Why aiming for zero community cases is still feasible

The latest virus outbreak in Victoria will probably not be the last, but it’s still possible to get back to zero cases of community transmission by February.

Elimination of local transmission has been achieved more than once in Australia, and again is the ambition in Victoria and NSW as the newly introduced virus again challenges health authorities and the wider population.

Can we get there again? And, if so, how long will it last?

There is a fine epidemiological line between controlled low-level local transmission and escalating cases that can quickly turn into a wave if control is lost. None of the recent clusters have reached the tipping point towards uncontained community transmission thankfully, even though all three were instigated with major seeding events; the Avalon venues and the BWS store in Berala making up the main Sydney clusters, and the Smile Buffalo restaurant in Black Rock, Melbourne.

Sydney’s northern beaches outbreak appears contained — no cases were reported for four days, then just one from the Croydon cluster on Tuesday and another from Avalon on Wednesday, both close contacts of known cases and already in quarantine when they became infectious.

Genomic analysis shows Croydon and Wollongong cases are all caused by the same strain seen in the northern beaches, and these two clusters have now been linked via the Swallowed Anchor restaurant.

The cases that emerged over the New Year period in the west of Sydney, centred on the Berala BWS, couldn’t be linked to the existing clusters.

Genomic analysis revealed it was in fact a different viral strain to that in the northern beaches, matching the variant contracted by a ground transport worker assisting in the transfer of returned travellers. It was then found a close contact of this person had contracted the virus and visited the BWS when infectious.

The MCG is a possible acquisition site for a new unlinked case. Picture: David Caird

While thousands were exposed at the busy BWS and nearby Woolworths, the good news is recent cases have been able to be linked to these exposure sites and, so far, there isn’t firm evidence of a wider, unmapped spread.

Tracking this many casual contacts and all the sites they subsequently visited is an enormous piece of work, but doable, and preferable to letting the virus get away.

One case travelled to regional NSW, but none of his travelling companions have tested positive, so hopefully this is a sign he may not have been very infectious.

The northern beaches virus did hitch a ride to Melbourne, sight unseen. This infected person, or someone they passed the virus on to, dined at a restaurant in Black Rock on December 21. To date, 27 people at the restaurant, guests on different tables or staff along with their close contacts, have tested positive. There are now some 1300 people in isolation or quarantine.

More worryingly, both NSW and Victoria reported an as-yet unlinked community case on Wednesday. The concern is one or more transmission chains linking back to known clusters may have been missed, or there is a new seeding event. Upstream acquisition searches are as important as downstream contact tracing in these situations.

The challenge with COVID is the incubation period you need to cover to identify possible sources is both long (up to 14 days) and variable (may be as short as 3-4 days). The process can be assisted by genomics which can narrow down the search window, for example when the NSW health department realised the western Sydney cases were linked to the transport worker and not Avalon, they could narrow their search for possible crossover points.

Both NSW and Victoria have modified public health settings to slow transmission which supports the public health response, and there has been good engagement in calls for testing which also helps us see where the virus is, how it is moving, and to more effectively prevent further transmission.

There is a fine epidemiological line between controlled low-level local transmission and escalating cases. Picture: Simon Dallinger

Less than three weeks has passed since the first northern beaches cases were notified, so it is early days yet for all clusters. But given daily case numbers sit at three or less, we can remain optimistic these are containable, and zero community transmission is still potentially attainable.

Is it a coincidence these clusters are associated with mini super spreader events? I think not.

Clusters are identified because many cases are generated in quick succession and the chance of someone developing symptoms and getting tested rises. The active contact tracing that follows identifies the others linked to the same exposure, and so a cluster becomes visible.

It is likely there have been other incidents where the virus crossed our international or state borders and was not detected.

Overseas research shows only a small proportion of cases, less than 20 per cent, ever pass the virus on to big numbers but they are responsible for 80 per cent of the next generation of cases. These are the clusters we get to know about, and they help us identify and address weak points in infection control and our international borders.

The latest unlinked cases may yet be linked back to known clusters, like most are, or they may remain as single unexplained cases that do not signal future risk of cases, but were identified through enhanced testing associated with the existing clusters. Let us hope these cases signal the end if unknown chains, if that’s the case, and not the start of new clusters.

Can we get back to zero community transmission by February? Possibly. Will this be the last community transmission we see? Probably not.

But as we contain these clusters, our confidence in our defences should continue to build, and we can at the very least learn to live with the virus in check in our communities, if not completely gone.

Catherine Bennett is chair in epidemiology at Deakin University

Originally published as Why aiming for zero community cases is still feasible