Non-pharmaceutical interventions and vaccinating school children required to contain SARS-CoV-2 Delta variant outbreaks in Australia: a modelling analysis

Milne, G. J.; Carrivick, J.; Whyatt, D.

Abstract Background: In countries with high COVID-19 vaccination rates the SARS-CoV-2 Delta variant has resulted in rapidly increasing case numbers. This study evaluated the use of non-pharmaceutical interventions (NPIs) coupled with alternative COVID-19 vaccination strategies to determine feasible Delta mitigation strategies for Australia. We aimed to understanding the interplay between high vaccine coverage levels and NPI physical distancing activation, and to establish the benefit of adding children and adolescents to the vaccination program. Border closure has limited SARS-CoV-2 transmission in Australia, however slow vaccination uptake resulted in Delta outbreaks in the two largest cities, and may continue once international travel resumes. Methods: An agent-based model was used to evaluate the potential reduction in the COVID-19 health burden resulting from alternative vaccination strategies. We assumed immunity was derived from vaccination with the BNT162b2 Pfizer BioNTech vaccine. Two age-specific vaccination strategies were evaluated, age 5 and above, and 12 and above, and the health burden determined under alternative coverages, with/without activation of NPIs. Age-specific infections generated by the model, together with recent UK data, permitted reductions in the health burden to be quantified. Results: Cases, hospitalisations and deaths are shown to reduce by: i) increasing coverage to include children aged 5 to 11 years, ii) activating moderate NPI measures, and/or iii) increasing coverage levels above 80%. At 80% coverage, vaccinating ages 12 plus without NPIs is predicted to give 1,162 hospitalisations; adding ages 5 and above gives 1,073 cases per million population. Activating moderate NPIs reduces hospitalisations to 705. Alternatively, increasing coverage to 90% reduces this to 684. Combining all three measures is shown to reduce cases to 398, hospitalisations to 2 and deaths to zero. Conclusions: Delta variant outbreaks may be successfully managed by an achievable 80% vaccine coverage rate and moderate NPI measures, allowing schools and many workplaces to remain open. This prevents use of hard lockdown measures, and consequential economic and societal damage. Activating moderate NPIs is shown to give a similar reduction in health burden as increasing coverage of ages 12 and above to 90%. If 90% coverage cannot be achieved, including children and adolescents in the vaccination program coupled with moderate NPIs appears necessary to contain future COVID-19 Delta transmission.