Effects of varying COVID-19 vaccination rates on population-level health outcomes across variant waves in the U.S.

In a recent article published in BMC Public Health, researchers analyzed the association between time-varying coronavirus disease 2019 (COVID-19) vaccination rates and COVID-19 case-hospitalization risk (CHR), a proxy for disease severity at an individual level and disease burden on healthcare systems at the population level across various severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) variant waves in the United States (U.S.).

Study: Association between vaccination rates and COVID-19 health outcomes in the United States: a population-level statistical analysis. Image Credit: Nhemz/Shutterstock.com

Background

There were 1.1 million deaths in the U.S. due to COVID-19 by 1 March 2023. COVID-19 vaccines most effectively curbed the disease and its impact, including the socioeconomic burden on the country’s people and health care system.

However, studies evaluating the effectiveness of COVID-19 vaccines relied upon individual-level data confounded by unquantified factors and inconsistent quality.

So, for the U.S., high-resolution data was unavailable at the population level, reflecting the real-world relative associations between available COVID-19 vaccines and COVID-19 CHR over time. 

About the study

The present study used Generalized Additive Models (GAMs) to investigate the relationship between COVID-19 vaccination rates and CHR in 48 U.S. states between 19 April 2021 and 1 March 2022.

The study model captured nonlinear dynamics, accounting for dynamic (time-changing) and static (temporally constant) factors potentially contributing to COVID-19 CHR and disease transmission.

The former were natural immunity derived from previous severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection, government policies, activity-related engagement levels in the population, and local healthcare infrastructure.

In contrast, the latter comprised the social vulnerability index (SVI), race/ethnicity, comorbidities, and healthcare expenditures of each state, considered significant in previous studies.

Additionally, the model reported COVID-19 case incidence rates (CIR) as a distinct outcome variable. The study framework used a variable transformation method to capture the spatial variations in relative associations.

The study period spanned the pre-Delta, Delta, and Omicron waves of COVID-19, and they evaluated all of these independently in this study. 

Results

The study results point to several key findings concerning population-level COVID-19 vaccine effects in the U.S. 

The GAMs using relative COVID-19 CHR (RCHR) as an outcome variable showed deviance explained values between 46.8% and 72.3% for variant waves.

Moreover, the correlation between observed and predicted RCHRs displayed strong positive correlations ranging from 0.67-0.81.

Population-level vaccination was significantly associated with reduced COVID-19 CHR.

Intriguingly, past SARS-CoV-2 infections (one to four months old) displayed robust negative associations with RCHR across different waves; however, this effect remained variable and inconsistent at both individual and population levels.

Activity-related engagement levels in the population (e.g., gym visits), government policies, and local healthcare infrastructure added to the explanatory power of the study model, favoring the significance of considering these on population-level outcomes of COVID-19 vaccines. 

However, their associations were inconsistent over time and across different variants. For instance, the association between COVID-19 CHR and hospital visits transitioned from negative to positive between the pre-Delta to Delta and Omicron waves.

Moreover, the observed correlation between the relative weekly testing rate and RCHR was negative and decreased from the pre-Delta wave to the Omicron wave. 

Additionally, U.S. states with higher SVI consistently showed higher RCHR, and Medicaid spending per person showed a consistent negative association with RCHR.

The GAMs using RCIR as the outcome variable demonstrated lower performance, suggesting a more dynamic relationship concerning COVID-19 transmission, particularly during the Omicron wave. 

The deviance explained for Model Omicron-Booster-RCIR was 17%, suggesting that booster vaccination conferred additional protection against severe COVID-19 during the Omicron waves. However, their effect on Omicron infection itself was limited.

Conclusions

The study provides robust evidence of the efficacy of COVID-19 vaccines against COVID-19 CHR across various variant waves in the United States. 

Despite the emergence of new variants, vaccines remained effective and remarkably mitigated adverse outcomes of COVID-19 and its socioeconomic burden on healthcare systems. This data could help inform future public health policies in the U.S.

Future studies should identify other factors that may capture the dynamics of COVID-19 transmission during the Omicron period.

Furthermore, studies should investigate the complex and evolving nature of COVID-19 transmission.