Stage four lockdown offers a chance to find our path out

The glimmer of light at the end of the tunnel is visible now, but it is critical we remain engaged in understanding, analysing and evaluating where we are now and what is working to help us navigate our way out of our lockdown in the safest and most efficient way possible.

Epidemiology is not only about tracking the spread of the virus and trends in cases, it is very much about prevention and understanding the distribution of disease, and of disease risk factors, within populations. This knowledge allows a more nuanced approach to managing risk and targeting prevention interventions.

The empty Emporium Melbourne shopping centre this week.Credit:Getty

Understanding infection risk is something we are getting better at over the course of the COVID-19 pandemic. The one upside of more cases is a richer data set to help us understand population risk, and tease out factors that place one person at a different risk of infection to another. Knowing what protects someone from infection can be equally important as knowing what puts someone at greater risk when trying to suppress the spread of a virus and prevent further cases.

We are aware of high-risk workplaces because that is where our largest clusters have been, but we also know these outbreaks are driven by background community transmission – as the risk of workers being positive in the community rises, so too does the risk that some will take the virus into work in just the right circumstances to trigger an outbreak.

So what have we learned about this background community transmission? Government analysts are currently carrying out a more detailed examination of cases. Their report next week should help us better understand the types of people at greater risk of infection, how it relates to occupation, and what might be learned from this.

Hopefully, information provided will cover those linked to large workplace outbreaks separately as these will dominate the data and may prevent us seeing the story about background community transmission risks.

The other barrier to understanding transmission outside the workplace is the number of "mystery" cases – those not linked to other cases, or known clusters, where we do not know where these cases picked up their infections. These are the cases that most worry the Health Department because they signal that other, even more mysterious, invisible cases must be out there.

Not being able to see all cases, or understand all transmission pathways, undermines our ability to bring the outbreak under control. The result is a need to resort to very blunt restrictions to close down community transmission.

Without a known source, or intact transmission chain, there is still a lot that can be potentially be learned from the 19 per cent of cases that are a "mystery" – what put them at greater risk of becoming infected? Case-control analysis is a useful method for seeing whether certain demographic, occupational or behavioural characteristics are associated with infection by comparing these cases with people who were not infected, or who were only infected because the mystery case brought the virus in to the household.

Such analyses won't link mystery cases to a specific person or place, but can identify if the data points to common activities or types of locations frequented that are associated with higher risk of infection. Was the mystery case more likely to do the grocery shopping, catch public transport, or go into an office to work? If patterns emerge, they could help identify the activities most associated with transmission risk and the ones to specifically target to suppress this.

The changes in restriction levels in regional Melbourne, with and before masks, also allow us to investigate patterns in community transmission against these changing backdrops. This helps us evaluate how our restrictions affected transmission, and identify persistent transmission weak points that should be considered as we start to relax restrictions, and be targeted in future control measures.

There are still so many unknowns, but now is our opportunity to learn more about how the virus moves through our communities, how to most effectively and surgically shut down transmission without the need for blunt restrictions, and how to chart a safe path ahead.

This is not only about how we relax restrictions for this wave, but what we learn from it so that Australia can be more precise with the type and timing of control measures and successfully manage the risk of future outbreaks.

Catherine Bennett is chair in epidemiology at Deakin University.

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