Covid-19 Omicron outbreak: New Zealand's reproduction number highest in the world, model shows

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New Zealand currently has one of the highest R values for Omicron in the world, according to the Covid-19 Modeller from Rako Science.

The Reff (effective reproductive number) is a measure of how many people each Covid case is infecting, on average, in a mixed population of susceptible and non-susceptible people.

A Reff of 2 means each person typically spreads the virus to two more people, giving rise to an exponential outbreak. A Reff below 1 would see case numbers dropping.

As of Sunday morning, New Zealand's Reff was 3.74, the highest in the world according to the model's calculations.

Other countries with a high Reff included Myanmar (3), China (2.37), Vietnam (2.29), Cambodia (2.25) and South Korea (2.22).

Countries with an Reff over 2.2 are represented in dark red in Rako Science's Covid-19 modelling.

Many regions that saw Omicron surge earlier this year have reached R values over 3 at their peak but have dropped below 1 as cases fall.

In Australia, Victoria's Reff is at 0.64 and New South Wales is at 0.75, both after peaking at about 3.5 at the start of the year.

But in Western Australia, where the Omicron wave is at a similar stage to New Zealand, the Reff is 3.07.

On Saturday New Zealand had 13,606 new cases, up from 12,011 the day before. The number of detected cases doubled between Thursday and Friday as RAT testing became widely used.

Rako Science founder and director Stephen Grice said earlier this week that New Zealand was among only a handful of countries experiencing an Omicron surge.

The country's surge began on January 7, the day that R0 climbed above 1.0, he said.

Rako Science, which offers saliva PCR testing, said its sampling across the country suggested a positivity rate of about 3 per cent nationwide as of February 22, with a higher rate in South Auckland.

Not everyone thinks Reff is a useful number when it comes to Omicron.

Te Pūnaha Matatini Covid modeller Dr Dion O'Neale said last month he felt R didn't really apply to Omicron.

He was more interested in the doubling time of around three days during an outbreak, which had been fairly consistent around the world.

"I keep trying to convince people that R is a really dumb measure to use, because it's become a mix of pathogen, and interaction, and observation, all rolled into one," he said.

"The one thing that's both directly observable and relatively consistent around the world has been that doubling time."