Vaccines designed to protect against everyday viruses like shingles and respiratory illnesses may also reduce the risk of developing dementia – and researchers are starting to ask why. A new study from the University of Oxford found that adults over 60 who received a vaccine for respiratory syncytial virus (RSV) , a common cause of coughs and colds, were nearly 30 per cent less likely to be diagnosed with dementia in the following year and a half. The surprising link between vaccines and dementia risk isn’t entirely new. Research has shown that people vaccinated against shingles are also less likely to develop dementia. The latest findings strengthen the link and point to a new potential explanation: It might not be the virus protection itself that helps to protect the brain , but a specific ingredient in the vaccine. Existing research shows greater dementia protection from the shingles vaccine now in widespread use, called Shingrix, than the previous one, called Zostavax. A key difference between these vaccines is that Shingrix contains an ‘adjuvant’ – an ingredient designed to enhance the vaccine’s effect. Both the RSV vaccine (Arexvy) and the shingles vaccine (Shingrix) contain the same adjuvant, known as AS01. It’s not clear how this ingredient might help lower dementia risk, but studies show it can stimulate cells of the immune system that scientists claim could help to protect the brain from some of the harmful processes that lead to dementia. “The findings are striking,” said senior author, Professor Paul Harrison, Department of Psychiatry, University of Oxford. “We need studies to confirm whether the adjuvant present in some vaccines contributes to the reduced dementia risk, and to understand how it does so.” The study looked at health records from more than 430,000 people in the US and found that dementia diagnoses were 29 per cent less in the 18 months following both RSV and shingles vaccination, compared with those who received the flu jab, which does not contain the adjuvant. These effects were even seen after accounting for factors like age, sex, and general health. “Our findings show that vaccines against two separate viruses, shingles and RSV, both lead to reductions in dementia,” said lead author, Associate Professor Maxime Taquet, NIHR Academic Clinical Lecturer, Department of Psychiatry, University of Oxford. “This gives another reason to have the vaccines, in addition to their effectiveness at preventing these serious illnesses.” It remains unclear whether the vaccines prevent dementia or simply delay its onset. Either way, scientists say the impacts of the findings could be meaningful, especially as there are currently no other reliable ways to prevent or slow down the condition. Prof Sir Andrew Pollard, Ashall Professor of Paediatric Infection and Immunity and Director of the Oxford Vaccine Group, University of Oxford, said: “It is premature to be too certain about the mechanism by which vaccines might reduce dementia risk, but these observations provide further incentive for those eligible to turn up for their scheduled vaccination visits to prevent the unpleasant and potentially serious and life-threatening infections for which they were designed, but with the added possible benefit of a longer dementia-free life-span. What’s not to like?” The RSV vaccine is recommended for adults over the age of 60, while the Shingrix vaccine is already widely used for shingles prevention. Commenting on the study, Dr Julia Dudley, Head of Research at Alzheimer’s Research UK, said: “Dementia is not an inevitable part of ageing. Identifying ways to reduce dementia risk is a priority for research, and vaccination offers an intriguing area of exploration. “There have been an increasing number of studies suggesting a link between people who receive certain vaccinations, like the Shingrix vaccine, and a decreased risk of dementia. This study offers a potentially different perspective on what might be linked to this finding.” She added: “However, as the study is observational and examined past health data, the researchers cannot conclude how the Shingrix and Arexvy vaccines may protect against dementia. We also cannot rule out that the link between vaccine and dementia risk is due to other factors not captured in this study, such as social and lifestyle factors.” The study was published in npj Vaccines .