“The inactivated vaccine [developed by Sinovac] is known to have fewer side effects but suffers from relatively lower antibody response. Our study offers a potential solution to enhance [its] efficacy.”

The researchers collected blood and stool samples from 138 people aged between 18 and 67 to measure their antibody levels in relation to their gut bacterial composition. The samples were taken before vaccination and one month after their second dose between April and August this year.

About 57 per cent of the participants who took the Sinovac shot and had suboptimal antibody response had inadequate Bifidobacterium adolescentis levels. Among the BioNTech recipients, the 25 per cent with the lowest antibody level also lacked the bacteria, the study showed.

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Dr Hein Tun, assistant professor of public health at HKU, noted that there were two possible ways for gut microbiota to boost antibody response to vaccines.

“First, some components of gut bacteria such as flagellin serve as natural adjuvants to enhance antibody production,” he said. “Second, gut microbiota produce metabolites such as short chain fatty acids that can enhance immune cell metabolism to support energy demands for antibody production.”

In an earlier CUHK study involving more than 2,000 residents, researchers found that 85 per cent either had very low levels of Bifidobacterium adolescentis (34.1 per cent) or had none at all (50.4 per cent).

Ng said while Bifidobacterium adolescentis was present in a person’s gut since birth, factors such as age, diets containing too much processed food, stress and the use of antibiotics would contribute to its decline.

“This bacteria is very sensitive to oxygen, so it dies very easily when exposed to it. It’s also very fragile in terms of the environment that it likes to live in,” she said.

“We can’t get it through our diet because it’s not sufficient to give you enough of this bacteria. A lot of what’s available in terms of probiotics may not have this bacteria because you need special technology to capsulise the bacteria so that it can survive and actually reach your gut when you consume it.”

Asked if the public should check whether they had an adequate amount of Bifidobacterium adolescentis in their body, Professor Francis Chan Ka-leung, dean of CUHK’s medical school, said it would be costly and recommended against it.

“This bacteria can be measured using metagenome analysis but this type of analysis is only available in university-level laboratories, so it is not widely available and will be pretty expensive,” said Chan, who is also a director of the Centre for Gut Microbiota Research.

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“[People] should focus on improving their overall immune system through [a balanced] diet, exercise and possibly taking supplements containing Bifidobacterium adolescentis.”

While many health products contained Bifidobacteria – a family of probiotic bacteria in the gut – Chan said very few specifically had Bifidobacterium adolescentis and advised the public to check product labels carefully to ensure it was present.

Ng noted that in a previous study, participants reported an increase in the level of Bifidobacterium adolescentis in their bodies after taking a formula containing the bacteria, but that dropped when they stopped consuming it.

“I believe that for this bacteria to ... stay inside your gut to colonise, you need at least three to six months [for it] to stay in there to actually boost your immunity,” she said, adding that further studies would provide more information.

The researchers said they were currently conducting a large-scale clinical trial involving about 500 high-risk people, including the elderly and those with chronic diseases such as diabetes, to study the correlation between gut microbiota and adverse effects from vaccination.