To perform calculations, and present illustrations of these, to inform us about the questions
- What is the delay until transmission of the infection gathers momentum in Australia in the absence of border control measures?
- What is a suitable trigger for the introduction of border control measures?
- What is the effect of border control measures on the delay until the transmission of the infection gathers momentum in Australia?
First, preventing pandemic influenza from entering Australia is included in our concept of delay, although this is thought to be an unlikely event.
Second, the delay may provide time to establish the precise nature of the threat and prepare specific responses. It may give time to estimate (from data in the source region) biological parameters such as R0 and the effectiveness of various interventions.
Third, it may enable the deployment of a vaccine. This is only a substantial benefit to regions which can hold the epidemic peak off until the vaccine is developed and distributed, and immunity has been able to build up in the vaccinated population.
Fourth, the virulence of a new pandemic virus may decline over time due to adaptation to a new host species, and possibly natural selection. For example, strictly isolating the more severe cases will select for viral variants with milder symptoms. It is believed that this phenomenon occurred in the 1918 influenza pandemic, and can explain the low case-fatality rate in Australia and US Samoa.