Covid 19 Omicron outbreak: High R value a result of widespread introduction of rapid antigen testing, says experts

The country is facing Coronavirus case numbers like it has never seen before, and currently we have the highest R values for Omicron in the world.

And, experts warn that these figures will continue to rise steeply.

Epidemiologist Michael Baker said he wasn't surprised that the current R-value figure is as high as it is, which sits at 3.74 according to the calculations of a 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.

Baker said our 3.74 R-value was significantly affected by the widespread introduction of rapid antigen tests.

"There was certainly a big jump, when those results came in that would have impacted our effective reproduction number," said Baker.

"The gradient is still meaningful, it is still telling us that we have got very intense transmission at the moment."

He said case numbers would continue to rise steeply.

"We know that because the Omicron variant may affect half of the people in New Zealand over the next two months it means we will be peaking with tens of thousands of cases a day when it peaks in different parts of the country but only a fraction of those people will get tested."

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

Associate Professor Siouxsie Wiles said the contrast of New Zealand's R-value to the rest of the world currently came down to the fact that the country had been able to delay the entry of Omicron by two months.

"Many countries are much further into this Omicron wave – many countries had this kind of spread a month or so ago," she said.

She said the use of RATs provided a faster process to finding out how many people test positive or negative.

"We're now in a phase where there is good access to RATs which means we get a better idea of how many people are coming through to get tested and how many of them have Covid."

Baker and Wiles both said it is also important to look at all measurement tools such as the doubling rate, the seven-day rolling average and the hospitalisation rate.

Wiles said it was too soon to tell what our peak could look like because RATs have only just been introduced.

She said if our rates were to start doubling every one to two days, then we'd see transmission similar to other countries that experienced very high transmission.

Once over the peak, Wiles feared that the virus would not drop off fast.

"I imagine this outbreak is going to have quite a long tail ... we're in for a difficult few months."