The probability (pt ) that a recently-infected traveler arrives on day t can be changed by reducing pt , the prevalence of recently-infected individuals in the source region, and it is in every nation’s interest to support this effort. The probability can be further reduced by screening passengers departing from the source region, which includes isolating cases upon diagnosis in the source region. Border interventions under Australian control include
- Reducing kt , the number of travelers from the source region to Australia.
- Providing all arriving international travelers with information on the disease and common signs of infection and actions to take if symptoms develop after their arrival in Australia.
- Requiring all international air travelers to complete a health declaration card on entry into Australia which will include a statement that they have, or do not have, any of the symptoms of the disease.
- Requiring crews of all international aircrafts to report any symptoms of illness that they observe among passengers on their aircraft.
- Applying passive surveillance technology to arriving travelers (e.g. thermal imaging, or direct or indirect body temperature measurement).
- Requiring high risk passengers (e.g. those arriving directly from a country affected by the disease) to submit to medical assessment or examination.
- Placing all high risk international travelers under surveillance (e.g. reporting for a daily medical examination).
- Placing all high risk international travelers into quarantine or partial home quarantine whereby travelers are required to stay at home, though other household members continue to mix in the community.
These measures will reduce Australia’s risk of an early local epidemic by reducing incoming travel from at-risk regions (measure i.), identifying some infected travelers at the borders (measures ii.-vi.), leading to early presentation of infected arrivals who are not identified at the borders (particularly measures ii., vi. and vii.) and reducing the mixing of high risk international travelers with community members immediately after arrival (viii.). We incorporated these effects by permitting restrictions on travel into Australia, by including a chance of being identified as a case at the borders (depending on the time since being infected), by including very timely presentation (and subsequent isolation) of infected arrivals and by including partial home quarantine of infected arrivals for measure viii. Clearly the full quarantine of all arriving passengers for a sufficient period should prevent the importation of disease [as was observed with the quarantine of ships arriving into Australia in late 1918; see McCracken and Curson (2003)], but whether this is implemented is a question of policy and logistics, which we don’t consider here.