The number five features prominently in Victoria's road map. Statistically, 5 per cent is the accepted tolerance for being wrong; the risk that findings are simply due to chance. Here, I unpack five epidemiological features of the road out of lockdown.
St Kilda Beach was a popular spot for exercise and social catch-ups on Saturday. Credit:Penny Stephens
The five-kilometre rule. More than 90 per cent of cases are now linked to outbreaks, mostly in permitted workplaces where this rule does not apply, or in workers' homes. Transmission across 100 kilometres – Chadstone, Frankston and Mitchell – was seen within a single outbreak last week. There is risk of further spread once a worker takes the virus home, but this is true whether contacts are restricted to five kilometres or not. The only advantage of this rule is if it reduces further exposures by forcing more overlap in locations frequented by household contacts. Even if true, this is unlikely to produce sufficient coincidence of visits in time and place to reduce risk of transmission. What counts is close contacts being kept to a minimum, however far people travel.
Five people from two households can meet outdoors. Why five? The response is rightfully a pragmatic one – we have to draw the line somewhere. However, when the sum of two households is greater than five and only a subset can meet, the "exposed" individuals still go home to the rest of their households. Those not in the five may separately, and legally, meet with people from another household, possibly a different one, increasing the number of households intermingling. Epidemiologically, the number of households interacting is what matters.
“Epidemiological difference” between Victoria and NSW. This is used to argue against the validity of public health response comparisons for similar case numbers. We are indeed different given the second wave, but not in the negative way often implied. Strict lockdown for 13 weeks equates to 15 or more incubation periods. The virus will have died out within households and bubbles, whether symptomatic or not, whether tested or not. Unlinked cases represent less than 10 per cent of Victoria's declining case numbers and, because of the restrictions, we have more assurance than NSW ever had of a very low risk of underlying community transmission.
Differing rules. Step 2 has been troubled by differing rules about jobs or activities with indistinguishable epidemiological risk. The transmission potential in a banned game of tennis using your own equipment on a court with no other facilities is no different to using cricket nets. You can take your pet for grooming, but a kerbside mobile service is not permitted. Our public health team has had the unenviable task of permitting only a fraction of similar activities where "risk" decisions cannot be based in evidence. When rules are inconsistent, we risk reducing compliance more broadly. The good news is that we know, when similarly "risky" activities progressively open up, their safety has already been tested.
A sure and speedy exit. Sentinel surveillance will be a game changer, protecting high-risk locations while providing active surveillance for community transmission. Symptomatic testing remains critical and numbers are looking good, with only one in a thousand tests coming back positive. Wastewater surveillance will come into its own as community transmission drops and revitalised, localised contact tracing and outbreak investigation teams take us to a new level of response. Can we go further? Let's test asymptomatic people when a local sewerage catchment returns a positive result, or for casual contacts at high-risk exposure sites such as Chadstone’s Butchers Club outbreak. If we routinely follow up both first and second-degree close contacts within the first 48 hours, it will help contain outbreaks within two chains of transmission.
We should be filled with confidence, yet messaging remains focused on rules and "explosive" outbreak potential; so much so that some are now anxious about easing restrictions. Others are frustrated at inconsistent rules, or fearful we will never leave lockdown because the road map is too conservative.
The recent Burnet modelling was only used to simulate opening up directly to the final step, but even so, they found moving to outdoor gatherings of 200, the return of community sports and opening of cafes and pubs, etc, carried only a 17 per cent risk of a resurgence in cases if restrictions were eased with fewer than 10 cases per day. This is without even stepping up our outbreak response as we are with Chadstone now.
Last autumn, we could have up to five people in our homes as many rules were eased on May 12. We had simple principles to follow and we were trusted. On that day we had a 14-day average of 10.8 cases and the Cedar Meats outbreak was still being contained. Yet, over subsequent weeks we successfully eliminated the virus circulating at that time.
What people need now is reassurance that our outbreak detection and response systems are up to the task in the absence of restrictions, and certainty that next steps will be safe and rational. We need positive and clear messaging to get everyone through this – fewer rules, more confidence.
Professor Catherine Bennett is chair in epidemiology at Deakin University's School of Health and Social Development.
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