A study of blood samples has revealed eight cases of Covid-19 that went previously undetected in the community.
Infections were likely missed early on when the virus first arrived in New Zealand, Associate Professor Dr Nikki Moreland says. Photo: 123RF
Auckland University Faculty of Medical and Health Sciences tested 9806 samples of blood donations from New Zealanders to see if they had antibodies for the virus.
Antibodies can tell if someone has been infected with Covid-19, it cannot identify an active case.
Associate Professor Dr Nikki Moreland said the study, which had not yet been peer reviewed, showed there was a low level of undetected community transmission.
From the almost 10,000 samples collected between December 2020 and January 2021, 18 came back with a positive result for a historical Covid-19 infection.
"We were able to back match six of those to people who had already previously been confirmed as being infected.
"And another four had extensive travel in 2020 to places that have a really high risk of Covid infection, so it's most likely that they were infected overseas.
"So that left eight people who were previously undiagnosed."
Moreland said the eight cases were spread between seven different District Health Board regions.
"I think it really reaffirms the success of the elimination strategy.
"When we set out to do this, we weren't sure what we would see in terms of what levels or what numbers of samples we would identify being positive for these antibodies.
"And the fact that we saw just this really low number, and we're sitting, you know, at 0.1 percent positivity suggests that, yes, there has been a level of undiagnosed infection or missed infections, but that level is really low."
Moreland said infections were likely missed early on when the virus first arrived in New Zealand and the testing criteria was very tight.
It had also been estimated that up to 15 percent of people who contract Covid-19 have no symptoms but remain infectious, she said.
The blood donors were anonymous and blood samples were collected through the New Zealand Blood Service.
Moreland said the study used three different assays, or tests, to triple confirm any positive case discovered in the research.
She said the first test was one using the spike protein that was synthesised in New Zealand by the scientists at Callaghan Innovation.