The following summarise the implications of our modeling of the effectiveness of border control measures.
Delay in the absence of travel interventions
The likely delay from a pandemic strain of influenza being recognised overseas and an epidemic taking hold in Australia in the absence of interventions ranges from a few weeks to several months, depending on the disease reproduction number and the number traveling from the infected region to Australia.
Effects of the disease reproduction number on the delay
A large reproduction number in the source region usually means that the epidemic starts with a period of rapid exponential growth, which shortens the delay until an Australian epidemic in two ways. First, the exponential growth increases the prevalence of infectious individuals, and therefore the expected number of recently-infected travelers rises rapidly. Second, at times during this period of rapid exponential growth, the time since infection among recently-infected individuals is considerably weighted towards short times, with the consequence that an infected traveler is more likely to be asymptomatic during travel and therefore escape border screening.
Effects of border screening on the delay
If R > 1 in the source region, then border screening cannot be relied upon to delay introduction of the infection into Australia, even if screening is 100% effective for individuals who are symptomatic at entry. The only conditions under which border screening can appreciably delay the initiation of the epidemic in Australia are so benign (e.g. low R0) that preventing the entry of such a disease may not be the most cost effective way to manage the threat.
Effects of reducing the number traveling
Severely curtailing the number of travelers (e.g. reducing it to 5% of the typical number) may delay the start of an Australian epidemic by several weeks, although the effectiveness of this intervention dissipates as the number of cases in the source country rises.
Effects of early presentation following symptoms
Like screening, attempts to achieve the earliest possible presentation of infected arriving passengers do not delay a local epidemic appreciably. In the absence of a complete halt to international travel, all forms of preventative interventions are eventually overwhelmed by the exponential rise in the cumulative number of infected arriving passengers.
When to initiate a response
Short of preventing international travel altogether or extremely effective quarantine of arrivals from source regions, eliminating a nascent pandemic in the source country appears to be the only reliable method of preventing the importation of a pandemic strain of influenza into Australia. It is therefore important to monitor the risk of importation into Australia, which can be done on the basis of data on the daily incidence of the cases in the source region and travel volumes from the source region into Australia, using methods outlined above.