Modeling the COVID-19 Vaccination Dynamics in the United States: An Estimation of Coverage Velocity and Carrying Capacity Based on Socio-demographic Vulnerability Indices in California

BackgroundCoronavirus 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 proposal 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). MethodsOverall 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. ResultsBased 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). ConclusionsThis 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. Key MessagesO_LIThe Social Vulnerability Index (SVI) and its four themes along with the daily proportion of vaccinated individuals across the 58 California counties were used to model, overall and by theme, the growth velocity and anticipated maximum proportion of population vaccinated. C_LIO_LIOverall, high vulnerability counties in California had a lower vaccine coverage velocity compared to low and moderate vulnerability counties. C_LIO_LIThe largest disparity in coverage velocity between low and high vulnerability counties was observed based on the SVI Theme 3 status (minority status & language). C_LIO_LIBased on the current trajectory, highly vulnerable counties based on SVI Theme 3 are expected to eventually achieve a higher proportion of vaccinated individuals compared to low vulnerable counterparts. C_LIO_LIUnderstanding the vaccine coverage dynamics and underlying sociodemographic factors is critical for achieving equity in public health outcomes during disease outbreaks and catastrophes. C_LI