Why are elderly people more susceptible to infections like the flu? As people age, the cells that make up their immune systems also begin to decline and cease to repair themselves. For this same reason, older folks don't respond as well to vaccines.

Vaccines work by inducing a small immune reaction so that the body 'remembers' and responds to attacking pathogens. In a new study launched on Thursday, a team of researchers here aims to work out the mechanisms by which this decline, called immunosenescence, occurs, and how to stem it.

The team is led by Dr Anis Larbi of the Agency for Science, Technology and Research's Singapore Immunology Network (SIgN), with Associate Professor Ng Tze Pin of the National University of Singapore's Yong Loo Lin School of Medicine, and will use a vaccine by drug firm Sanofi Pasteur.

The three-year study will examine some 240 people, including both healthy and frail elderly aged 65 to 90, and younger people as control subjects. It will look for biomarkers, or clues, in the blood that predict healthy ageing or something going wrong with the immune system, and devise a simple test for doctors to use routinely, said Dr Larbi.

"The outcome of the study will give us clues on the approaches we can take to reduce or restore dysregulated immunity in the elderly to ensure healthier longevity," he said.

By 2020, those over 65 are expected to make up 20 per cent of Singapore's population.