Coronavirus: Eight previously unknown NZ COVID-19 community cases found in blood samples

Eight previously unknown cases of COVID-19 in the community have been discovered during testing of blood donor samples.

All the cases are historic, and experts say it's further proof undetected spread of the virus in New Zealand was limited. 

Blood from nearly 10,000 donors collected in December and early January was tested for antibodies for the disease, which has killed 26 people in New Zealand and more than 3 million around the world to date. 

Just 18 had antibodies - six were previously known and four had travelled to hard-hit Europe and the UK and likely infected there.

The remaining eight were "unexplained", study author Nikki Moreland of the University of Auckland Faculty of Medical and Health Sciences said. They were spread across the country, covering seven different district health board regions. 

Overall, the study estimated 0.1 percent of Kiwis have been exposed to the virus, compared to 10 percent or more in similar seroprevalence studies overseas. 

"We found a very low rate of COVID antibodies in our survey population," said Dr Moreland.

While the research is yet to be peer-reviewed, it's the largest serology study on COVID-19 to date in New Zealand. 

"It's encouraging to see such a low number of previously-undiagnosed cases," said Amanda Kvalsvig, a senior research fellow at the University of Otago's Department of Public Health. 

"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."

Dr Kvalsvig said there are numerous reasons why people who caught the virus might not have even realised. 

COVID-19 symptoms can differ wildly from person-to-person, with some never experiencing symptoms at all - such as the airplane cleaner whose infection was picked up in regular testing earlier this week. But asymptomatic people can still spread the disease to others, making it a tricky virus to track.

"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." 

She said the true figure is likely higher, particularly in children who can't donate blood, though still much lower than most other countries. 

"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."

She said there might be some people out there unable to donate blood because they've been diagnosed with chronic fatigue syndrome - when what they're suffering is in fact long COVID. 

Microbiology professor James Ussher of the University of Otago said the research confirms New Zealand's zero-tolerance strategy not only flattened the curve, but crushed it.

"This study confirms that lockdown and border restrictions were highly successful in limiting SARS-CoV-2 infection in New Zealand," he said, using the virus' formal name. 

How much protection a previous infection gives against future exposure isn't yet clear. People who have had the virus before should still get a vaccine, experts say.

The study has been published online ahead of peer review.