The over 65s are eligible for flu vaccine in the UK
One of the main benefits of flu jabs for the elderly - protection against pneumonia - may not exist, US scientists have claimed.
They say the risk of pneumonia for older people stayed the same, whether they received the jab or not.
The study, published in The Lancet journal, looked at pneumonia rates among thousands of vaccinated and unvaccinated people.
The Health Protection Agency said the flu jab played a vital role.
Pneumonia represents one of the biggest threats to elderly people during the annual winter flu season in the UK.
It is either caused by the direct action of the influenza virus within the lungs, or by a separate bacterial infection taking hold while the person is weakened by flu.
In the UK, the over-65s, alongside those with chronic illnesses such as asthma and diabetes, are encouraged to have a yearly flu jab in the autumn.
This is not the first study to cast doubt on whether the vaccination programme actually does any good, even though some other studies have suggested that vaccinated older people are less likely to be admitted to hospital for pneumonia.
The latest research followed 3,500 people through three flu seasons to see if it made any difference, and while slightly fewer vaccinated patients were diagnosed with pneumonia, the difference was not statistically significant.
'Still worth it'
Dr Michael Jackson, from the Group Health Center for Health Studies in Seattle, said that this was the largest study of its type looking at flu vaccination and the elderly.
He said that previous studies had overestimated the benefits in terms of pneumonia prevention by disregarding the fact that older, frailer people - more vulnerable to pneumonia - were less likely to get the vaccine in the first place.
However, he said that it was still worthwhile getting the vaccine: "Despite our findings, and even though immune responses are known to decline with age, I still want my grandmother to keep getting the flu vaccine.
"It is safe, so it is worth getting, even if it might lower the risk of pneumonia and death only slightly."
A spokesman for the Health Protection Agency, which monitors the uptake of flu vaccine in England and Wales, said "at-risk" groups should carry on getting immunised.
"This paper does not negate the need for influenza vaccine.
"Other studies that were specifically designed to address this question have shown benefits from vaccination in reducing both infection and subsequent morbidity and mortality in the elderly.
"Anyone over the age of 65 or who falls into an at-risk group, including those with heart problems, diabetes or asthma, should always make sure they are vaccinated every year."
Dr Keith Prowse, chairman of the British Lung Foundation said the study was interesting, but had been carried out on healthy adults.
He added: "It should not deter people from getting the flu vaccine because influenza is an unpleasant illness in itself and the vaccine is particularly important for elderly people with chronic diseases who have a weakened immune system and are susceptible to developing pneumonia."