It's the new UK variant of COVID-19 that has caused the country to be on such high alert.
The B 1.1.7 variant, first found in the UK, is raging overseas, and a new study shows it's probably more deadly.
The danger is, it's 50 percent more infectious than the original COVID-19.
"That means if you were going to infect two people last year, you'd actually infect three people this year if you had the new variant," says University of Auckland Prof. Shaun Hendy.
The B 1.1.7 variant was first discovered in the UK in September, causing a huge surge in cases there.
Easier to spread and according to a new study - more deadly.
"There's a suggestion that it's resulting in higher rates of hospitalisation and death in the UK at the moment," says University of Otago professor of public health, Michael Baker.
And it's harder to close down an outbreak, prompting fast action from the Government.
"We are responding really aggressively by going to [alert] level 3 and making sure that we're doing a really wide reach with our contact tracing," says Associate Minister of Health, Dr Ayesha Verrall.
B 1.1.7 became the dominant variant in the UK within months.
It's now rapidly spreading through the US.
Australia is taking no chances, plunging Melbourne into lockdown when it turned up there.
So why is it more contagious? Shaun Hendy says it's mutated to better infect our cells.
"The transmissibility seems to be associated with changes in the protein that's on the outside of the virus particle and it uses this protein to attach to our cells and get inside those cells," said Prof. Hendy.
The good thing is, the Pfizer vaccine that arrived on our shores today appears to be effective against it, but there's more evidence to come.
Prof Baker says the best way to deal with it is to not let it in in the first place.
"The only way the virus arrives in New Zealand is that it's imported, so this is another reminder that we need to work much harder at our borders," Prof Baker says.
So far today, we've been lucky, with no new community cases, the Director-General of Health says.
"It's an encouraging start."
But the Prime Minister says we're far from out of the woods.
"That is heartening but I think all of us want a wider set of tests, we want to really rule out with some confidence additional potential chains of transmission, anything further in the community," says Jacinda Ardern.
If a wider outbreak of this variant is discovered, level 3 may contain it. But to stamp it out would require tougher and potentially longer measures.