Cartoon image of a man showing his arm where he received a vaccination

Boosters and bivalents

Evidence continues to clearly show the effectiveness of a course of the mRNA Comirnaty COVID vaccine against severe disease, but it is less effective for mild disease or transmission. So many of those vaccinated will still catch COVID, but most will avoid severe disease.

The virus has evolved from the original strains through to a range of omicron strains. It is clear that a primary course of two doses plus a booster is needed to reduce the risk of severe disease ie being hsopitalised or dying from COVID disease for all adults. The vaccine does have some effect against mild disease and reduction of 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, 2 doses plus one booster (2 + 1) gives important protection against omicron strains. Having mixed immunity is good too, that is, those who have had vaccinations and then also COVID. Even if it’s many months from the first booster shot, for most protection from severe disease continues.

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

Bivalent vaccines for COVID (that includes 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 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 – 84% (versus no booster). They are also expected to be effective against recent strains of the Omicron XBB, which is dominant in the United States now.

An Israeli study including 700,000 participants showed 81% reduction in hospitalisations and 86% reduction of deaths using the bivalent in adults 65 years and older, while data from the CDC in November shows 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 of hospitalisation among seniors vaccinated but without a bivalent booster.

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.


• CDC Data Tracker

• Early Estimates of Bivalent mRNA Vaccine Effectiveness in Preventing COVID-19-Associated Hospitalisation Among Immunocompetent Adults Aged ≥65 Years – IVY Network, 18 States, September 8-November 30, 2022: here

• Effectiveness of the Bivalent mRNA Vaccine in Preventing Severe COVID-19 Outcomes: An Observational Cohort Study: here.