Coronavirus: There may be a 'reduced risk' of outbreak in Wellington thanks to Sydney COVID-19 case's first vaccine dose

The news that a person infected with COVID-19 travelled to New Zealand and visited many of our capital's most popular establishments over the weekend has set alarm bells ringing.

Since the news broke on Tuesday, Wellington's coronavirus response has been escalated to alert level 2, testing centres have been set up across the region and health officials have been frantically chasing up potential contacts.

So far, no further cases have been found - and the boss of a Kiwi COVID-19 surveillance company says the risk of a potential outbreak may have been reduced by the fact the man has received his first dose of the vaccine.

"We hope that the Sydney visitor to Wellington was not one of these super-carriers even if infected with the more transmissible Delta variant," Rako chief science officer Dr Stephen Grice said.

"It is good news the visitor had a first vaccination shot as it's possible this reduced the risk of being a super-carrier, which supports Chris Hipkins' comment that it is a low risk but not no risk."

A Public Health England study published in the British Medical Journal backs this up, finding adults infected with COVID-19 three weeks after receiving one dose of the Pfizer-BioNTech or Oxford-AstraZeneca vaccine were 38-49 percent less likely to pass the virus on.

The study found protection against COVID-19 was seen from around 14 days after vaccination, with similar levels observed regardless of the age of cases or contacts.

However should the Sydney case prove to be a COVID-19 'supercarrier' - an individual carrying unusually high amounts of the SARS-CoV-2 virus - it could spell bad news for Wellington.

A Colorado University study found just 2 percent of COVID-19 cases carry 90 percent of the circulating SARS-CoV-2 virus in a population. That means most people with COVID-19 don't infect many if any others, but a few people infect others in large numbers.

"Coming in contact with a super-carrier is a game of chance so you have to continue to be careful. And some asymptomatic people are carrying a viral load as high as someone with COVID-19 who is very sick in hospital," Dr Grice said.

"If you do come into contact with a super-carrier there is much higher risk - and there is no difference whether they are coughing or not."

For most of the pandemic, no one has known what makes someone more likely to spread COVID-19 than others.

But researchers earlier this year identified that older and less healthy people generate more droplets than their young and healthy counterparts, making them more likely to spread the virus.

New Zealand health officials have not found evidence of community transmission since February 28. If a community case was to be detected on Friday, it would be the first since the emergence of the last case linked to Auckland's Valentine's Day cluster, 117 days ago.