Covid19 coronavirus: What if we never solve the infection mystery?

Auckland's latest two community cases are students at Papatoetoe High School, and linked to the three cases initially announced on Sunday. Photo / Alex Burton

Scientists suspect New Zealand's latest Covid-19 scare may again end as an unsolved mystery, with blood tests offering one of the few ways to tell how the virus entered the community.

Covid-19 Minister Chris Hipkins told Parliament's Health Select Committee that investigators were exploring every potential source of the new community cases.

"It would be fair to say that all of the scenarios sit at the more unlikely end of the spectrum, and hence we are just looking at every potential opportunity that there could have been for infection."

Hipkins said there was a potential link to the border, through the mother's job in the laundry at LSG Sky Chefs, which he described as "one of the more likely of the unlikely scenarios".

Experts told the Herald it meanwhile remained likely that the source investigation could come to the same dead end as the probe into the Auckland August cluster.

In the wake of that outbreak, some researchers suggested it may have even been sparked by someone returning a "false negative" test on leaving the border or MIQ.

The latest wastewater testing results showed no signs of an ongoing community outbreak - and scientists had found no genetic match with any positive cases detected in MIQ.

ESR and Otago University virologist Dr Jemma Geoghegan said that if the so-called "index case" that originally passed on the virus had recovered, there was no chance of getting that genomic link.

If the case was an international import of the virus, it was possible the record of that specific genome might be loaded into the public database from somewhere overseas.

"We're going to continue to monitor it and if there ever is a match, hopefully we'll find it."

Geoghegan said there was another avenue in serology testing, which could show if a person may have once had been infected without knowing.

University of Auckland immunologist Associate Professor Nikki Moreland agreed this approach could well help inform inquiries.

"Because we know that it takes people several days, or up to a couple of weeks, to develop a good level of antibodies and therefore be positive by serology," she said.

"A normal swab test can only tell you who is infected right now, whereas serology testing enables you to walk back in time and see who was infected some weeks ago."

But that still wouldn't yield the all-important genetic match.

"All it can tell you is someone has been infected - it can't tell you exactly what strain they were infected with."

University of Auckland infectious diseases expert Associate Professor Siouxsie Wiles said a dead end would be "frustrating" - but this didn't change our ability to manage outbreaks.

"If this did come through laundry services, it just leaves us with another question mark as to how rarely this happens. Are there processes we need to stop that?"

While there were no documented cases in the world of the virus being transmitted through contaminated surfaces - so-called "fomites" - such rare occurrences might be observable in a country like New Zealand, where there was otherwise no virus.

"By trying to chase down every potential way the virus is transmitted, we can help the world understand what the risks are, as well as helping countries like ours in mitigating those risks."

Otago University epidemiologist Professor Nick Wilson said, without knowing the source, we'd have to assume there had been some weakness in border processes.

"Given that transmission via imported food or other goods is extremely unlikely, I think this raises another argument to strengthen our border processes."

And he said there was much more that could be done.

In a recent analysis, Wilson and colleagues recommended steps including slashing the number of infected travellers arriving at MIQ, mandating QR code scanning and daily saliva tests for MIQ workers, and introducing post-MIQ home quarantine for five to seven days.