New survey finds eight undetected cases

Zealand Blood Service to collect the samples, which were taken from people aged 16-88.

They then used serology testing, which reveals antibodies that remain many months after infection.

Standard Covid-19 DNA tests only detect the virus when it is present during an infection.

Of the 9806 samples, 18 tested positive for antibodies.

Six of those were identified as previously-confirmed Covid-19 infections, while four came from people who had travelled from the United Kingdom and Europe, suggesting infection had happened outside New Zealand.

The remaining eight positive samples were unexplained, suggesting they were previously undetected cases of community transmission.

Those eight cases were spread out and came from seven district health board regions throughout New Zealand.

Associate Professor Nikki Moreland said the low rate of antibodies found in the survey meant undetected community transmission ‘‘has been very low’’ and was evidence ‘‘New Zealand’s elimination strategy has been a success’’.

Dr Amanda Kvalsvig, a senior research fellow at the University of Otago, said it was likely the overall percentage of missed cases was higher than the eight found in the

Auckland University study.

This is because people who donate blood are not usually a typical sample of the population.

‘‘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.’’

Despite this, she said it was ‘‘encouraging to see such a low number of previously-undiagnosed cases’’.

The results of the study compared favourably to surveys conducted in Europe and America, where the presence of Covid-19 antibodies has been reported to be larger than 10 per cent.