Almost all covid cases in NZ are now caused by the latest variant JN.1, according to the latest data, with an immunity expert saying it became the dominant strain "very rapidly".

Immunisation Advisory Centre founder Dr Nikki Turner says the strain most likely arrived in October.

"It's only got a small single change in the spike protein, so it's very similar to previous ones. It's just become more dominant very rapidly."

Dr Turner says the traditional vaccine is holding up well against the new strain, but an updated vaccine expected to arrive in a few weeks "closely matched" JN.1.

Medsafe approved Pfizer's application for an updated shot for the XBB.1.5 strain in December and it will be available in time for the upcoming winter season.

"American data is showing the vaccine looks highly effective to the strain, so that's very reassuring."

Dr Turner says the new variant is slightly different to the previous one.

"We do have to remember about all vaccines with Covid, they're more effective against severe disease.

"So the vaccines are preventing a lot of severe disease."

Dr Turner says mild disease is expected, even after getting vaccinated.

"But the vaccines are having a very good effect against hospitalisation."

She says the virus is spreading, even though people had been spending a lot of time outside thanks to a warm summer.

"Even though we go outdoors, we do mix closely.

"We sit together and cars, we sit together and transport, we cuddle up in our barbecues and, you know, we hug.

"It's pretty easy to spread this little bug around."

Dr Turner says vaccines offer the best form of protection.

"Particularly [for] people who at high risk, you really at this stage still need a vaccine every six months, six months since you had the disease or six months since you had a last vaccine."

The elderly, people with medical issues, those concerrned about getting severe flu or covid symptoms should not delay getting the vaccine, she says.

But covid is not the only virus we need to worry about, she says.

"Flu is nasty and it kills."

She says most flu cases are in winter, but recently there have been some non-seasonal outbreaks.

The new flu vaccine will be available in March.

Dr Turner says it's essential people get it, especially because it's updated each year to match the variance.

She says it's safe to get the covid and the flu vaccine together.

"There is data to show that they are both effective if you have them at the same time. And I'd suggest to many people prior to winter, think about having both of the vaccines at the same time."

-RNZ.