The number of new COVID-19 cases in New South Wales each day is likely to be up to 10 times the figure reported by authorities, according to one epidemiologist, as fears about community transmission in the state grow.
- Experts say there are undetected cases of community transmission in NSW
- While Victoria has widespread community transmission, NSW is working to keep numbers contained
- Experts say two weeks of mandated mask use would help stop asymptomatic and pre-symptomatic cases from spreading
University of Melbourne epidemiologist Tony Blakely made the estimation and said the presence of cases without a known source was “a good signal” actual case numbers were a lot higher.
“So, if they’ve got an average of 20 [cases per day] in the last week, that means that at any one point in time, there’s about 200 other cases out there that we don’t know about. Basically, 10 times the daily count,” Professor Blakely said.
He said some of those cases would have an identifiable source, but others would be “mystery cases”.
“If you’ve got a case that pops up and you can’t trace it back to its source, and you assume that tracking has been done well … it means it’s come from silent transmission,” Professor Blakely said.
On Thursday, NSW reported 12 new cases of COVID-19, including three that were acquired locally without a known source.
In total, there were 380 cases in the state that were acquired locally, but the source had not been identified.
Leading epidemiologist Marylouise McLaws agreed NSW had some level of undiagnosed transmission of COVID-19, with at least one undiagnosed case for every diagnosed person.
“I am not sure I would put it as high as 200, but I accept the argument that for every diagnosed person, there is probably at least one other that we don’t know about, or possibly two or three,” Professor McLaws said.
“There are probably at least 20 cases that we are not aware of because it takes people a while to realise they are unwell and to get tested.”
Professor Blakely said “silent transmitters” could be people who were infected with COVID-19, but were asymptomatic, as up to a third of all infections were.
“And then there are other people who are also silent transmitters who are so mildly symptomatic — they don’t realise they have it,” he said.
Both experts noted people who had COVID-19, but were pre-symptomatic, would be spreading the virus unknowingly before symptoms hit, contributing to community transmission.
“Eventually, a chain of transmission that is silent will end up with a case that’s symptomatic. Someone … is going to get crook enough to present with symptoms and get tested,” Professor Blakely said.
Professor Blakely said after a couple of weeks of cases reappearing, it was “almost inevitable” some of those would be “mystery cases” without a known source.
NZ’s elimination vs NSW’s suppression
Victoria has widespread community transmission, but is starting to see its daily new case count decrease.
Western Australia, Queensland, South Australia, Tasmania, Northern Territory and the ACT have all successfully eliminated community transmission of the virus.
NSW has a suppression strategy, so some new cases are to be expected, but to move to elimination, the state’s response would have to be very different.
When four cases emerged among an Auckland family, the New Zealand Government moved quickly to place the city under stage 3 restrictions and the rest of the country under stage two.
Those were the first cases of COVID-19 recorded in New Zealand in more than 100 days.
On Thursday, the country recorded another 13 cases, but they were all linked to the Auckland family.
“New Zealand gives a perfect example of what you do when a mystery case pops up and you want to maintain elimination or get to elimination — you go hard and you go early,” Professor Blakely said.
“Because you need to hit it that hard to get ahead of the virus and stop transmission and stamp out all cases, including both the ones you’ve been able to find where they come from as well as the silent mystery cases.”
NSW has a different strategy, one that aims to keep the caseload at a “manageable amount” and if authorities can manage to keep case numbers down, Professor Blakely said it would provide a “blueprint”.
“NSW is running a game at the moment whereby they’re accepting there will be ongoing community transmission and they want to keep it low,” Professor Blakely said.
Masks critical to stop COVID-19 ‘wildfire’
To keep the caseload low, both Professor Blakely and Professor McLaws recommended widespread use of masks during this delicate time in NSW.
Professor McLaws, who is a member of the World Health Organization experts advisory panel for its COVID-19 response and an infection-control expert, urged everyone to wear a mask for the next two weeks.
“In NSW, over a two-week period, we are now up to 175 cases, which is high,” she said.
That figure excludes returned travellers in quarantine who cannot spread the virus.
“A simple mask-wearing exercise for the next two weeks, which is slightly more than two incubation periods, should be able to flush out more of the asymptomatic cases or yet undiagnosed,” Professor McLaws said.
She said mask use was effective in stopping the virus from spreading.
“It won’t hurt and if we can all do it for two weeks, we will see those numbers go down,” Professor McLaws said.
Professor Blakely agreed, saying: “Wearing a mask has no real effect on the economy, it’s just a slight imposition on the public.”
The NSW Department of Health has been contacted for comment.