Covid 19 Omicron outbreak: Positivity rate jumps as New Zealand reports record new cases

New Zealand’s test positivity rate is jumping up and moving closer to a World Health Organisation benchmark suggesting a widespread outbreak, Herald analysis shows.

The rate, according to one expert, is now far higher than at the peak of the Delta outbreak that shut down Auckland for months last year.

Using the latest available case numbers and tests processed, Herald analysis shows the country registered a 4.2 per cent positivity rate on Tuesday, near the 5 per cent threshold set by the WHO at the start of the pandemic.

It comes as cases broke through the 1000 mark yesterday, with a record 1160 positive results announced by the Ministry of Health.

Test positivity is the percentage of positive tests amongst total tests taken.

“Positivity rates will vary day to day but, as the outbreak progresses, we expect test positivity rates to trend upwards which is what we are seeing currently in New Zealand,” said a Health Ministry spokesperson.

The figures are still much lower than reported in other countries and regions at the peak of their outbreaks.

Reports show New South Wales, for instance, recorded a 20 per cent positivity rate at the start of the year, meaning one in five people who were tested returned a positive result.

According to the Health Ministry, the positivity rate for community cases only was 4 per cent on Feb 12, and 4.28 per cent on Feb 13.

A spokesperson said interpreting the positivity rate needs to consider how many tests are taken, who is being tested, and whether enough exposed people are being tested.

The test positivity rate is one of a range of measures indicating how widespread an outbreak is.

The Ministry also considers other factors like the geographic spread of cases, any increase in unlinked cases, and hospitalisations as a proportion of total cases.

University of Otago mathematician Dr Matthew Parry says daily fluctuations in the positivity rate may seem alarming but the figure can flatten out over the week.

He said the average per week is about 3 per cent, much higher than the 1.4 per cent average test positivity when the Delta outbreak was at its peak in New Zealand.
“Something is definitely changing.”

It means our testing and tracing system is starting to struggle to keep up with new cases, as confirmed by director general of health Dr Ashley Bloomfield at midday.

Bloomfield told reporters testing capacity is around 20,000 tests a day, already stretched by the current 22,000 tests being processed a day on average. And this could surge to about 60,000 and 70,000 a day.

But looking at test positivity on its own to understand an outbreak is like reading tea leaves, Parry cautioned. Many variables can change the test positivity, for instance, the number can be higher on weekends because fewer people turn up for testing.

As infections increase, the number of people who come forward to get tested is also likely to drop. “Test positivity may not be a super useful guide in the future because of the way we test and trace is changing.”

“For example in the UK, only a third of cases are being detected,” he said.

Parry and other experts have said the hospitalisation rate may become more important when reported case numbers are no longer as accurate.

“This is the ratio of the people in hospital to the number of active cases,” and he said it is also drifting up, but hospital admissions tend to lag case numbers by up to a week so it’s still a “wait and see” how the Omicron surge will affect this rate.

Fifty-six people were reported in hospital with Covid today, none in ICU or HDU.

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