Quantifying the direct and indirect impact of COVID-19 vaccination: evidence from Victoria, Australia

TL;DR

This study introduces a new method to quantify both direct and indirect effects of COVID-19 vaccination, demonstrating significant population-level benefits and herd immunity effects during the Delta outbreak in Victoria, Australia.

Contribution

The paper develops a novel approach to estimate indirect vaccination effects at the population level, improving assessment of vaccination program impact.

Findings

Approximately 316,000 infections were averted due to vaccination.

Vaccination prevented around 33,500 hospitalizations and 4,900 deaths.

Indirect effects accounted for about half of the infections and a quarter of hospitalizations and deaths averted.

Abstract

Vaccines not only directly protect vaccinated individuals but also contribute to protect the entire population via indirect herd-immunity benefits. However, researchers have long struggled to quantify these indirect effects at the population level, hindering assessment of vaccination program effectiveness. We developed a new method to estimate these effects, thereby markedly improving measures of the number of infections, hospitalizations, and deaths averted by vaccination. Our population-based analysis of 6,440,000 residents of Victoria, Australia reveal strong indirect effects during the Delta outbreak (September-November 2021). By modelling a non-vaccination counterfactual, we conservatively estimate 316,000 infections were averted (95\% BCI: 232k-406k), as well as 33,500 hospitalizations (95\% BCI: 22.2k-46.2k), and 4,900 deaths (95\% BCI: 2.9k-7.3k). These are 4.0, 7.5, and 8.0 times higher, respectively, than observed. Half of the averted infections and around one-quarter of hospitalizations and deaths were attributable to indirect protection. Homogeneous vaccination across LGAs could have reduced outcomes by approximately 25\%.