Covid-19

Understanding NZ’s Covid-19 curve

Analysis: On the face of it, New Zealand's epidemic curve is going exponential. But are case numbers really rising for the same reasons as overseas?

If you've been following any news about Covid-19 over the past three months, you've heard of The Curve. It looks, variously, like a sky-high pillar representing thousands of cases overwhelming our healthcare system or a squat hill representing a system operating at, but not above, peak capacity - the famously flattened curve.

The Curve is explained in this graphic from the United States Centers for Disease Control.

Although New Zealand has moved away from attempting to flatten the curve, opting to quash the thing entirely, we're still going to see some form of bump as we try to root out whatever cases entered the country in the weeks prior to the imposition of various border restrictions.

Since the first days after the arrival of the virus in New Zealand, commentators have attempted to map our curve onto international precedent. In mid-March, it was asserted that New Zealand was just 10 days behind Italy.

That is clearly no longer the case, but with our cases now doubling every two to four days, new international comparisons are being made. Are these fair? Is New Zealand really on the same path as some of our international peers, who are seeing skyrocketing cases and deaths?

In short, no, not yet. Although the number of cases continues to rise each day and New Zealand has moved from eight cases to 589 in two weeks, this isn't due to the virus spreading rampant in our communities.

When people talk about the virus spreading exponentially, they are talking about community transmission. If one person transmits the virus to three other people - which is the reproduction rate modellers have deduced for Covid-19 - and then those three transmit it to three more, the situation gets out of control. But that's not what is happening in New Zealand.

Of the 455 cases the Ministry of Health has gathered a body of information on, 57 percent - a majority - contracted it overseas or have a close link to overseas travel, Director-General of Health Ashley Bloomfield said Monday. Another 26 percent are close contacts of a confirmed case and 15 percent are both a close contact and have an overseas travel link. Just 2 percent of cases - around 10 - are being treated as community transmission.

NZ Covid-19 cases Cause of infection
Accurate at time of publication.

There are still 145 cases where more information is still pending and it is likely that some of these will be community transmission, but the vast bulk of New Zealand's cases are linked to overseas travel or just one or two degrees removed from an international element.

Why haven't we seen community transmission? In part, it's because we've moved early. The best practice is to adjust your policy settings for where you think you will be in two weeks, not where you are today. That's why New Zealand introduced stringent travel restrictions on March 14 when it had just six cases - the same restrictions were first implemented in Israel when the country had 42 cases.

Norway, a country of similar size to New Zealand, took the same action on March 12 when it had 598 cases and Australia did so two days after New Zealand, when it had 401 cases.

New Zealand total Covid-19 cases
Accurate at time of publication

Less than a week later, New Zealand closed its borders entirely. We had just 28 cases that day. By comparison, Norway had 1254 cases when it closed its borders and Australia had 756 cases.

By the time the Prime Minister announced a total lockdown, New Zealand had 102 cases of Covid-19, while Norway announced its lockdown at 598 cases. Australia still has yet to go into full lockdown, but stricter measures were announced by their Prime Minister Scott Morrison on March 25, when the country had 2676 cases.

Now, it's certainly possible that community transmission is more widespread in New Zealand than we think and that we just haven't found it yet. It remains unclear how many of the 1700 or so tests conducted each day are for patients who have no link to overseas travel or other confirmed cases - in other words, are we actually testing to find community transmission?

"I can't give you the exact proportion but it's very clear that there is a lot of clinical judgment being applied to the application of testing," Bloomfield said on Sunday.

"Not everybody has a history of overseas travel or is a close contact. That's good to see - we have got testing capacity there, we continue to increase it and as I've said right from the start, we want to find the cases so we can act accordingly and break the chain of transmission."

Even if we aren't finding that community transmission, however, the existing curve of known cases doesn't represent an exponential increase through community spread.

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