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

Several vaccine candidates for control of the Coronavirus disease 19 (COVID19) are in phase three trials, with some being approved in different countries. A limited initial supply of vaccine doses is likely in most countries, and will require a judicious vaccination strategy to be developed until supply is unlimited. Vaccines may be effective as post-exposure prophylaxis (PEP) in contacts, but no data are yet available for COVID19 vaccines. If vaccines do have efficacy as PEP, this provides more options for strategies with limited supply. We simulated an epidemic response with limited supply (targeted or ring vaccination) and mass vaccination for the State of New South Wales (NSW) in Australia. For targeted vaccination, the effectiveness of vaccinating young adults was compared to older adults. For mass vaccination, we tested outcomes for varying vaccine efficacy and distribution capacities, and for ring vaccination, we tested different contacts tracing rates. With a limited vaccine stockpile of 1 million doses in NSW, if there is efficacy as PEP, the most efficient way to control COVID19 will be ring vaccination, however at least 90% of contacts per case needs to be traced and vaccinated. Age based strategies make minimal impact on the epidemic due to low coverage rates, but vaccinating older people prevents more deaths. Herd immunity and interruption of community transmission can only be achieved with mass vaccination using a high efficacy vaccine. With a 90% efficacy, herd immunity can be achieved by vaccinating 73% of the population. If the vaccine stockpile is unlimited, distributing at least 60,000 doses per day is required to achieve herd immunity. Slower rates of vaccination will result in the population living with COVID19 longer, and higher cases and deaths.