The janitor who tested positive for Covid-19 was working on the Green and Red zone aircraft on the same day. Video / Mark Mitchell
Scientists have analyzed nearly 10,000 blood samples to show that there are very few undetected cases of Covid-19 in New Zealand.
Learning, just released online prior to peer review, has also offered a useful new basis for the upcoming mass rollout of Pfizer’s vaccine.
To create a picture of the undetectable spread across the country, scientists looked for specific antibodies among 9806 samples collected from New Zealand Blood Service donors between the ages of 16 and 88.
Antibodies play an important role in the immune system against pathogens such as the coronavirus.
Once a new virus is recognized, antibodies are specially made to bind to the “spike protein” and stop it from entering our cells – while signaling other parts of the immune system to destroy foreign invaders.
Antibodies can also act as useful markers of past infections.
It is the sampling of antibodies that scientists have made possible to reveal last month a previously unknown cluster that predated New Zealand’s first officially reported Covid-19 case.
A co-author of the new study, University of Auckland PhD student Alana Whitcombe, said determining a positive sample count for the Sars-Cov-2 antibody could essentially help measure the rate of undetectable transmission and the potential spread of the virus across the country.
That is important, given that there is evidence to suggest that up to 15 percent of people who contract Covid-19 do not show symptoms, but can still be contagious.
When it came to sampling, Whitcombe and colleagues measured the antibodies using a test based on the viral spike protein.
They will previously used the same method to show you how, in a group of 112 New Zealand patients, the Sars-Cov-2 antibody can persist for months after infection.
“That’s important because the survey sample was collected at the end of 2020 – but the first cases of Covid-19 were reported in New Zealand in February 2020,” Whitcombe said.
“Basing our survey on long-lived spike antibodies means we are likely to detect all possible past infections, even those occurring as early as 2020.”
From the total sample, the researcher identified 18 positive samples.
To ensure they can confirm their results, the team also reconfirmed positive samples using two commercially produced surge-based assays.
Six of the 18 people were confirmed to have previous Sars-CoV-2 infection and the other four were from people who had traveled from the UK and Europe, indicating infection outside New Zealand.
The remaining eight “seropositive” samples came from seven different district health councils – equivalent to a total seroprevalence rate of 0.1 percent of the population.
Study co-author and research associate from the University of Auckland, Dr Reuben McGregor said this was only slightly higher than a previously reported prevalence of 0.04 percent.
While their results suggest some undiagnosed infections have occurred, McGregor said it is much lower than the seroprevalence estimate of more than 10 percent reported in an overseas study where the virus was poorly controlled.
“Overall, the survey shows that undetectable community transmission appears to have been restricted in New Zealand and provides serological evidence for our country’s successful elimination,” added study author Associate Professor Nikki Moreland, an immunologist and biomedical scientist at the University of Auckland.
The data also provide a good basis for vaccine rollouts, he said.
“Although the seroprevalence detected in the survey was very low, there is no reason to think that our immunity levels may be affected by a lack of viral circulation.”
Clinical trials and real-world studies abroad have shown that two doses of Pfizer’s vaccine produce antibody levels and an immune response that are in line with – or often superior to – those seen after Covid-19 infection.
“Achieving high vaccine coverage and increasing seropositivity for spike antibodies through vaccination is much safer than uncontrolled circulation of Sars-CoV-2 in the community.”
The researchers note that the study has several limitations – notably that it does not represent all Kiwis, but only those eligible to be donors, and cannot always be relied on to extrapolate from a small number of seropositive cases.
University of Otago clinical and infectious disease microbiologist Professor David Murdoch also emphasized that blood donor populations usually differ from the total population, in terms of age and ethnicity.
“However, the study findings provide evidence that community transmission of undetected Covid-19 has been limited in New Zealand, and is consistent with similar findings from Australia,” he said.
“In contrast, the prevalence is much lower than in regions of North America and Europe where the pandemic is not well controlled.”
Professor James Ussher, Associate Immunologist at the University of Otago, said the findings also highlight the effectiveness of our bold actions to eliminate the virus.
“This study confirms that border locks and restrictions have been very successful in curbing SARS-CoV-2 infection in New Zealand.”