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In people with multiple health conditions, a booster (third) dose of a Covid vaccine was associated with a 90 per cent reduction in death when compared to two doses, a new study has shown.

"We found a substantially reduced risk of Covid-19 related death in adults with multimorbidity who received a homologous booster dose of BNT162b2, an mRNA vaccine, or CoronaVac, an inactivated whole-virus vaccine," said Dr Esther Chan of the University of Hong Kong, with co-authors, in the study.

The team of researchers compared data on 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 booster dose between November 11, 2021, and March 31, 2022, compared to people who received only two doses, according to a study published in CMAJ (Canadian Medical Association Journal).

The study included 1,20,724 Pfizer-BioNTech vaccine recipients (87,289 of whom received a booster) and 1,27,318 CoronaVac recipients (94,977 who received a booster).

CoronaVac recipients died at a higher rate than Pfizer-BioNTech recipients, said the study.

Moreover, the study's findings "highlight the potential benefit of booster vaccination, particularly in vulnerable populations living with multi-morbidity, and support the recent focus on older people and those with chronic conditions for future SARS-CoV-2 vaccine booster doses beyond the first booster".

"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 multimorbidity," said Francisco Lai, first author and a scientist at the University of Hong Kong with coauthors.

The study further suggests that the robust results will contribute to the evidence base that getting boosted provides strong protection against death from Covid-19.

"As the data on SARS-CoV-2 vaccination records used for this study was provided by the sole operator of vaccine roll-out in Hong Kong, with a unified recording system, and with linked clinical records provided by a territory-wide public health care provider, our data should be highly reliable and representative," the authors said.