By Jodie McVernon and James McCaw
Mathematical models of infectious disease are representations of the way infections spread between individuals, in households, and through society. They can be used to anticipate the likely future impacts of a disease, and to consider how well public health interventions, treatments and vaccines might reduce infection transmission, and limit severe outcomes.
Models have a particular role to play in planning for the emergence of novel diseases, such as pandemic influenza or COVID-19. By definition, we don’t know when or where they will emerge, how infectious they will be, or how severe. Without such knowledge, we can model different scenarios to think in advance about different disease control strategies that would be effective and proportionate. These models can also be used to estimate requirements for essential resources, like hospital beds and personal protective equipment.
The Australian Health Management Plan for Pandemic Influenza (AHMPPI) is a living document developed over many years to guide preparedness activities in Australia. While focused on influenza, its principles are relevant to other respiratory viruses. Response strategies are framed around modelled pandemic influenza scenarios with different clinical impacts. In a real influenza pandemic, early assessment of the virus’s growth rate and severity helps decision makers identify which scenario they’re in, informing targeted response actions.
The Australian Government’s response to COVID-19 has built on that approach, tailored to increasing understanding of this new virus. New models of COVID-19 infection have been developed and benchmarked against the AHMPPI’s preparedness scenarios. This virus, if allowed to spread uncontrolled, is much more infectious and severe than any recorded influenza pandemic. Early observations from China were aligned with “worst case” impact scenarios, since confirmed by the experiences of Europe, the United Kingdom and the United States.
The World Health Organisation has supported an international modelling network since mid-January 2020 to rapidly share information and emerging insights into COVID-19. Early knowledge was based on publicly available information, media reports and sharing of “pre-print” publications. Australian modellers have participated in these calls and incorporated this evidence into preparedness scenarios developed for the Australian Government. Based on our advice since early February 2020, the Commonwealth has worked with jurisdictions to prepare for a scenario worse than those previously envisaged, in an accelerated timeframe.
Stringent border measures were implemented early to reduce the risk of imported infections. Australians repatriated from high-risk locations were kept in quarantine until confirmed uninfected. Jurisdictional public health units have recruited many more staff to identify and isolate cases and quarantine contacts. Nationwide, laboratory testing capacity and resources are being strengthened and expanded.
Hospitals and intensive care units are re-prioritising surgery and adjusting work-practices to ensure existing beds are available and more patients can be cared for in readiness for an influx of cases. Local manufacturing of ventilators, personal protective equipment and sanitisers has rapidly increased. Special attention has been paid to protecting those in our community at greatest risk of severe disease outcomes.
The majority of COVID-19 infections reported in Australia so far have been imported, with identified chains of spread to close contacts that have been effectively shut down. The rapid rise in overseas acquired cases in recent weeks reflects the growth of epidemics elsewhere and poses a genuine threat to management of COVID-19 in Australia. All efforts are being made to reduce the ongoing spread of infection, but there is clear evidence of emergent community transmission in several states.
The widespread social measures that have been enacted in recent weeks will reduce the spread of all respiratory infections, including COVID-19 and influenza. These measures are essential over a period of many months to ensure that our health system is not overwhelmed and that all Australians can access the care they need, including for COVID-19.
As we pivot from preparedness to targeted response, models based on local epidemiological data will be used to estimate how quickly our epidemic is growing and how well interventions to reduce spread are working. These estimates will help predict where we’re headed next and anticipate the burden on the health system. This knowledge will guide decisions on strengthening of existing measures and, when appropriate, their carefully staged relaxation. Australia is contributing to global efforts to identify effective antiviral drugs that will reduce COVID-19’s impact and to development of vaccines that may be able to definitively stop the outbreak.
Meanwhile, COVID-19 remains highly infectious, and it is clear that our best efforts to be ready must now be reinforced by every Australian if we are to successfully “flatten the curve”.
Professors McVernon and McCaw are, among other roles, members of the Victorian Infectious Diseases Reference Laboratory Epidemiology Unit, The Peter Doherty Institute for Infection and Immunity, The Royal Melbourne Hospital and The University of Melbourne.
Related reading: Why some countries have few COVID-19 cases