A third booster dose of a COVID-19 vaccine was associated with a 90 per cent reduction in death in people with multiple health conditions compared to two doses, according to a study conducted in Hong Kong.

The research, published in Canadian Medical Association Journal, compared data from people aged 18 years or older with two or more chronic conditions, such as high blood pressure, diabetes and chronic kidney disease, who received a third dose between November 2021 and March 2022, compared to people who received only 2 doses.

"We found a substantially reduced risk of COVID-19–related death in adults with multi-morbidity who received a homologous booster dose of BNT162b2, an mRNA vaccine, or CoronaVac, an inactivated whole-virus vaccine," said Esther Chan from The University of Hong Kong.

"These results support the effectiveness of booster doses of vaccines of two different technological platforms in lowering mortality among those with multi-morbidity amid the Omicron epidemic," Chan said.

As the Omicron (BA.2) variant epidemic hit Hong Kong in late 2021, the city reported the highest COVID-19 mortality rate worldwide relative to its population of 7.5 million people, the researchers said.

Since November 11, 2021, older people, healthcare professionals and other priority groups were able to receive a booster dose of either the BNT162b2 mRNA (Pfizer-BioNTech) or CoronaVac (Sinovac) vaccine.

As of January 1, 2022, all others were eligible, resulting in more than three million people receiving booster doses in the first four months of 2022.

"Our findings suggest that this timely, massive public health measure has plausibly played a pivotal role in lowering the mortality rate amid the epidemic, especially among people living with multi-morbidity," sad Francisco Lai, first author and a scientist at The University of Hong Kong.

The study included 120,724 recipients of the Pfizer-BioNTech vaccine (87 289 who received a booster) and 127,318 CoronaVac recipients (94,977 who received a booster).

There were more deaths among CoronaVac recipients than Pfizer-BioNTech recipients, the researchers said.

The findings "highlight the potential benefit from booster vaccination, specifically in vulnerable populations living with multi-morbidity, and support the recent focus on older people and those with chronic conditions for future booster doses of SARS-CoV-2 vaccines beyond the first booster."

The study will contribute to the evidence base that getting boosted provides strong protection against death from COVID-19, the researchers added.