Analysis of NZ serology study

In the year since New Zealand closed its border and adopted an ‘elimination strategy’ against SARS-Cov-2, only one reliable serology test has been conducted. During this period at least 47 serology studies have been conducted throughout the world. Serology tests were banned from import or sale in NZ.

The result of the authorised study of 9806 blood samples taken in December 2020, was pre-print published (not peer reviewed) on April 19:

The headline result is that it found antibodies to SARS-CoV-2 in 0.1% of samples.

This is lower than we expected – especially when compared to the prevalence found in other nations of studies conducted earlier in the pandemic (as high as 50% in India). It is also much lower than the NZ prevalence of H1N1 (30% positive antibodies), which triggered health authorities to abandon elimination plans.

The title and commentary of the paper suggests this low level is explained by elimination of the virus. It is directly explained by the estimated 3-month half-life of antibodies (S and RBD, compared to month long half-life of N protein). Our reference paper on seropositivity is That means ‘fresh’ infections have been falling. This undoubtedly means that border closure has cut off supply of renewed infection but tells us very little about how much infection existed in NZ at the time of the border closure.

Even if you would like to believe the result shows the elimination strategy has throttled infection rates, you cannot ignore that it simultaneously proves that elimination is impossible. The 0.1% prevalence is double the number of identified positive tests. For every identified case, there is at least one other person with covid-19 who has not been identified. That means there has been at least 5000 cases in NZ (5,000,000*0.001).

Worse still, community infection is higher than thought. The study shows the ratio of previously detected locally acquired cases to known cases is 6:8. The number of locally acquired cases from the Ministry of Health is 2600 – 865 in MIQ = 1,735. This indicates that there were 2313 (1,735*8/6) extra locally acquired cases that were not detected.

If we wanted to ascertain true cumulative exposure to infection, then 0.1% is certainly an underestimate, compared to influenza antibodies. The study makes no mention of the possibility of infection that can be found in T-cell levels e.g. from Karolinska. Those studies suggest that if the true infection rate could be, conservatively, 1.5 times the antibody prevalence.

We note that the eight undetected cases claimed in media coverage were widely geographically distributed, so could not have been from a localised cluster. Covid-19 was evidently widespread across NZ, breaking the fiction of being contained by lockdowns and tracking into ‘clusters’.

A big implication of the study is that we now have a more definitive infection fatality ratio (IFR) for NZ of 0.5% (26/~5000). Only a month or two before this serology survey Rod Jackson and the NZ Herald refused to retract articles that told New Zealanders the IFR was at least double that (over 1%). We trust they will now delete those articles. Most other NZ experts have been more recently citing the CDC’s IFR of 0.65% – which is now clearly too high in NZ.

Our search for an accurate IFR now has a more certain starting point. We know that about one quarter of the NZ deaths were attributed to covid without evidence of a positive test. We also know that given the half-life of antibodies, the real infection level must be higher than 5000.  A conservative level would be about 10,000 infections. So NZ’s IFR could be as low as 0.2% (20/10,000). This figure is concordant with median estimates from summaries of serology studies.

In summary, the study reveals a lower antibody level than we expected. It’s a surprise that indicates a likely waning of fresh transmission. But it reveals that we have had at least one undetected case for each detected case. This means:(a) the virus is not as deadly as first thought as these cases were not diagnosed since they didn’t come to clinical attention and(b) it is a fiction that New Zealand has detected each and every case of covid-19 and so can declare the virus ‘eliminated’.