The positive COVID-19 case who travelled to Wellington recently, sparking fears of New Zealand's first outbreak since February, was infected with the Delta variant of the virus.
This has experts concerned, as it's more infectious and potentially more harmful than the original virus and other variants.
Here's everything you need to know.
What is the Delta variant?
A strain of the SARS-CoV-2 virus, which causes COVID-19. It was first detected in India in late 2020, and is believed to have been responsible for the massive wave of infections and death the country experienced in early 2021, after making it through the early waves of the pandemic relatively unscathed.
How did it come to be?
Putting it simply, evolution. There are two main ways viruses evolve - mutation and recombination.
Mutation happens when a virus makes a mistake while replicating. The new virus has a different genetic code to the original, which may offer it an advantage - perhaps it finds it easier to attach itself to host cells, or it isn't as susceptible to vaccines. Viruses with an advantage are more likely to survive and thrive.
The more chances a virus has to replicate, the more likelihood mutations will emerge - hence why epidemiologists widely back elimination strategies like the one New Zealand pursued. The sheer size of the outbreak in India gave the virus more chances to evolve.
Recombination happens when two viruses infect a cell at the same time and swap genetic information. This is how a virus can pick up new abilities,such as the ability to jump from one species to another. It's been suggested this is how SARS-CoV-2 got the ability to infect humans in the first place.
Why is it called 'Delta'?
When it first made headlines in early 2021, Delta was dubbed the 'Indian variant' or the 'double mutant' virus. The World Health Organization (WHO) discourages naming a disease or virus after places, as this can lead to stigma.
In May the WHO announced the COVID-19 variants would all have names taken from the Greek alphabet. The one discovered in India was dubbed 'Delta', being the fourth major variant found since the start of the pandemic.
"No country should be stigmatised for detecting and reporting variants," said WHO COVID-19 technical lead Maria van Kerkhove.
The variant first found in the UK was dubbed 'Alpha', South Africa's 'Beta', and the one spotted in Brazil, 'Gamma'. These three, plus Delta, have been labelled 'variants of concern' by the WHO.
Others found since dubbed 'variants of interest' - a step below 'concern' - include Theta (Philippines), Iota (New York), Lambda (Peru) and Kappa (India).
How much more infectious is Delta than previous strains?
Very, it would appear.
"Delta seems to be around 60 percent more transmissible than the already highly infectious Alpha variant (also called B.1.1.7) identified in the United Kingdom in late 2020," scientific journal Nature reported this week. Alpha is between 40 and 80 percent more infectious than the original SARS-CoV-2 strain, and drove the UK's massive outbreak over the northern hemisphere winter months.
Delta's ability to infect people was demonstrated in the current Australian outbreak when one person infected another during a "scarily fleeting" encounter at a Sydney mall. They merely walked past each other, CCTV footage showed, and that was enough for the virus to infect its new host.
"Literally people not even physically touching each other but fleetingly coming into the same airspace has seen the virus transfer from one person to another," NSW Premier Gladys Berejiklian said. "That's how contagious it is."
The Delta variant is now more common in the UK than Alpha, driving a worrying new surge that prompted the government to delay the lifting of some restrictions. It's also gone from making up 10 percent of all infections in the US to 20 percent in the space of a week.
Delta also appears to be much better at infecting children than previous strains.
New Zealand is currently in winter, so people are spending more time indoors. The SARS-CoV-2 virus is known to spread much more easily indoors than out, particularly if there isn't sufficient ventilation.
"The difficulty with this variant is we have not had it circulting in New Zealand to date outside of MIQ facilities," University of Otago epidemiologist Michael Baker told Newshub on Wednesday. "We don't know what it is capable of, and of course it has arrived in a period of winter when people are indoors so there is a higher risk of transmission."
"It is really important we scan with the contact tracing app, have the Bluetooth on and wear masks in public," added microbiologist Siouxsie Wiles. "Let this be a reminder the virus is still out there and we should be taking these precautions."
What are the symptoms?
A new infection of the Delta variant has slightly different symptoms to classic COVID-19, according to a UK researcher.
"Cough is rarer and we don't even see loss of smell coming up in the top 10 anymore," Prof Tim Spector of King's College London said earlier this month.
Instead, mild cases of the disease in young people present more like a bad cold, he says. The fear is people will think it's just a cold and not get tested or isolate themselves, further spreading the virus.
"The number one symptom is headache, followed by runny nose, sore throat and fever. Not the old classic symptoms... people don't realise that and it hasn't come across in any of the government information.
"This means that people might think they've got some sort of seasonal cold and they still go out to parties and might spread around to six other people and we think this is fuelling a lot of the problem."
Is it deadlier than previous variants?
UK data suggests people infected with the Delta variant are twice as likely to end up in hospital than those who contract Alpha.
Even if it turns out not to be deadlier, the WHO has warned Delta has the potential to kill more people "because it's so infectious, and "will eventually find those vulnerable individuals who will become severely ill, have to be hospitalised and potentially die", Mike Ryan, executive director of the WHO's health emergencies programme, said earlier this week.
Are vaccines effective against it?
So far, the answer appears to be yes - but not as effective as they were against the original or other strains.
The Pfizer-BioNTech vaccine being rolled out in New Zealand has been shown to be about 94 percent effective against the original strain, two weeks after the second dose. But data collected so far suggests a two-dose regime only offers 79 percent protection against infection with Delta - high enough that it's still a wise move to get it, but a significant drop.
Data collected during Israel's early and quick vaccine rollout suggested even a single dose of the Pfizer vaccine offered up to 85 percent protection against infection with original COVID-19, prompting some countries to roll out single doses to as many people as possible, rather than fully vaccinating fewer people.
But newer research published at the end of May has found the Pfizer vaccine is only 33 percent effective against Delta after a single dose.
"What is clear from this research is that the main thing we can do to reduce the spread of this variant is to ensure that we get our second dose of vaccine, whatever vaccine we had for our first injection," said Paul Hunter, professor of medicine at the University of East Anglia, said at the time.
Other vaccines have also shown reduced efficacy against Delta. AstraZeneca's two-dose vaccine offers just 60 percent protection against infection with Delta, and a single dose just 30 percent.
The good news is even a single dose appears to protect well against serious illness requiring hospitalisation - 94 percent for Pfizer and 71 percent for AstraZeneca - not dissimilar to their efficacy against the Alpha variant.
Are there any worse strains?
"From an evolutionary perspective it makes no sense for a pathogen to harm the host on which it depends for its survival," Ed Feil and Christian Yates of the University of Bath wrote in February.
But that doesn't mean it's guaranteed, the pair noting "ancient diseases" like tuberculosis and gonorrhoea "are probably just as virulent today as they ever were".
On Wednesday (NZ time) there were reports of a sub-variant emerging in India dubbed 'Delta-plus', which officials said might be even more infectious than Delta and was resistant to an antibody cocktail recently approved for use.