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Blood testing finds undiagnosed Covid cases in NZ – Expert Reaction

non-peer reviewed analysis of more than 9,800 blood donor samples suggests there were eight previously undiagnosed Covid-19 cases that were likely infected within New Zealand. 

The blood samples were collected between December 3rd 2020 and January 6th 2021. The researchers say the very low rate of cases implies undetected community transmission has been limited.

Note: this study is a preprint, and therefore has not yet been peer-reviewed. 

The SMC asked experts to comment on the study. 

Dr Amanda Kvalsvig, Senior Research Fellow, Department of Public Health, University of Otago, Wellington, comments:

“It’s encouraging to see such a low number of previously-undiagnosed cases. It does support the evidence from other data sources such as community testing, which have consistently shown low rates of positive tests. So far there’s been no indication that Aotearoa New Zealand has experienced large undetected outbreaks in the community.

“It’s likely that the overall percentage of missed cases is higher than indicated by this study. As the authors are of course aware, people who donate blood and other tissues aren’t a typical sample of the population. Importantly for any study of the current pandemic, some people who are at increased risk of Covid-19 aren’t eligible to donate because they have a medical condition – for example people who have diabetes and need insulin to control it. People over the age of 70 years are also less likely to be blood donors while generally more at risk of infection than others, and Pasifika might be missed in the same way.

“At the other end of the age range, children under 16 years aren’t eligible to donate blood, and that same age group is more likely to have missed out on being tested for Covid-19, especially early on in the pandemic. It’s likely there are quite a few child cases that we don’t know about.

“Another really important group who won’t appear in the blood donor data is people with chronic fatigue syndrome. They’re not eligible to donate blood, which means that people experiencing long-term effects of Covid-19 illness – or ‘Long Covid’ – wouldn’t be picked up in blood donor testing. If they also weren’t tested during their acute illness, they may not even be aware that the symptoms they’re experiencing are caused by Covid-19. For that reason I hope that the newly diagnosed cases will be made aware of their diagnosis. People with Long Covid can struggle to have their symptoms taken seriously if they don’t have proof of infection.

“There are a number of reasons why Covid-19 cases can be missed during an outbreak. One is that people might not experience symptoms so they don’t know they need a test. They might be a contact of a case but not know it, and put their symptoms down to a cold or hay fever. Even if people feel unwell, they may be tested too early or too late in their illness so the test is negative. Sometimes the test just doesn’t work.

“That’s why population-based testing is a useful way to count the cases that slipped through the net. Testing donated blood is a practical addition to a population testing strategy because it’s possible to test large numbers of healthy people who might not otherwise be having blood tests. I hope there’ll be more studies in future that aim to fill in the rest of the gaps, with a particular focus on identifying those who typically wouldn’t be blood donors but might be unknown Covid-19 cases.”

No conflict of interest declared.

Professor David Murdoch, Dean and Head of Campus, University of Otago, Christchurch comments:

“A recent study detected antibodies to SARS-CoV-2 (the virus that causes COVID-19) in a very small number of samples from New Zealand blood donors collected during December 2020 and January 2021.
The study report, which has not yet been peer-reviewed, documented 18 positive samples from 9,806 tested. Of these, 6 were from previously confirmed COVID-19 cases and 4 from people who had recently travelled to countries with high COVID-19 transmission. The remaining 8 positive samples may have come from otherwise undiagnosed (and potentially asymptomatic) infections.

“Detection of antibodies provides evidence for infection with SARS-CoV-2 in the previous weeks or months. Antibody detection tests can be susceptible to false positive results, although the researchers in this study took measures to minimise this occurrence. It is also important to acknowledge that blood donor populations typically differ from the total population of a country in such characteristics as age and ethnicity distributions, thereby limiting the generalisability of the findings.

However, the study findings do provide evidence that undetected community transmission of COVID-19 has been limited in New Zealand, and is consistent with similar findings from Australia. In contrast, the prevalence was much lower than in regions of North America and Europe where the pandemic has been poorly controlled.”

Conflict of interest statement: Member, COVID-19 Vaccine Strategy Taskforce, NZ Government. Member, COVID-19 Vaccine Strategy Scientific and Technical Advisory Group, NZ Government. Member, Advisory Group, Vaccine Alliance Aotearoa New Zealand (VAANZ). Independent Member, Clinical Trials Steering Committee, University of Oxford COVID-19 Vaccine. Member, COVID-19 Expert Advisory Network, Ministry of Health.

Associate Professor James Ussher, Department of Microbiology and Immunology, University of Otago, comments:

“This paper confirms that there was limited undiagnosed infection with SARS-CoV-2 in New Zealand in 2020. In contrast to similar studies in countries with widespread outbreaks where more than 10 per cent of blood donors have previously been infected with SARS-CoV-2, this study found that only 0.1% of New Zealand blood donors had previously been infected with SARS-CoV-2.

“In total, 18 people were found to have antibodies against SARS-CoV-2, of which six were previously known to be infected and four were likely acquired overseas. This study confirms that lockdown and border restrictions were highly successful in limiting SARS-CoV-2 infection in New Zealand.”

Conflict of interest statement: Associate Professor Ussher is Science Director of the Government-funded Vaccine Alliance Aotearoa New Zealand – Ohu Kaupare Huaketo, a partnership between the University of Otago, the Malaghan Institute and Victoria University of Wellington. He is also on the Government Vaccine Taskforce’s Science and Technical Advisory Committee.