One year on from when many Kiwis got their first Covid-19 booster vaccine, how is the country's immunity holding up?

"It is now around a year since many New Zealanders received their last Covid-19 vaccine,” says Te Pūnaha Matatini and University of Canterbury Professor Michael Plank.

'A large proportion of the population has since been infected by the virus, providing a natural boost to immunity albeit by an unwelcome route.”

The professor says there is probably still at least half the population whose last vaccine dose or infection was more than six months ago.

'Covid-19 is still causing or contributing to 30-40 deaths every week and will continue to inflict a significant health burden. Alongside antiviral treatments, vaccines are by far the best tool we have to minimise illness and death.”

He says New Zealand has at least two routes available to increase levels of immunity using vaccines.

'The first is to continue efforts to reach those who are currently eligible for a vaccine dose, especially groups where risk is high or uptake is low. These include Māori and Pacific peoples, and the elderly.

'More than half of over-50s are still due for either their first or second booster. Using vaccination to top up immunity levels among older adults aged 50-plus will go a long way to reducing the health burden of Covid-19.”

He says the second potential route is to start offering so-called bivalent vaccines as many countries are now doing.

'These updated vaccines contain mRNA-encoding proteins from both the original 2020 strain of the virus and an Omicron variant (either BA.1 or BA.4/5).

'Although the virus continues to evolve its ever-growing family tree, using Omicron-based vaccines will provide a better match to currently-circulating variants, which are all part of the Omicron family. They may also broaden our immune response, meaning our immune system is less likely to be blindsided by a future new variant.

'England has offered these bivalent vaccines to over-50s since September 2022 and around 65 per cent have since had a dose.”

He says data shows that these vaccines halve the risk of hospitalisation for Covid-19 compared to people whose last dose was more than six months ago.

'I hope New Zealand will start to use these updated vaccines in the near future."

University of Auckland Immunisation Advisory Centre Director Professor Nikki Turner says evidence continues to clearly show the effectiveness of a course of the mRNA Comirnaty Covid-19 vaccine against severe disease, but says it is relatively less effective for mild disease or transmission.

'So many of those vaccinated will still catch Covid, but most will avoid severe disease,” says Professor Turner.

'The virus has evolved from the original strains through to a range of Omicron strains. It is clear that for all adults, a primary course of two doses, plus a booster, is needed to reduce the risk of severe disease - i.e. being hospitalised or dying from Covid disease.”

She says the vaccine does have some effect against mild disease and reducing transmission - but to a lesser extent, 'and it is this protection that wanes fairly fast”.

'There are still a lot of people who haven't had their first booster and for healthy adults we should concentrate on getting that first booster, rather than offering extra boosters.

'For healthy adults, two vaccine doses plus having one booster (2 + 1) is important protection against Omicron strains.”

She says having mixed immunity is good too, that is, for people who have had vaccinations and then also had Covid infection.

'Even if it's many months from the first booster shot, for most people protection from severe disease continues,” says Professor Turner.

'A second booster is used for people at higher risk, the elderly or those with other health problems, i.e. when their immune systems are compromised in some way. The extra booster helps them to lift their immunity so it's closer to that of a healthy person with a first booster.”

She says bivalent vaccines for Covid - that include a component of the original strain of Covid and an Omicron strain - have been tested internationally.

'The results to date do show a good immune response that has the potential to offer even broader protection than the current 'monovalent' vaccine.”

The New Zealand Covid-19 Technical Advisory Group will be considering these data in the coming weeks.

'So far the research done in the United States and Israel suggests that the use of a bivalent vaccine as a further booster can reduce hospitalisations in those 65 years and older by 73 to 84 per cent, compared with no booster,” says Professor Turner.

'They are also expected to be effective against recent strains of the Omicron XBB, which is dominant in the United States now.

'An Israel study including 700,000 participants showed 81 per cent reduction in hospitalisations and 86 per cent reduction of deaths using the bivalent in adults 65 years and older, while data from the CDC in November shows a 13.5-fold reduction in hospitalisations for adults 65 years and older who received a further booster as a bivalent versus those who were unvaccinated.

'There was a 2.5 fold higher rate among seniors vaccinated but without a bivalent booster.”

She says there is a clear body of consistent evidence now to show the bivalent's benefit in a booster dose particularly for high risk individuals, and there is nothing to suggest the bivalent is inferior.

'All data points to added benefit."