Modelling of COVID-19 vaccination strategies and herd immunity, in scenarios of limited and full vaccine supply in NSW, Australia

Abstract

Several vaccines for SARS-CoV-2 are expected to be available in Australia in 2021. Initial supply is limited and will require a judicious vaccination strategy until supply is unrestricted. If vaccines have efficacy as post-exposure prophylaxis (PEP) in contacts, this provides more policy options. We used a deterministic mathematical model of epidemic response with limited supply (age-targeted or ring vaccination) and mass vaccination for the State of New South Wales (NSW) in Australia. For targeted vaccination, the effectiveness of vaccinating health workers, young people and older adults was compared. For mass vaccination, we tested varying vaccine efficacy (VE) and distribution capacities. With a limited vaccine stockpile enough for 1 million people in NSW, if there is efficacy as PEP, the most efficient way to control COVID-19 will be ring vaccination, however at least 90% of contacts per case needs to be traced and vaccinated. Health worker vaccination is required for health system resilience. Age based strategies with restricted doses make minimal impact on the epidemic, but vaccinating older people prevents more deaths. Herd immunity can only be achieved with mass vaccination. With 90% VE against all infection, herd immunity can be achieved by vaccinating 66% of the population. A vaccine with less than 70% VE cannot achieve herd immunity and will result in ongoing risk of outbreaks. For mass vaccination, distributing at least 60,000 doses per day is required to achieve control. Slower rates of vaccination will result in the population living with COVID-19 longer, and higher cases and deaths.

Keywords: Australia; COVID-19; Epidemic; SARS-CoV-2; Vaccines.

Fig. 1

Model diagrams for targeted, mass and ring vaccination.

Fig. 2

Targeted vaccination strategies with a restricted supply for 1 million people: From left to right is the epidemic curve, the cumulative case and deaths numbers by targeted age group vaccinated.

Fig. 3

The impact of speed of mass vaccination with a vaccine of 90% efficacy. From left to right is the epidemic curve, the cumulative case and deaths numbers by number of number of people vaccinated per day in NSW.

Fig. 4

From left to right is the epidemic curve, the cumulative case and deaths by vaccine efficacy (VE), with 125,000 people per day vaccinated until 70% of NSW population is vaccinated.

Fig. 5

The impact of contact tracing and ring vaccination on epidemic control for a vaccine with 45% efficacy as post-exposure prophylaxis: From left to right is the epidemic curve, the cumulative case and deaths numbers by percentage of contacts traced and vaccinated.