Background
The role of infections in dementia remains to date insufficiently explored. Studies have reported that pneumonia and sepsis were either an immediate cause of death or increased mortality in people with dementia. However, it is unknown whether association with mortality is long-term and whether it is dependent on the type of infection. We aimed to investigate the association between several infections and the short- and long-term mortality in people with and without dementia.

Method
We conducted a nationwide registry-based prospective cohort study using data from Danish national registries. Follow up was from 1 January 2000 or the 65-year birthday (whichever came later) until death, immigration or 31 December 2015. Primary exposure was incident dementia and a first infection post start of follow-up. The primary outcome was all-cause mortality. Mortality rate ratios (MRR) were calculated in all exposure groups (Dementia/Infection; Infection before or after dementia, Dementia/No Infection, No Dementia/Infection and with No Dementia/No Infection as the reference group) and stratified by sex and infection site categories. MRRs were also calculated by time since first infection and by infection site.

Result
A total of 1,496,436 people were followed with a total of 12,739,135 person years. A total of 575,260 people died during follow-up (82,573 with dementia). MRR for the Dementia/Infection group (infection after dementia) was 6.52 (95% CI: 6.43 to 6.61) times higher compared with the reference group and was higher in men. MRRs for the Infection groups in all of the site categories were markedly higher in the Dementia than No Dementia group (highest ratios for sepsis and lowest for ear infections). MRRs were highest the first 30 days after onset of first infection in the Dementia group and remained higher than the No Dementia group until 10 years after first infection (similar trends in the analyzed infection site categories).

Conclusion
Mortality is substantially increased in people with dementia following infections of all sites. Excess mortality is both short- (within 30 days) and long-term (after10 years). Our findings identify people with dementia and infections as a vulnerable group who need clinical attention.