A recent modelling study has estimated the impact of the state’s vaccination campaign in 2021.
Victoria’s vaccination approach during the Delta outbreak of the covid pandemic prevented over 300,000 infections, 33,000 hospitalisations, and almost 5000 deaths over a two-month period.
That’s according to researchers from RMIT and Monash Universities, who developed a novel modelling approach to estimate the effects of vaccination during this timeframe.
“We applied an innovative Bayesian logistic model inspired by [susceptible-infected-recovered]-derived theoretical insights to analyse how period attack rates varies with overall vaccination level,” the research team wrote in the Journal of the Royal Society Interface.
“The results showed a statistically significant negative correlation between period attack rate and the vaccination level, providing strong empirical evidence of indirect vaccination effects.”
Researchers focused on infections occurring the Delta period, specifically from 26 September to 21 November 2021, as vaccine coverage data at the local government area level was not available prior to this period.
“Victoria, Australia, provided a unique context for evaluating the indirect effects of vaccination owing to the almost complete absence of natural immunity before the Delta wave in 2021, a scenario rarely available elsewhere,” wrote the researchers.
“In addition, the rapid pace of vaccination and the short interval (one to two months) between vaccination and exposure to the Delta variant, recognising that vaccine-induced immunity wanes rapidly over time, distinguished it from other settings.”
LGAs with a population less than 10,000 were excluded from the analysis, as were LGAs with fewer than 10 total covid infections during the study period. Consequently, data from 6,440,000 individuals across 67 of the 79 LGAs were included in the analyses. Individuals were categorised as either vaccinated (if they had received two doses of the vaccine) or unvaccinated (if they had received fewer than two doses).
Between 26 September and 21 November 2021 there were 79,000 infections, 4500 hospitalisations, and 600 deaths recorded in Victoria. The results of the modelling, based on available vaccination data, suggested that without the vaccination campaign, there would have been an estimated 394,000 infections, 37,900 hospitalisations, and 5500 deaths – a five-fold, eight-and-a-half-fold, and nine-fold increase, respectively, of the actual number of infections, hospitalisations, and deaths during this period.
“Of the [315,000] infections averted, an estimated 48.3%… would have occurred in those aged 16–49, 35.5% in those aged 50+ and 16.2% in those under 16,” wrote the authors.
“Of the 33,400 averted hospitalizations, 77.1% were expected to occur in individuals aged 50+ years, while 21.9% were expected in those aged 16–49. Furthermore… of the 4900 averted deaths, approximately 97.7% would have been in the 50+ age group, with 2.3% in the 16–49 age group.”
Related
The researchers emphasised that these estimates only applied to the study period analysed in Victoria, and that they “should not be interpreted as estimated of the total long-term effects of vaccination beyond this period”.
Further analyses revealed that roughly half of the infections that were averted because of Victoria’s vaccination coverage could be attributed to the indirect effects of vaccination. Around a quarter of the number of hospitalisations and deaths that were averted could also be attributed to the indirect effects.
“Although the magnitude of the indirect effect on infections is substantial, it is plausible given that the effective reproductive number in Victoria could be low owing to high vaccination level, with all LGAs above 40% and some above 60%,” the researchers said.
“However, such large indirect effects do not imply that the unvaccinated individuals were equally or better protected than vaccinated individuals… In fact, unvaccinated people had a much higher observed risk of infection, hospitalisation, and death than those who were vaccinated.
“While the number of infections averted by direct and indirect effects may be similar, the number of hospitalisations or deaths averted by direct effects is significantly higher. This is because direct protection includes not only the prevention of infection, but also the reduction in severity among breakthrough infections.”
The indirect effects of vaccination – for covid or other emerging infectious diseases – should not be overlooked, according to the researchers. Direct effects often become the focal point as they are easier to measure. But indirect effects, although harder to measure, are critical to understanding the broader impact of vaccination programs.
Journal of the Royal Society Interface, 30 September 2026
Comments for this article have been turned off.
