Covid 19 Omicron outbreak: More New Zealanders may get reinfected amid waning immunity and new variants

New Covid-19 variants and waning immunity could mean more New Zealanders are reinfected with the virus as an expert warns there is no guarantee a second infection will be milder than the first.

There were 9109 community cases of the virus and a further 20 Covid-related deaths reported yesterday. Ten of the 481 cases in hospital were receiving intensive care treatment.

Immunologist Dr Dianne Sika-Paotonu of the University of Otago said many people who had recovered from Covid-19 would have developed immunity for at least the initial months following the infection but there was a risk of reinfection.

"Although previous infection with the SARS-CoV-2 virus does offer some immune protection, this does wane over time."

A study from Imperial College London showed the risk of reinfection with Omicron was more than five times higher compared with Delta, Sika-Paotonu said.

Ongoing work was needed to understand more about whether reinfections for Omicron and its subvariants were more or less severe when compared to the primary infection.

"It is still important that those who may have already had a Covid-19 infection, strengthen their immunity with vaccination as well."

Sika-Paotonu said as issues with unequal global Covid-19 vaccine coverage, availability, accessibility and distribution persisted and remained unaddressed, new variants would continue to emerge, develop and spread.

To date, 70.9 per cent of eligible New Zealanders have had a Covid-19 booster. For Māori and Pacific peoples, these rates are 54.5 per cent and 56.9 per cent, respectively.

Professor Michael Plank of Te Pūnaha Matatini and the University of Canterbury said it was likely New Zealand would experience waves of Covid-19 infections driven by waning immunity and evolution of the virus.

There are different factors that could start a second wave of the virus, Plank said, including behavioural change, such as increased contact between people, and waning immunity.

If the second wave was driven mainly by waning immunity, it's expected a large fraction of those infections would be reinfections, Plank said.

"It's tempting to think that getting Covid-19 means you 'get it over with'. However, it's likely that immunity against getting reinfected starts to wane after a few months and some people could be reinfected even sooner than that."

New strains of the virus were emerging that might reinfect people more easily, Plank said.

"With borders open these variants are likely to quickly make their way into New Zealand, as has happened with the BA.4 sub-variant recently."

New Zealand's border reopened to international tourists from around 60 visa-waiver countries on Monday, marking the latest step in the Government's border reopening plan.

On Monday, the second border case of the BA.4 sub-variant of Omicron was detected in a person who had travelled from overseas to New Zealand.

The sub-variant has been reported in Europe, southern Africa and in New South Wales.

The Ministry of Health said its arrival in New Zealand was not unexpected and the public health settings in place to manage other Omicron variants were appropriate for managing BA.4 and no changes were required.

The arrival of a new variant could mean New Zealand gets a second wave "sooner rather than later", Plank said, however, it was not likely to be as dramatic as the way Omicron overtook from Delta unless a completely new variant exploded onto the scene.

University of Auckland cellular immunologist Dr Anna Brooks said the circulation of more than one variant paired with waning immunity from previous infection or vaccination left many at risk of being reinfected.

"The best approach is to avoid exposure, regardless of your vaccination or pre-infection status."