There are many questions still yet to be answered about how some of New Zealand's Covid-19 outbreaks occurred, but they may never be answered. Photo / File
They've caused lockdowns and job losses, sparked widespread public anxiety and mobilised an army of health workers.
But experts say there are still unanswered questions surrounding some of our Covid-19 community outbreaks and we may never know the origin of six clusters that spread the deadly virus.
Of the 19 clusters listed by the Ministry of Health, the origin of six are classed as "unknown", while two - Auckland's August 2020 and February 2021 clusters - were still "under investigation". The remaining 11 were caused by overseas exposure.
That is in addition to several examples of infections going unexplained at managed isolation facilities and airports.
Microbiologist Siouxsie Wiles, recently crowned New Zealander of the Year/Te Pou Whakarae o Aotearoa, said sometimes people were unaware of how difficult it was to determine with certainty how virus transmission occurred.
"We're still in the very early stages [of the pandemic] and it would be nice to have an appreciation for how complicated things are and that, sometimes, there are no simple answers."
Referencing a dearth of study into infectious diseases, Wiles said what science did know about viruses similar to Covid-19 was sourced years ago and needed time to be strengthened.
She cited a prior understanding of viral droplets, where it was believed the heavier the droplet, the faster it would fall to the ground - which explained a strong focus on surface transmission early in New Zealand's pandemic response.
However, Wiles said it had since been discovered that large viral droplets could hang in the air for hours depending on airflow - a discovery that had prompted a greater focus on ventilation in managed isolation facilities.
Asked what she believed deserved further investigation, Wiles said she was interested to know more about how transmission occurred for symptomatic and asymptomatic people, and the long-term health consequences for those infected.
Wiles believed branding unknown aspects of outbreaks as failures did a disservice to the people working at the frontline.
"I find it offensive actually when everybody refers to a transmission happening as being a massive failure, because actually that is completely downgrading their work."
Here are some of New Zealand's most puzzling Covid-19 outbreaks and the questions that still persist.
On August 11, four members of a South Auckland whānau tested positive for the virus - becoming the first community cases in New Zealand for 102 days.
The discovery sent Auckland almost immediately into alert level 3 while the rest of the country moved to alert level 2. The cluster grew to 179 cases before it was deemed closed.
Its origin is still under investigation according to the Ministry of Health. Analysis from Wanaka-based phylogeneticist Dr James Hadfield deemed the cluster was connected by a single viral source.
Three possible sources for the outbreak have been suggested: a border incursion from overseas, namely from the UK or Ecuador; undetected community transmission; and the virus arriving via foreign goods.
Hadfield explained that a border incursion was the most likely source, but admitted it was quite possible the cause would never be confirmed due to lack of evidence.
Questions still remain over how the virus was transmitted from a member of the New Zealand Defence Force working at an Auckland quarantine facility to a shop assistant who lived, worked and studied in the CBD.
The Defence Force worker is believed to have caught the virus from the Jet Park facility in November, but the source is unknown. Six cases were confirmed in total, three of whom were NZDF workers.
Two NZDF staff were in the Auckland CBD in early November, spending two hours at a bar 100 metres away from the woman's work, A-Z Collections, on High St.
However, no evidence was found of any connection between the two parties.
New Zealand faced an anxious wait as health officials scrambled to determine whether Covid-19 was spreading in the community in January after a Northland woman tested positive for the virus after leaving managed isolation.
The 56-year-old, who lived south of Whangārei, was released from the Pullman Hotel in Auckland on January 13, developed mild symptoms on January 15 and got tested on January 22 after her condition deteriorated.
She visited 30 southern Northland locations, including cafes, restaurants, retail outlets, tourist attractions and holiday hotspot Mangawhai. However, all 16 close contacts she encountered while travelling tested negative.
It was New Zealand's first reported community case since November. As at January 28, there were no firm leads into the transmission method, but later studies suggested the virus may have been transmitted through the air.
Professor Michael Plank, of science research centre Te Punaha Matatini, said it was fortunate the woman hadn't attended any potential "super-spreading" events such as large gatherings.
Later that month, two people who completed managed isolation at the same time as the Northland woman tested positive, which then led to the temporary closure and review of the facility. It reopened in February with greatly improved ventilation, an issue highlighted in the review.
Instead of Cupid's arrow, Auckland was struck by another volley of Covid-19 on February 14 this year leading to a cluster which saw 15 people infected and forced the City of Sails back into alert level 3 twice.
The cluster centred around Papatoetoe High School, which closed on multiple occasions as the virus spread to four families in the school community. The three initial cases doubled in three days, but the outbreak appeared contained and Auckland moved to level 2.
It wasn't until six further cases had been linked to the cluster by February 26, with one person - who worked at KFC Botany while infectious - receiving condemnation from Prime Minister Jacinda Ardern for not following instructions to self-isolate. The worker later denied they were told to self-isolate.
Another stint of alert level 3 in Auckland was confirmed following the discovery of the cluster's penultimate case, which had visited a gym and the Manukau Institute of Technology while infectious.
Despite one further case, Auckland found its way back to alert level 1 by March 12.
The second case of the cluster, who worked at a catering and laundry service in Auckland International Airport, is believed to have been the source of infection. However, the method of her infection was still not known and is classed as under investigation by the Ministry of Health.
A flight attendant with the national carrier tested positive for a Russian Covid strain in early March after returning from Japan on February 28.
The Auckland Airport Countdown was listed as a location of interest, with all casual contacts asked to monitor their health. They were one of three airline staff to have become infected around that time. The other two had worked on flights from Shanghai and Los Angeles.
The late February infection prompted calls from epidemiologist Michael Baker for a review into rules around airline staff not having to quarantine after arriving from overseas.
An airport worker who picked up the virus while cleaning a plane that flew to New Zealand on April 10 provides another unanswered question around the source.
The worker shared the same viral genome sequence of an infected passenger aboard the "red-zone" flight. Three locations of interest were released following the positive test; Westfield St Luke's Mall food court, Bunnings New Lynn, and Movenpick Dominion Rd.
As the cleaner was wearing full PPE, including an N95 mask, and there was no face-to-face contact with passengers, how the worker became infected was unclear.
This prompted estimations from experts that airborne transmission was the likely source and calls for reviews into protection of border workers following multiple instances of staff being infected despite adhering to strict protocols.
The Grand Millennium managed isolation facility was closed for an investigation into virus transmission through ventilation systems following three positive cases originating from the facility.
In late March, a cleaner at the Grand Millennium tested positive who then passed the virus onto a security guard linked to multiple locations of interest in Mt Roskill and the CBD. It was later discovered the security guard was not vaccinated, having missed multiple opportunities to do so due to personal reasons.
However, the link between the cleaner and the guard remained unknown. Epidemiologist Michael Baker postulated a "missing link" was a likely explanation for the transmission.
Speculation the September cluster at the Christchurch Crowne Plaza Hotel, which totalled six people, originated from a rubbish bin touched by a returnee at the facility has been strongly questioned.
On October 2, the Ministry of Health released the theory before it was criticised by experts who believed it was much more likely the virus was transmitted via the air.
Later investigations and studies seemingly agree with that conclusion as it was proposed that during MIQ swabbing, masks have to be lowered briefly to expose the nose - increasing the risk of hotel occupants being exposed to infectious aerosols that would've been hanging in the air.
While recent studies pointed to aerosols being a more likely source in unventilated spaces, the case of the Rydges Hotel maintenance worker was still believed to have been infected by surface transmission inside an elevator at the managed isolation facility.
OPINION: New Zealander reflects on delivering speech at St James' Palace.