By Catherine Bennett
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Victoria will enter its sixth lockdown from 8pm on Thursday in response to the emergence of new mystery cases of COVID-19 in recent days.
Contact tracing is no longer enough to contain an outbreak of the Delta variant – people who have contracted the virus just can’t be found fast enough.
Even with the best QR systems and contact tracing, the infection cycle is now so fast that it is not possible to identify potential cases and test and isolate before they have time to become infectious.
We are still waiting on the genomics, but given the recent mystery case was a Delta case, we know we do have virus in the community despite our last lockdown.
Recent cases have largely been in isolation throughout their infectious period as either close contacts of known cases or who had been at Tier 1 exposure sites, or their household contacts.
The worker at the testing site could not be directly linked back to the outbreak other than via a genomic match. Now we have more “mystery” cases, some linked to each other, but at least three index cases that we can’t link back to the outbreak.
So where were they infected? It is possible they are not related to the known Lockdown 5 outbreaks and instead link to different importations of the virus across state or international borders.
Given the timing, it is also likely these are associated with the recent two seeding events, the removalists and the family in Hume. The larger outbreak was thoroughly followed up, but there were important gaps in the investigation that mean we cannot be sure that we did track and trace every case, or identify every exposure site.
As we now know, not everyone who goes into 14-day quarantine gets tested and therefore health authorities can’t work out if they might have been infectious before they quarantined.
If they were infected, and we know a number of cases were found among the three-quarters who did test, any missed cases could have been infectious and exposed others. On average most of the cases that were picked up on testing were infectious in the community for more than a day before quarantining.
Genomics will help unpack the most likely transmission routes involved, but whether newly imported or older chains of transmission, we now have five of Thursday’s cases infectious before they were discovered.
Lockdown 6 is upon us, and once again state-wide as we also have positive test results from the sewerage surveillance in northeast Victoria.
Seven days will allow authorities to map and then work out what is needed to contain this outbreak, but if this is a signal that more cases are yet to be found, or that we have transmission chains running back some weeks, then there is a lot more work to do.
A number of restrictions have been maintained in place since lockdown, so we will have supressed transmission risk while these latest cases were infectious, as well as those others yet to be found that they caught the virus from.
That is exactly why they were in place, and should make this work in front of us all that little bit easier and more swift.
Catherine Bennett is chair in epidemiology at Deakin University.