The spread of the more infectious Delta variant of Covid-19 throughout Australia can be contained with a combination of strong adherence to public health guidelines and good luck, infectious diseases and virology experts say.
On Monday the number of locally acquired cases in New South Wales grew to 130, with 124 of those linked to the Bondi cluster. In Queensland, premier Annastacia Palaszczuk said the next 48 hours would prove crucial, as the number of local cases grew to 10. The Northern Territory health minister, Natasha Fyles, said authorities expected more cases, with the total there at six, while two cases have emerged in Western Australia since Sunday.
“A virus in the wild in winter is a hard one to get a handle on,” said Kirby Institute virologist at the University of NSW Stuart Turville.
Not only do people tend to congregate indoors together in colder months, giving the virus more chance to spread and survive, he said, but data was revealing additional challenges associated with Delta.
The secondary attack rate is the probability of an infection occurring in a close group of unvaccinated, previously unexposed people, such as a household or close contacts. Public Health England data suggests the Delta variant has a secondary attack rate of 11.4%, compared with 8% for the Alpha variant, originally identified in the UK. Delta soon dominated over Alpha.
But data on Delta is still being collated from the UK, and much of the data so far was gathered during the warmer months. A different and more concerning picture of secondary attack rate may emerge from countries where Delta is spreading throughout winter.
An example of just how high the secondary attack rate of the Delta variant can be was revealed in the cluster which emerged after a house party in the western Sydney suburb of West Hoxton. On Monday, the NSW health minister, Brad Hazzard, said: “Of the more than 30 people that were at that party, 24 of those people are now positive for Covid-19.” Six fully vaccinated health workers at the party have not yet developed the virus, highlighting the importance of vaccination.
Turville said this was why luck also played a role; there were always outlier events, such as super-spreading events, that could occur, bucking the data trends.
“There is luck and bad luck,” he said. “There are opportune moments that the virus has where someone with a high viral load may be in a setting that can seed onwards in large numbers. Other times, the contact tracers are on top of those people in time to get them safely into isolation and out of the community.”
Despite a few exceptions, with some people fleeing across borders to avoid lockdowns and travelling outside lockdown zones, Turville said Australians were overall “a cautious bunch” who began limiting their movements even before lockdowns were officially announced.
“We have done this well in the past,” he said. The more infectious variant that dominated Victoria’s second wave of Covid-19 also hit during winter, and was a much “fitter” variant than the S-strain that emerged after the first mutation of the virus.
“And we crushed it,” Turville said. “It no longer exists.”
While Delta is a fitter strain again, Turville said: “We know these strategies well now and can apply them again. It will be like turning off the dripping tap. We’ll turn it harder and harder the worse this thing tracks.”
Turville, who leads the Kirby Institute’s “containment lab”, where samples of the virus from cases throughout Australia are analysed and grown, said discussion comparing the responses to outbreaks between different states, and questions as to whether NSW should have locked down sooner, were unhelpful. He described the NSW chief health officer, Dr Kerry Chant, who has three decades of experience working in public health and communicable diseases, as “brilliant”.
“We have brilliant people advising us,” he said. “Let’s give them the respect to guide us through this response. My hope is that we have got this but we need to keep an eye out for those opportune viral spread moments and also not let pandemic fatigue lower our guard.”
James Wood, a mathematical modelling expert with the UNSW School of Population Health and who has expertise in infectious disease epidemiology, said the next few weeks would be a particularly “challenging period” for jurisdictions unaccustomed to Covid outbreaks, such as WA and the NT.
“This is about 40% more infectious than the Alpha strain, and about twice as infectious as the strains Australians dealt with last year, including in NSW with the Crossroads Hotel cluster,” Wood said.
“That’s a problem, because we had systems that worked pretty well against those strains, but once you’re looking at a strain that’s twice as infectious, those same systems may not be sufficient. Particularly if you have a bad start, like what’s happened with Bondi, and cases occurring in a mass gathering setting where it’s difficult to track who’s been exposed.”
It was not necessarily just the increased infectiousness that had seen the variant spread throughout multiple states so fast, Wood said.
“Every outbreak takes on the characteristics of the population it lands in, and if you think about somewhere like Bondi Junction, it’s a huge shopping centre near Bondi Beach, so it’s the sort of location which is going to be potentially connected to travellers,” he said. “That’s different to other outbreaks where it lands in a community that doesn’t travel a lot, where you see more local spread in families. Unfortunately, in this case, it started off in a setting where you don’t need many cases before it moves states.”
But Wood said Australia was still in a good position to contain and control the virus.
“But it doesn’t mean we won’t have a couple of dicey periods in the next couple of weeks as different states adapt to what’s going on,” he said. “Hopefully, the NSW outbreak is a bit of an outlier and the other ones will be more easily brought under control.”