Study highlights key role of COVID-19 boosters in preventing serious illness

Analysis of health data for 5.3 million Kiwis shows vaccine effectiveness against COVID-19 infection drops significantly after six months, highlighting the importance of booster shots.

Person getting COVID-19 vaccination
Credit: Colin McDiarmid, Image Services

New research analysing health data for more than five million Kiwi residents shows COVID-19 vaccinations have been most effective at preventing infections and hospital admissions in the first month after they’re administered.

“By six months, vaccine effectiveness drops significantly, highlighting the important role of booster shots in maintaining protection against COVID-19,” said study co-author Professor Colin Simpson, an associate dean at the University’s Faculty of Health.

The study’s findings are based on comparing health outcomes for vaccinated and unvaccinated people for the period from 8 December 2020 to 28 February 2023 when the Omicron variant of COVID-19 was dominant.

The researchers assessed the effectiveness of the first and second vaccine doses, plus the subsequent two boosters, in preventing infection, hospitalisations, and death from COVID-19.

Health data for 5.3 million people were included in the study. Of those:

  • 82 percent received the first vaccine dose and 78.9 percent got the second
  • 52 percent received the first booster and 14.3 percent got the second
  • 18 percent remained unvaccinated during the study period.

Analysis of health data showed vaccine effectiveness in preventing infection and hospital admissions from COVID-19 was the highest in the first 30 days after vaccination, said co-author Dr James Mbinta, a research fellow in the School of Health.

“For example, the initial booster shot reduced hospital admissions by 81 percent in the first month after it was administered. By month six, its effectiveness had declined to 57.4 percent.

“Similarly, the second booster reduced hospitalisations by 81.8 percent in the first month. By month six, this figure had dropped to 49 percent,” said Dr Mbinta.

Protection against COVID-19 infection also dropped over the same six-month period, from 57.4 percent to 9.9 percent.

Vaccine effectiveness in preventing deaths from COVID-19 also declined, but the drop was small.

“When we look at the first booster, it was 92.9 percent effective in preventing deaths from COVID-19 in the two-month period after it was administered. By months five and six, effectiveness was still high at 87.2 percent,” said Dr Mbinta.

The study findings mirror those of international research that show vaccine effectiveness declines over time.

Professor Simpson said the results will be useful in evaluating the COVID-19 vaccination programme in Aotearoa New Zealand.

“Our findings suggest vaccination provides longer-term protection against hospitalisation and dying from COVID-19, and shorter-term protection against infection. Further research is now needed to assess vaccine effectiveness beyond the six-month period post-vaccination that we reviewed. Research will also be needed to assess the effectiveness of future booster doses,” he said.

The study is published in the New Zealand Medical Journal. The research was funded by the Ministry of Health.