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Key findings:

• Vaccinated travellers are less likely to enter the community while infectious, and less likely to spread COVID-19 to others if they do, even under shorter quarantine durations.
• Families with children too young to be eligible for vaccination (i.e. less than 12 years) present a level of risk that is marginally greater than fully vaccinated arrivals.
• Ensuring high compliance with home quarantine plays a greater role in preventing infection from arriving travellers than whether they are quarantined for 14 or 7 days.
• If we assume that arrivals will return to levels approaching those pre-2020, infectious travellers who remain undetected by quarantine and screening procedures provide a continuous source of new infections. However, with vaccine coverage above 80% and ongoing public health measures, these infections do not lead to large outbreaks.

Out of scope:

The primary purpose of the model was to consider the relative impact of different international arrival capacities and quarantine pathways on local epidemiology. In estimating this impact, we therefore did not consider:

• Changing patterns of immunity in the community, such as waning and ongoing vaccination (primary or boosters).
• Changes in the public health and social measures in response to local epidemiology.
• Arrival of travellers via managed pathways such as international students, agricultural workers, diplomatic travellers, etc.
• The potential impact of domestic travel on importation of infection.

Limitations

• We estimated risks and impacts based on transmission characteristics consistent with the Delta strain of COVID-19. Variant strains with different infectiousness or vaccine efficacy may modify estimates of importation risk and impact.
• Our scenarios considered each quarantine pathway in isolation. In reality, we would anticipate arrivals to follow a blend of quarantine pathways depending on, for example, their source country, vaccination status, etc.
• We consider stylised family units consisting of two vaccinated adults and two unvaccinated children. Inclusion of other arrival group compositions may modify estimates of importation risk and impact.
• The baseline rate of arrival of infectious travellers into the quarantine system was not calibrated to disease prevalence or vaccination rates in specific source countries. Ideally, these parameters would be refined over time based on observed prevalence of infection in arriving travellers.

Frequently Asked Questions:

What does the modelling say about the potential introduction of new variants of COVID-19?
All assumptions about infectiousness and vaccine efficacy are based upon current knowledge about the Delta strain of COVID-19. Our estimates of the effectiveness of quarantine pathways and the epidemiologic consequences of infections entering the community would be revised in light of a new strain emerging, particularly if it was more transmissible than Delta or if vaccination provided less protection against this strain. The possibility of new variants of concern highlights the importance of ongoing testing of arrivals and monitoring of the global epidemiological situation.

How could rapid antigen tests complement quarantine scenarios?
From the perspective of preventing case importation, the primary advantage of rapid antigen tests (RATs) over PCR is faster delivery of results and subsequent case isolation. In scenarios where result delivery time is not limited by test processing queues, our modelling showed that use of RATs could marginally reduce the time to detection of imported cases. This advantage is likely greatest when screening for cases immediately upon arrival. Returning a result within one hour, rather than after a 24-hour turnaround, could limit exposure during arrival processing and transit to final destination.

What does the modelling say about unvaccinated arrivals?
The risk presented by unvaccinated arrivals is clearly higher than that presented by vaccinated arrivals. The modelling shows that continued hotel or dedicated-facility quarantine will minimise the impact of breaches from these individuals in low-prevalence settings, although this will necessarily cap the number of arrivals. In high prevalence settings, unvaccinated arrivals will still have relatively little impact on the epidemic dynamics. Again, our findings assume that arrivals are not carrying a COVID strain that is more infectious than local viruses, or that escapes vaccine immunity.

What if a large outbreak should occur elsewhere in the world?
As with the potential emergence of a new strain, ongoing monitoring of global COVID-19 epidemiology is essential, to enable arrivals procedures to be adapted accordingly. If appropriate, quarantine arrangements for arrivals from regions of heightened risk may need to be made more stringent; for example, by limiting numbers or making use of dedicated quarantine facilities.

How does the number of cases in the community affect the risk of outbreaks from arriving travellers?
The risk of outbreaks from arriving travellers is dependent upon the current epidemic behaviour in the community of interest. In a community with widespread transmission, the added risk of outbreaks from arriving travellers is minimal. This is because it is much more likely for an infection to arise due to the large number of active local cases than the comparatively small number of infected arriving travellers (due to testing protocols for arriving travellers there is only a small chance the individual will be exposed or infected upon arrival). In a “zero-COVID” setting, high vaccination levels and ongoing public health and social measures will limit the number of secondary infections that occur from arriving travellers.

What about domestic travel?
Our estimates of infection risk in the community are focused on international arrivals and do not incorporate the epidemiological consequences of domestic travel (for example, in introducing infection into “zero-COVID” settings). While we have framed our model in the context of international arrivals, the workflow used to estimate infection risk associated with travellers could be adapted to domestic travel scenarios, enabling quantification of the potential risk of spread between states.

Why do you assume quarantine breaches will continue to happen?
Australia has experienced multiple quarantine breaches over the last 18 months as a result of staff exposure, unauthorised departure, and other reasons. While quarantine processes have been continually improved to minimise these risks, they remain present. A shift to home quarantine reduces some risks, as there is less interaction among travellers and between travellers and staff. However, there are additional risks associated with home quarantine arising from incidental interaction with delivery people, public health officers, and others, and the risk of travellers not complying with quarantine orders. These risks have not been robustly quantified, and we conservatively assume that on any given day, one in ten people in home quarantine will pose an exposure risk to the broader community.

Why don’t quarantine breaches lead to large outbreaks?
In the quarantine scenarios that we consider, the effect of breaches is minimal. This is because a high level of vaccination in the community has reduced the transmission potential to a point where ongoing public health and social measures (contact tracing, mask wearing, etc) are effective at limiting transmission.

Why do the models only run for 1 year?
The quarantine scenarios presented in this phase of work show the relative risk associated with different arrivals pathways, for different volumes of travellers, and different levels of vaccine coverage. We ran the models for one year to provide sufficient time to demonstrate the relative trade-offs across those factors, and how quickly we might expect an outbreak to grow. These model outputs are not predictions of what will happen. Other factors that would affect outbreak dynamics will change over this period in ways that are not included in these scenarios. For example, vaccine coverage, booster doses, global epidemiology, the possible emergence of new variants, and so on. Ongoing situational assessment will be required to evaluate future epidemiological dynamics.