Why are NZ’s Covid-19 cases spiking?

Analysis: Covid-19 cases in New Zealand have spiked at the same time our testing has ramped up, but is it correlation or causation?

In the past week, the number of Covid-19 cases in New Zealand has quadrupled. In the same period, the number of completed Covid-19 tests has also nearly quadrupled.

Is this correlation or causation?

The implications of the answer to this question are massive: If more tests lead to more cases being identified, then Covid-19 could be far more widespread than previously anticipated. If it's coincidence, then we can have much more confidence in the current testing regime.

In order to answer the question, we have to understand the two reasons testing has ramped up so dramatically. First, the criteria were recently changed to allow for testing cases with a plausible connection to Covid-19 - overseas travel or contact with a confirmed case - even if they don't have a fever but display other symptoms.

Second, a provision of the criteria that allows doctors to use their clinical judgment - essentially, discretion - to order a test even if the patient doesn't fit the strict criteria is now being used more liberally. This is after Jacinda Ardern stepped up efforts to inform doctors of their ability to do so, although the criteria have allowed such discretionary testing since March 8 and Ardern said as much on March 9.

That message seems to have finally filtered through, with labs completing 620 tests on Tuesday, compared to 582 for the entire previous period.

If the eight new cases announced on Wednesday and the four on Tuesday wouldn't have been caught under the old testing regime, then that could signal more cases are out there, waiting to be found as testing numbers continue to rise.

The Ministry of Health was unable to provide a breakdown on which of the eight newest cases would have been caught under the old regime, and the information presented to the public has been too sparse for us to do our own detailed analysis.

"The Ministry's assessment is that all tests would meet the case definition. The case definition allows leeway for clinicians to use clinical judgment in applying the tests - answering more specifically would mean talking to each of the health professionals who tested their patients," a spokesperson told Newsroom.

However, all eight involve people who travelled from overseas, satisfying what is likely the stricter of the two criteria: proving a plausible connection to Covid-19.

It seems likely that many or even most of the new cases would have been captured by the old regime, although this cannot be stated for certain. That's cause for celebration but also a reminder that, as 80,000 New Zealanders abroad are urged to return home, there's potential for significantly more cases arriving on our shores.

Ensuring this doesn't translate to community transmission - which New Zealand does not yet seem to have - will require vigilance and an increasingly robust testing regime capable of processing thousands of tests a day.

Read more of Newsroom's Covid-19 coverage here. 

Covid-19 is transmitted like the flu. The Ministry of Health recommends that all New Zealanders wash their hands frequently and refrain from touching their face in order to protect themselves and others. Call Healthline on 0800 358 5453 if you have any symptoms and have been to any countries or territories of concern or have been in close contact with someone confirmed with Covid-19.

Help us create a sustainable future for independent local journalism

As New Zealand moves from crisis to recovery mode the need to support local industry has been brought into sharp relief.

As our journalists work to ask the hard questions about our recovery, we also look to you, our readers for support. Reader donations are critical to what we do. If you can help us, please click the button to ensure we can continue to provide quality independent journalism you can trust.


Newsroom does not allow comments directly on this website. We invite all readers who wish to discuss a story or leave a comment to visit us on Twitter or Facebook. We also welcome your news tips and feedback via email: Thank you.

With thanks to our partners