stages of distribution, and these populations were determined to be at risk of direct exposure to the virus and were prioritized to receive the vaccine first [35]. The recommendation was made anticipating the federal government’s vaccine distribution efforts across the nation as the FDA issued Emergency Use Authorizations (EUA) for the Pfizer-BioNTech vaccine on December 11, 2020 [36] and for the Moderna COVID-19 vaccine on December 18, 2020 [37]. At the time, these vaccines were authorized for individuals aged 16 and older and for individuals aged 18 and older, respectively. With the first shipment of Pfizer-BioNTech vaccines, California Governor Gavin Newsom launched the “Vaccinate all 58” campaign on December 14, 2020 in an effort to distribute vaccines safely, fairly, and equitably for all 58 counties of the state [38]. On January 13, 2021, CDPH issued a statement opening up vaccine eligibility to individuals aged 65 and older. This population was identified as part of Phase 1b, and while vaccine demand was still much higher than the supply, the demand among healthcare workers declined among the initial Phase 1a populations [39]. Phase 1b populations included individuals aged 65 and older as well as workers in food and agriculture, education and childcare services, and emergency services [40]. Because the state gave counties the power to decide when to adopt vaccine phases based on vaccine availability, parts of California moved to Phase 1b earlier than others, and the various populations in Phase 1b were eligible on different dates across the state. A third vaccine, Johnson & Johnson/Janssen COVID-19 vaccine, was issued an EUA by the FDA on February 27, 2021 for individuals 18 and older [41]. This further increased vaccine availability in California as vaccine demand continued to outpace supply. A brief pause in the Johnson & Johnson/Janssen vaccine starting April 13, 2021 [42] was issued jointly by the FDA and CDC as a result of concerns that the vaccine led to six rare cases of blood clot development, but this pause was lifted on April 23, 2021 jointly by both agencies after a safety review determined that the potential benefits of receiving the Johnson & Johnson/Janssen vaccine outweighed the potential risks [43]. Further updates to vaccine eligibility for Californians were made in the next two months. On March 11, 2021, vaccine eligibility was extended to residents and workers in facilities such as prisons and homeless shelters (including the homeless population). These populations were determined to be in facilities at great risk of spreading the virus and around individuals likely to have medical conditions that increased the risk of developing negative effects from the virus. Eligibility was also extended to public transport, airport, and commercial airline workers because of their risk of contracting the virus at work. These workers were also determined to be working critical operations [44]. On April 1, 2021, individuals aged 50–64 were eligible to receive vaccines, and Californians aged 16 and older were eligible shortly after beginning April 15, 2021. The Moderna and Johnson & Johnson vaccines were still authorized for individuals 18 and older only, but the FDA expanded the EUA on the Pfizer-BioNTech vaccine May 10, 2021 for vaccinating individuals aged 12 through 15 [45]. This was followed by CDPH announcing vaccine eligibility for Californians aged 12 and older beginning May 12, 2021 [40] (LTC: Long-term care, FDA: U.S. Food and Drug Administration, EUA: Emergency Use Authorization, CDC: U.S. Centers for Disease Control and Prevention)

Fig. 3

Distribution of overall Social Vulnerability Index ranks across 58 California counties and the cumulative rate of individuals who received at least one dose of vaccine per 100,000 capita (circles) at the study endpoint. Numbers in the circles represent the vaccination rate

Fig. 4

Proportion of persons with 1+ vaccination dose is shown for each county, with the model-fit estimates for each Social Vulnerability Index (SVI) category shown in bold

Fig. 5

Model estimates of K and v parameters for each theme and Social Vulnerability Index (SVI) category, with 95% confidence intervals

Fig. 6

Vaccination rate ratios for low and moderate SVI vs. high SVI counties, by date and SVI category. Presented are rate ratios with 95% confidence intervals. *Indicates a temporal reduction in the rate ratio for the given date vs. January 1 at p < 0.05