Modelling direct and herd protection effects of vaccination against the SARS-CoV-2 Delta variant in Australia.

Objectives To analyse the direct and indirect effects of vaccination by vaccine type, age-strategy, and coverage on Australia's Coronavirus Disease 2019 epidemic. Design Our model incorporates age-specific mixing, infectiousness, susceptibility and severity to assess the final size of the epidemic under different public health intervention scenarios in Australia. Main outcome measures The model predicts the final infected population, cumulative hospitalizations, deaths and years of life lost following an epidemic wave, taking into account vaccination and additional mitigation measures. Results Assuming a pre-vaccination effective reproduction number for the Delta variant of 5, we show that the current "Mixed" program (vaccinating over 60s with AstraZeneca and under 60s with Pfizer) would not achieve herd immunity, unless 85% of the population is covered (including many 5-16 year-olds). At a lower effective reproduction number of 3, the Mixed program can achieve herd immunity at 60-70% coverage without vaccinating 5-15 year-olds. An effective reproduction number of 7 precludes achieving herd immunity with existing vaccines, however vaccination still prevents over 85% of deaths compared with no vaccination. Conclusion Vaccinating the vulnerable first is the optimal policy at low coverage, but vaccinating high transmitters becomes more important in settings where reproduction numbers are lower and coverage is higher. Vaccinating over 85% of the population including children would likely be needed to achieve herd immunity at the most plausible effective reproduction number of 5. Even without herd immunity, vaccines are highly beneficial at reducing deaths.