The effectiveness of vaccination on the COVID-19 epidemic in California

The COVID-19 pandemic has caused overwhelming morbidity, mortality, and hospitalization worldwide, including in the state of California. Vaccination efforts have been an important measure in curtailing the adverse outcomes of COVID-19.

To quantify the effectiveness of COVID-19 vaccinations in California, we conducted a retrospective cohort study investigating how vaccination has impacted the extent of COVID-19 contraction, hospitalizations, and death totals. We compared outcomes of the Delta Wave, Omicron Wave, and Pre-Delta Period.

Vaccinated individuals have far-lower incidence risk ratio (IRR) of and odds of contracting a COVID-19 case (Delta IRR: 0.197) being hospitalized from COVID-19 (Delta IRR: 0.105), and dying from COVID-19 compared with an unvaccinated individual (Delta IRR: 0.0941). The preventive fraction of the unexposed and population-preventive fractions for cases, deaths, and hospitalizations also showed significant proportions. All tests showed P < .001.

Vaccination was most effective in the Delta Wave, then in the Omicron Wave, and least effective in the Pre-Delta Period. Deaths were the most prevented outcome, followed by hospitalizations, then cases.

This study exposes the massive impact of vaccinations in California in reducing COVID-19 outcomes and the potential for fewer adverse outcomes had there been greater vaccination compliance, demonstrating the need to increase vaccination efforts.