Waning immunity and complacency have contributed to the fifth wave of Covid-19 in New Zealand, an epidemiologist is warning.

The current summer wave of Covid-19 is larger than the last, with 6558 new cases reported in the past week.

Forty-eight scouts who were attending a week-long jamboree in Hamilton caught Covid.

Epidemiologist Michael Baker said wastewater testing indicated there was a rise in cases throughout November and December, but there had not yet been a result in the new year.

"The hospitalisations did appear to reach a peak just before the end of last year and may be declining slightly, but it's very hard to know with more infectious variants like JN.1 circulating we may see more peaks."

The latest Covid wave started before JN.1 had arrived in New Zealand, he said.

"So there's other factors such as waning immunity and I think people getting a bit more complacent about the virus and obviously a succession of slightly more effective variants arriving."

Baker said many people seemed oblivious to the pandemic's existence - but complacency would not keep people safe.

Professor Michael Baker says hospitalisations appeared to have reached a peak towards the end of last year, but the new JN.1 variant may mean a new peak. File Photo.

"The reality is it's here the whole time, it seems to be causing a couple of waves a year now and I think this wave is a surprise in that it's quite a bit larger than the fourth wave earlier in the year."

It was important for people to continue to socialise, but they should take a few more precautions than usual at big events, like rapid antigen testing for attendees, he said.

Both having had Covid and getting vaccinated increased your immunity to the virus, he said.

"The problem with this virus, as we're learning, is that immunity fades and so you become very vulnerable again within a few months, so this virus keeps on infecting and re-infecting people who are exposed."

Getting recurrent Covid infections left "a scarring effect on some organs and the lining of your blood vessels" which was one of the theories about what caused long Covid, he said.

"People should do everything they can to avoid getting infected over and over again and that's regardless of your age."

Older people and people who were immuno suppressed were vulnerable to serious illness with Covid, he said.

"These are the people who are going to hospital, particularly people over 75 years of age and unfortunately dying every day."

Older, more vulnerable people will likely need to get a vaccination booster annually, he said.

- RNZ