Some vaccines may reduce the risk of dementia through a double effect in which the jabs help protect the brain through their added “adjuvant” chemicals as well as by suppressing viruses, according to a study by Oxford university.

The analysis of health records of 430,000 people in the US, published on Wednesday, found that vaccination against respiratory syncytial virus (RSV) with GlaxoSmithKline’s Arexvy was associated with a 29 per cent decrease in diagnoses of Alzheimer’s and other forms of dementia during the following 18 months.

The findings add to growing evidence of a link between viral infections and dementia, but the Oxford scientists were most excited about their conclusion that a significant part of the dementia reduction might be due to the AS01 adjuvant — a mixture of fatty molecules that are added to enhance the vaccine’s effectiveness by priming the recipient’s immune system.