Health differences between older people who have the flu jab and those who do not may complicate findings on the effectiveness of the vaccination.
Despite ongoing campaigns to encourage more elderly people to protect themselves against the seasonal virus, new studies have cast doubt over the benefits of mass vaccination, reports Bloomberg.
Researchers at the University of Washington found that healthier people are more likely to go for the jab, making it difficult to establish the exact level of protection provided by the treatment.
The study showed that while the jab was thought to have reduced the risk of death during the flu season by 44 per cent, it was also found to be associated with a 61 per cent drop in the months before the season.
"The reduction in risk of death in the pre-influenza period indicates the presence of bias due to preferential receipt of the vaccine by relatively health seniors," explained the authors of the study, published in the International Journal of Epidemiology.
However, the World Health Organisation believes that at least 75 per cent of elderly people should be immunised over the next five years and has encouraged governments to continue with free jab campaigns.