A number of Kiwis deemed 'probable' cases of COVID-19 probably never had the disease, a new study has found.
Researchers looking at the Southland outbreak - the country's largest in early 2020 - initially wanted to figure out how many cases might have been missed.
In April, a separate study found 0.1 percent of Kiwis had antibodies to the virus - suggesting there may have been twice as many cases as health authorities have confirmed.
The Southland study saw 1214 people undergo blood tests between June and August last year, between four and 10 weeks after the last known community case in the region. Of that group, 78 were confirmed COVID-19 cases and nine probable. The remaining 1127 were considered high-risk for contracting the virus, eg. frontline workers.
In the latter group, nine new previously unknown infections were detected.
"All had epidemiological risks including travel to Europe during their outbreak, and/or being a close contact of a known case," the study, published in the journal Pathology, said.
"Only two of these individuals had PCR testing performed; the remaining seven did not as they were symptomatic overseas or did not meet the original case definition."
The pair who'd been tested but told they didn't have the disease was probably because the initial tests that were available at the time "demonstrated sub-optimal sensitivity".
Overall, the researchers "found little evidence of undetected infection among the individuals in the [Southern] DHB region who were considered to be at higher risk than the average resident, due to contact with a PCR-confirmed case, or because of workplace duties, or because they were Queenstown residents".
Seven of the nine probable cases tested negative for antibodies.
"An unexpected finding was that seven of the nine individuals diagnosed with ‘probable’ infection, and included in NZ’s official tally, were sero-negative... While acknowledging the delay (approximately three months) in serum collection and the possible impact on sensitivity, it is likely that at least some of these individuals did not have infection.
"This highlights the role of serology in the diagnostic algorithm where PCR is negative despite symptoms and epidemiological risks, and further testing of NZ's remaining 341 probable cases may be warranted."
New Zealand has had one of the lowest death and infection rates in the world, after successfully eliminating community transmision of the virus. A study published earlier this week found New Zealand was the only wealthy country in the world to actually suffer fewer deaths than normal in 2020.