Catherine Bennett, Deakin University

It’s the last thing anyone wanted to hear, a week out from Christmas. But the growing cluster of COVID-19 cases linked to Sydney’s northern beaches has put Australia on alert.

There are now 28 cases associated with the cluster, with 25 linked directly either to the Avalon RSL, Avalon Bowlo, or both, NSW Health confirmed earlier today. A further two cases are direct contacts of the 25. One case identified via contact tracing, who had been at one of the Avalon venues, had subsequently flown to Queensland, but has since returned to New South Wales.

Genomic testing shows the strain from the northern beaches cluster to be an international variant of the virus, one currenty circulating in the United States. But what we don’t yet know is exactly how and when this international strain entered the country and how it spread to the community. One traveller still in quarantine, who has a similar strain and arrived in early December, is being investigated as a possible index case.

The Sydney van driver who tested positive earlier this week, after transporting international air crew, was also likely infected with a US strain of the virus, but a different variant. There have been no other cases associated with him so far and his close contacts remain in quarantine. In other words, it looks like he did not seed the transmission we’re seeing in Sydney’s northern beaches.

Travel plans disrupted

Today’s news further complicates Sydneysiders’ interstate travel plans, with most states and territories announcing various, and rapidly changing, travel restrictions.

If this cluster continues to grow, it looks likely northern beaches residents will have to rethink their Christmas plans and avoid large gatherings, especially of multi-generational families.

Some of the cases who acquired their infection in Avalon are from different areas in Sydney, and so there is still a risk the spread might be wider than the immediate northern beaches area. This will become clearer over the next couple of days as the contacts of these cases are tested, and as people who attended possible exposure sites across Sydney are tested.

NSW Premier Gladys Berejiklian also said residents of other parts of Sydney should be on high alert:

Nobody should be getting on public transport without wearing a mask, nobody in Greater Sydney should be going to a supermarket or a place of worship without wearing a mask[…] It would just be crazy.

A list of sites where people may have been exposed, and are required to come forward for testing, is being updated regularly.

Where did this strain come from?

Genomic testing involves looking at the genetic sequence of various viral isolates to see if they are related. This can tell us whether cases within a cluster are linked, and can help identify the source of the outbreak — whether linked to other local cases, or a new introduction of the virus into the community, as in this case.

Genomic testing has already told us the virus at the centre of the northern beaches cluster is likely a US strain of SARS-CoV-2, the virus that causes COVID-19. But we still don’t know how it ended up in the community and in the northern beaches.

The traveller in quarantine, who has a similar strain, arrived in early December. If this is the source, it limits the time the virus has been circulating in Sydney to two weeks or so, which would be somewhat reassuring — there may only be one or two cases in the chain of transmission between the arrival of the virus and the cluster we now know about.

At this stage, we can’t rule in, or out, the possibility of other sources of this virus strain, including infected international aircrew. But news that quarantine arrangements for this group will be tightened, recognising it is a “weak link” in our chain of defence, is very welcome.

Air crews will now quarantine in one of two police hotels between flights rather than being spread across 20 or more city hotels where oversight of compliance with quarantine was left up to the airlines.

Contact tracing is also important

Evidence so far points to a significant “seeding event” on December 11. In other words, a person (or persons) who attended the Avalon venues at this time is thought to have been infectious, infecting a number of people while there. Some of these cases then attended a second local venue a couple of days later and more people were infected there.

The initial “index” case who attended the Avalon RSL on December 11 while infectious has not yet been identified. This person might not have been tested and remains unknown, or they did test this week but are no longer positive as they had recovered from their infection.

The index case could also be one of the cases we do know about — they may have been still asymptomatic on December 11 but infectious, and not developed symptoms till a few days later. Other secondary cases may have developed symptoms at a similar time if they had a particularly short incubation period, therefore it can be difficult to pinpoint who the index case might be.

We already have at least two secondary cases reported among contacts of those cases who attended these venues. That’s why it’s important to trace not just the immediate contacts of those infected, but the contacts of those contacts so that people who have already been exposed and infected, but are not yet infectious, are in quarantine pending the results of COVID testing.

If the evidence points to people in Avalon being infected more than a week ago where PCR tests (the type usually used to diagnose people) might not still work as the virus has been cleared, then authorities can use serology tests to check for past infection. This helps trace backwards, to understand the source of infections, and importantly, how this particular strain was introduced into the community. This also helps identify and shut down any “silent” chains of transmission that might have been missed upstream from the current cluster.

It’s really important to do this as quickly and as aggressively as possible.

What should we be doing in the meantime?

Northern beaches residents will need to continue following current NSW Health advice: limit movement of people into, out of and around the area; work from home where possible; avoid gathering in large numbers or in high-risk venues; and wear masks when they can’t physically distance.

NSW Health is doing all the right things to suppress this cluster. That includes identifying all the key sites where people may have been exposed, especially in Avalon, starting the process of identifying contacts of contacts, and opening up more testing clinics. This, together with the fact that all cases so far can be linked back to the key exposure sites means we are in a good position.

But a lot can happen in the week between now and Christmas. If the cluster spreads more widely, expect further travel restrictions, border closures and a very different Christmas to the one we were expecting only a few days ago.

Catherine Bennett, Chair in Epidemiology, Deakin University

This article is republished from The Conversation under a Creative Commons license. Read the original article.