The Ministry of Health said the highly-transmissible BA.5 subvariant of Omicron would become the dominant strain of the virus in New Zealand in a matter of weeks.
Yesterday the Ministry of Health reported 6223 new Covid cases, 254 of which were reinfections of people who had had the virus before.
Immunologist and Malaghan Institute director professor Graham le Gros said the virus has moved on and people had been infected with previous versions of Covid-19.
"The one that is circulating now has changed so much that that immunity doesn't work quite so well and when you add in the factor that the immunity that's been generated by either the previous virus infection or vaccine, it runs down over a period of six months - it seems that it's very short-lived immunity."
That meant people were susceptible to getting infected by new variants, Le Gros told First Up.
Health advice now suggests reinfection can now occur within just 29 days of recovering from the Covid-19.
People were in a better position than when the virus emerged two years ago, Le Gros said, but immune systems were playing catch-up in trying to deal with the new variants.
"With the new variants, it's just trying to get ahead and that's the nature of the evolutionary race of the immune system versus the virus."
It was a tough time with the flu virus also circulating in the community, Le Gros said.
But people needed to do what they could to avoid getting viruses and the most important thing to avoid Covid-19 was getting a booster shot, he said.
"The other one is to avoid places where a lot of people are talking very excitedly and without masks."
People with suppressed immune systems needed to be particularly careful about not getting infected with Covid-19 as they may take a lot longer than usual to get rid of the virus, Le Gros said.
Everyone needed to be aware of this though and it was why mask-wearing was so important, he said.
An increasing number of older people were now catching Covid-19 as the country was no longer locked down, Le Gros said.
"But I think we have to work with that cause we can't stay locked down forever."
Le Gros' tips for avoiding the virus were ensuring a person had both boosters if qualified, ensuring they were sleeping and eating well, not drinking too much, wearing a mask and staying away from large unmasked crowds.
NZ's Covid per capita death rate higher than Victoria's
Australian-based epidemiologist Tony Blakely said New Zealand had a higher Covid-19 death rate per capita than Victoria - and the reason was not obvious.
Blakely has co-authored an article with New Zealand epidemiologist Professor Michael Baker, arguing there is a real risk the current settings will not be enough to prevent both countries' health services being overwhelmed.
The per capita hospitalisation rate of people with Covid-19 in New Zealand and the state of Victoria was currently about the same, Blakely told Morning Report.
But he said per 100,000, New Zealand's death rate was 20 percent higher than Victoria's.
The reason for this was unclear, although it could be due to a number of factors, he said.
It was surprisingly difficult to attribute a cause of death and usually it could take up to a year for authorities to do so, but this had been sped up due to the pandemic, he said.
Unlike New Zealand, Victoria tried to split the deaths into people who died with Covid-19 and people who died from Covid-19, Blakely said.
"New Zealand may have slightly higher rates due to a larger proportion of the population Māori and Pasifika and also lower socio-economic status, where-by that's kicking up your mortality rates, people with comorbidities, with high obesity rates, it could be things like that as well."
Another reason could be that Covid-19 infections rose in Australia earlier than in New Zealand which stayed with zero Covid-19 for longer, he said.
That meant that New Zealanders might have less overall immunity to the virus because "if you've been naturally infected and vaccinated, you've got a bit better immunity than if you're 'just vaccinated'".
However, compared to other countries both New Zealand and Australia were doing well and had a low accumulated death rate due to their zero Covid strategies earlier on, Blakely said.
"We're now learning to live with the virus which is by definition messy."
Both sides of the Tasman were doing well on the rollout of vaccinations and the rules for third and fourth boosters and on rolling out anti-viral drugs, Blakely said.
"Where there is contention and debate is about whether masks should be mandatory or encouraged."
Blakely said a game changer would be vaccines that stopped people from getting infected, rather than just stopping them from becoming seriously ill.
"What we've got at the moment is sort of a long battle with this virus to try and keep the total morbidity and mortality as low as possible without disrupting society too much."
