An epidemiologist says he’s not surprised the Wellington visitor with Covid-19 didn't appear to have spread the virus even though they’re suspected to have the highly-transmissible Delta variant.
University of Canterbury associate professor Arindam Basu warned, however, that it was still early days. He said people should remain vigilant as only a week had passed since the Australian man was in the capital.
But, there was cautious optimism so far after two days of no new Covid-19 cases in the community after mass testing and negative test results in the Wellington region’s wastewater. Basu has a few theories about why that’s been the case.
The first is that most people who contract the virus don’t tend to spread it widely, and that’s been true throughout the pandemic, he said.
Basu said only about 10 to 20 per cent of people turn into “super spreaders”, but they result in about 80 to 90 per cent of overall infections.
Unless a person expels a sufficient amount of the virus into the air — such as through singing, shouting or sneezing — that then comes into contact with people’s mouth, nose or eyes, quiet breathing is “not likely to result in people infecting others”, he said.
“Rather than focusing on an individual [as a super spreader], what usually happens is there is a gathering of many people in a setting that does not have good ventilation.”
Basu’s second theory is that with about one million Covid-19 vaccine doses already given in New Zealand, with about 382,000 people receiving two doses already, it helped stop the virus’ spread.
He said the fact that the traveller had already received one dose of the AstraZeneca Covid-19 vaccine also “definitely” made a difference.
“Their viral load is much lower than if he had not taken any vaccine whatsoever. So, to start with, he had a low viral load that he could emit,” Basu said.
“The third thing is that if you note the pattern of this person’s movement around Wellington, there were a number of different places visited.
“So it's quite possible that the opportunity of actual transmission, the duration or proximity to other people, may have been cut short enough that they didn't have the opportunity of wider transmission.”
Basu said for a new Covid-19 variant to spread in a community with no prior infection, or had eliminated it, it would usually take “several rounds of introduction”.
New Zealand, at the moment, was now in a comparable state as it was in early 2020, where it took multiple people to come into the country with Covid-19 to start spreading it widely, he said.
“It can work both ways. [This state] can be dangerous, or it can be protective. In this case, it was protective because the new infection could not get a stronghold in the country.”
He said as long as the country’s border was maintained properly, the risk of seeing novel variants in the community could remain low.
The traveller is suspected to have the Delta variant of Covid-19, although New Zealand health authorities are still awaiting confirmation from their Australian counterparts.
The man’s infection is epidemiologically linked to Sydney’s Bondi cluster, which saw the Delta variant spread rapidly.
Estimates as to how much faster the particular variant spreads varies. Basu said the variant is estimated to be between 50 to 67 per cent more contagious. Meanwhile, University of Otago evolutionary virologist Jemma Geoghegan puts that figure between 50 to 100 per cent .
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