Melbourne’s latest Covid outbreak a cluster of curveballs

Just when things were looking up for Melbourne, we’ve been hit by a cluster of Covid cases with a dangerous new variant. What does this mean for the city’s lockdown?

The Covid-19 outbreak in Victoria is taking many twists.

False positives are not a common feature of Covid-19 testing, but their possibility increases as the infection rate falls.

The Health Department has a robust review process, and two positive cases were removed this week as a result.

Lowering case numbers is a good thing, but especially when these were two significant cases that highlighted concerns over the casual infection risk of this Kappa variant.

One “case” suggested a woman was infected upon entering a display home well after a case had left.

Removing this aerosol transmission worry was almost as significant as removing a large number of exposure sites and primary contacts from the investigation.

But just when things were looking up, we were hit by an epidemiological curveball, an emerging cluster of cases with a different Covid-19 variant.

It may be fortuitous that Melbourne was locked down as this latest cluster was discovered. Picture: David Crosling

It’s not the first time we have had two distinct Covid-19 outbreaks overlap in the community.

We saw it in the second wave in Victoria with two separate seeding events into the community from two quarantine hotels; one cluster was contained after 37 cases, the other expanded into the second wave.

We saw it also in NSW during the northern beaches outbreak when the Berala cluster was discovered to be a separate seeding event from patient transport linked to returned travellers.

Now in Melbourne we have an outbreak, triggered by a returned traveller who picked up an infection with the Kappa variant in hotel quarantine, running concurrently with another smaller outbreak of the more transmissible Delta variant, also associated with India.

The Delta variant has been seen outside quarantine only once in Australia, and that was in the NSW case that was with a variant genetically matched to a returned traveller in quarantine, but a direct physical link was never established.

Genetically the Delta virus in the West Melbourne cases does not closely match any of the Delta cases seen in returned travellers.

The answer to the question about the source of the cluster may still be going on long after restrictions have eased. Picture: David Crosling

Could it have been transmitting at low levels in the community since the NSW case and only now appeared with enough mutation to look different?

Given Delta is associated with more severe infection in Britain, with more than twice the hospitalisation rates of the UK strain, and is 50 per cent more infectious, if this had been circulating for some time more cases should be visible. Therefore, it’s unlikely.

What does this mean for Melbourne’s lockdown?

It may be fortuitous that we were locked down as this latest cluster was discovered, and that we had mass testing and vaccinations prompted by the Kappa outbreak. But this is not why we are in lockdown. If we are sure that all close contacts who need to be in quarantine are, then we will have achieved our goal.

The new West Melbourne cluster will hold us back a few days as the investigation unfolds, but if we find no more cases outside close contacts the need for restrictions might abate.

The answer to the question about the source of the cluster may still be going on long after restrictions have eased.

Catherine Bennett is Deakin’s chair in epidemiology