Influenza vaccination is widely promoted, particularly the elderly. One argument for flu vaccination in the elderly is that it is said to reduce their risk of dying – primarily through its ability to prevent flu (which can be deadly) or a potentially deadly complication of flu such as pneumonia. Some evidence appears to show flu vaccination can halve risk of death.
However, the types of studies which yield these sorts of claims are ‘epidemiological’ in nature, which means they look at death rates in vaccinated and unvaccinated treatment. We have a problem here, because, generally speaking, individuals who get their flu vaccination as instructed each year tend to be more health-conscious and in better general health than those who do not. It might be, then, that it is this factor (sometimes referred to as the ‘healthy user effect’) that explains why vaccinated individuals are less likely to die than those who are not.
We actually have some evidence that this is indeed the case in the form of evidence which finds that flu vaccination is associated with a reduced risk of death even outside the flu season. Outside the flu season flu should not cause any deaths at all, so flu vaccination should not reduce the risk of death. The fact that it does lends support for the idea that those who opt for flu vaccination are generally healthier and hardier than those who do not.
A recent study published in the Archives of Internal Medicine teased away at this issue. It examined data for 9 flu seasons in individuals aged 65 or older in Ontario, Canada [1]. The researchers used a form of analysis known as ‘instrumental variable’ that is believed to counter the bias referred to above. In short, it is believed instrumental variable analysis is believed to give a much truer reflection of the benefits of a treatment than conventional analysis.
The researchers applied this approach and looked to see whether flu vaccination reduce the risk of hospitalisation due to flu and/or pneumonia as well as overall risk of death. When more than one outcome are analysed together (referred to as a ‘composite’ outcome) it makes it more likely that a statistically significant result will be found. In this case, flu vaccination was found to reduce the risk of the composite outcome in the flu season by 14 per cent.
The researchers also, however, analysed risk of death on its own. Here, flu vaccination was not found to reduce risk. What this suggests is that flu vaccination does not save lives in the elderly, even though this is often claimed.
This is not the first time the effectiveness of flu vaccination has been questioned. Back in 2010, a review by the respected Cochrane Collaboration concluded that “ “The available evidence is of poor quality and provides no guidance regarding the safety, efficacy or effectiveness [of flu vaccination] for people aged 65 years or older.” This is in stark contrast to the image flu vaccination has. See here and here for other posts which question vaccine effectiveness in the elderly
To know just how effective flu vaccination really is we’d need to conduct randomised controlled trials where some individuals get the true vaccine and others get a placebo. Some argue that such trials are unnecessary, as vaccines are like parachutes – you don’t need a trial to know they’re effective. The evidence, though, suggests otherwise.
References:
1. Wong K, et al. Estimating Influenza Vaccine Effectiveness in Community-Dwelling Elderly Patients Using the Instrumental Variable Analysis Method. Arch Intern Med. 2012;172(6):484-491.
