Aotearoa has been experiencing another wave of Covid-19 over the last few weeks, and hospitalisations are now back to the levels seen last January.

This week, health officials warned of their concern that increased social gatherings and events in the summer holiday period could lead to both an increase in the rate of Covid infections, and potentially put vulnerable populations at risk – prompting a call for those at highest risk to get a summer booster.

So, what’s happening?

Covid-19 modeller, University of Canterbury mathematics professor Michael Plank​ said Covid-19 numbers have been heading upwards over the last month or so, and remain at “relatively high levels” compared to most of the last year.

As of midnight Sunday, there were 336​ people in hospital with Covid-19 – up slightly from 332​ the week prior.

National Public Health Service interim clinical lead, William Rainger,​ told Stuff that hospital admission rates for Covid-19 have increased since October, and remain higher than what we saw in the last peak, in April – but lower than this time last year.

As of December 3, hospital admission rates were highest among Pacific peoples and Māori; those aged 65+; and those with high deprivation levels – groups which have “consistently been the most impacted” in terms of adverse health outcomes when Covid-19 cases increase, Rainger said.

The current increase in cases and hospital admissions is happening within all regions, but the most recent data shows admission rates are highest in the Northern region (Northland and Auckland), he said.

Plank said going into Christmas it’s possible the end-of-year party season and increased social mixing is “playing a bit of a role in pushing those [case] numbers up”.

So it may be that once we’re well into the school holidays, that might start to drop off, he said.

On the other hand, JN.1 – another new Omicron sublineage – could potentially play a role in what happens with this wave, Plank said.

Plank said JN.1 is present in our sequenced data and is growing relatively quickly in other countries.

If it increases to a higher percentage of cases, it could “potentially contribute to pushing those numbers up again” – both potentially prolonging, and increasing the size of the wave, Plank said.

JN.1, a descendant of BA.2.86​, was first detected in the United States in September, according to the Centers for Disease Control and Prevention (CDC).

When BA.2.86 (nicknamed ‘Pirola’) emerged, it was regarded as a “bit of a wildcard” due to its heavily mutated nature.

There is just one change between JN.1 and BA.2.86 in the spike protein.

The continued growth of JN.1 suggests it is either more transmissible or better at evading our immune systems, the CDC said last week.

The health agency said, at that stage, there was no evidence that JN.1 presented an increased risk to public health relative to other currently circulating variants, and there was no indication of increased severity from JN.1.

What is wastewater showing us?

There have been elevated levels of SARS-CoV-2 in wastewater on a national scale since about the beginning of November, Dr Joanne Hewitt​, Science Leader, Health and Environment, at the Institute of Environmental Science and Research (ESR) said.

Hewitt said while the results are not as high as they were in last December’s wave, surveillance data showed the levels of virus in wastewater have been tracking higher than they have been since about January.

The most recent data – released on Thursday – shows things have “plateaued a little bit, at this elevated level”, she said.

Hewitt said they’re keeping an “eye on” JN.1.

The latest wastewater variant analysis data, up to December 3, showed JN.1 was detected in the majority of surveyed sites and was up on the week prior.

From clinical samples, and what’s going on overseas, Hewitt said it’s expected the prevalence of JN.1 will continue to increase in Aotearoa over the next few weeks.

Where to from here?

Current modelling of Covid-19 hospital admissions provided by ESR suggests hospitalisations will peak in the week prior to Christmas Day, with approximately 450​ people in hospital across Aotearoa at that time, Rainger said.

This modelling is through until December 27​ and takes into account case numbers, hospitalisations, wastewater levels, time of year, variants, and vaccination, he said.

Plank said if JN.1 wasn’t in the mix, “I’d feel fairly confident we’d see a peak by Christmas, if not before. But JN.1 does sort of complicate things a little bit”.

If it causes cases to continue increasing, it’s possible we might see a double peak: perhaps one prior to Christmas, “before JN.1 has a chance to really take over”, followed by a second peak if that happens, he said.

University of Otago evolutionary biologist and virologist Dr Jemma Geoghegan​​ said JN.1 appears to be “rapidly growing” overseas.

Given the season we’re entering – with more parties, social gatherings, weddings, and music festivals which have acted as “super-spreader” events in the past – Geoghegan said she would expect JN.1 to increase in frequency amongst sequenced cases.