Since Sydney went into lockdown, there have been many figures thrown at us, from the daily case numbers to the number of people infectious in the community, the number of hotspot LGAs to the percentage of fully vaccinated adults needed before restrictions can ease. However, one number that hasn’t received a lot of airtime in the premier’s daily press briefings is the ‘R0’, but as Sydney’s Delta outbreak continues to surge, it is fast becoming the most crucial stat to keep track of.
The R0 was talked about fairly regularly during the initial outbreaks at the beginning of the pandemic last March, as Australia attempted to “flatten the curve”. Since then, because of the almost unparalleled success (until recently) of Australia’s suppression strategy, which has allowed Sydneysiders to enjoy almost totally unimpeded freedom for more than a year, it has been largely irrelevant on Antipodean shores. Elsewhere around the world, however, it has been one of the most talked-about metrics, and for good reason.
What is the R0?
Pronounced “R-nought”, this number relates to the virus’s ability to replicate and spread, quantifying the number of people on average a single infected person will pass the virus onto. This number is not fixed, as it is not only a gauge of how infectious the virus is but also of how local environmental factors, like lockdown settings, are helping to curtail the spread.
Why is it important?
Another dynamic of the pandemic that has not been discussed much in Australia for a while is the question of exponential growth. The R0 is essentially a proxy for establishing if an outbreak is growing, shrinking or holding steady – and crucially, whether lockdown settings are having an impact. The higher the R0, the more infections a community can expect. Any R0 greater than 1 is an indication that lockdown measures are not containing the spread, as this means that every single person diagnosed has almost certainly passed on the virus to more people. An R0 of more than 1 means the increase of cases has become, or is fast becoming, exponential.
What is the R0 in Australia?
Until recently, Australia had managed to maintain an R0 of less than 1 for more than a year, meaning containment measures were able to prevent infections from spreading out of control. However, Sydney’s R0 is now 1.3, which means every single infected person is on average infecting 1.3 others, or put another way, for every 10 cases diagnosed, 13 more have been infected. With an R0 of 1.3, the numbers in Sydney will continue to climb, with the potential to get far worse than the already record-breaking numbers the city is currently recording. It’s also an indication that contact tracing and other lockdown measures are not yet effectively suppressing the spread.
To put that into perspective, as of the time of writing this piece, more than 4000 cases in Sydney’s current Delta outbreak are unlinked, meaning chains of transmission cannot be accurately determined and cut through contact tracing. In an interview with the Sydney Morning Herald, one of the federal government’s top medical advisers, epidemiologist James McCaw, said that Sydney’s outbreak was likely to continue climbing for several more weeks, and could reach a peak of 2,200-2,500 per day before stabilising.
The good news, however, is that other countries have recorded a far higher R0 with Delta than Australia has so far experienced, with outbreaks in certain parts of the UK and the US clocking an R0 average of 3-4 and other parts of the world an R0 as high as 6-7. This shows that, while not totally arresting the spread, lockdown measures are slowing it, preventing local health infrastructure from being overwhelmed and allowing Australia’s sluggish vaccination rollout time to reach the 70-80 per cent coverage the Doherty Institute report says is required to retire lockdown measures.
What this means is that lockdowns are still absolutely vital to Australia’s recovery, in tandem with vaccination, in order to prevent huge numbers of hospitalisations and deaths. As premier Gladys Berejiklian has said in several recent press briefings, Australia, and Sydney in particular, is entering the darkest days of the pandemic, and September and October are going to be brutal for all of us. However, Australia has something that no other country had when they faced similarly bleak scenarios last year: two (soon to be three) highly effective vaccines. The message is clear: get vaccinated, stay home, stop the spread. We can do this.