Covid-19 is, by its very nature, unpredictable, and it can be difficult to compare outbreaks across settings. However, it is worth contrasting epidemic dynamics and public health responses in Sydney today with those in Melbourne in June given concerns that we might be on the verge of another large wave.
By 20 June in Victoria, 25 new infections were reported, with three of these in hotel quarantine staff, taking that outbreak to 10 and the state’s weekly total to 107. Even though the hotel quarantine breaches were a known source of community transmission, only 25% of new community cases were linked to known cases or locations at the time of report, meaning many infections might have been missed. The proportion of cases that could be linked dropped further to less than 10% soon after.
In New South Wales on Monday, while we still do not know the point at which this overseas strain was introduced into the community, local transmission cases are linked to two critical seeding events in the northern beaches – 83 cases are all linked to one of two Avalon venues on December 11 to 13, either as attendees or their close contacts. Two remaining community cases may yet be linked when investigations are completed as they too are from Avalon.
Back in June, Victoria’s public health capacity for chasing down community transmission had not yet been fully stress-tested as symptomatic people in the community were not tested unless linked to a known case, or if they had community onset pneumonia. NSW Health, on the other hand, has been chasing down community transmission cases and clusters since the virus was reintroduced into NSW from Victoria in July, and looked to have finally eliminated that transmission with more than 28 days free of local transmission of the Melbourne strain.
NSW Health also routinely alerts the wider community of potential exposure locations to encourage testing of those at highest exposure risk, something Victoria was not doing until the tail of the second wave. Venue records are also now required across Australia. In June Victoria was testing up to 20,000 per day, NSW exceeded 38,000 on Sunday. NSW knows only too well what played out in Victoria over subsequent months, and this too should encourage people to engage, test and isolate.
For all these reasons, there is likely to be more complete case identification in NSW now than in Melbourne in June, and so daily case counts may not be directly comparable.
Another important difference lies in where the initial seeding into the community occurred. The hotel staff in Melbourne took the virus home to sections of the community with large extended households and casual workers often working across multiple locations. By 20 June, two aged care facilities were already in lockdown. It was the involvement of workplaces and healthcare settings that accelerated transmission and led to the rapid rise in Victoria’s case numbers. These outbreaks then seeded the virus into more households as workers took the virus home, spreading the virus across Melbourne and into regional Victoria.
The restrictions in place on movements and mixing outside the home across Sydney, Central Coast and the Blue Mountains, and the hard lockdown in the northern beaches, provide a strong outbreak control position for NSW Health, allowing the public health response and contact tracing efforts to be more rapid, efficient and effective. Restrictions and lockdowns buy precious time for NSW Health to get ahead of the virus, map the edges of the cluster, and make sure the outer ring of exposed contacts are all in quarantine before they are infectious.
Covid-safe behaviours – distancing from others, practicing good hygiene and wearing masks when around others – all lessen the risk that a virus exposure will translate to infection. Masks are now strongly recommended in Sydney, but arguably it is time for mandatory masks on public transport and crowded indoor spaces. This should also be considered across states when we have active community transmission in the country. Some casual and close contacts of Avalon cases and people who visited hotspot venues reside outside the northern beaches and have returned home. We usually don’t know about active cases in our community until after they have tested positive and have been infectious while out and about. Wearing a mask reduces the risk of getting infected if exposed and, when done en masse, prevents secondary clusters.
It is cruel this has happened so close to Christmas. We must now all be ready to change plans at short notice. Those in the northern beaches will soon know if they will be in lockdown at Christmas, but regardless the message will still likely be that it is not safe to bring households together indoors, especially for older relatives. For greater Sydney, Blue Mountains and Central Coast, travel limitations will remain, and travel and holiday plans for all Australians will be impacted and contingency plans the order of the day in case rules changes at short notice. But 15 cases on Monday, all linked, is encouraging news. We can also take heart that this is more akin to the outbreaks at the closing stages of Victoria’s second wave, not the start.
• Catherine Bennett is chair in epidemiology at Deakin University
