Although vaccines are still the best tool available to combat this seasonal scourge, their ability to protect lies well below that of other vaccines, such as those given in childhood.
The inactivated influenza vaccine (flu jab) is the only one licensed for use in Australia.
To determine how well these vaccines protect people from the flu, scientists use results from two different types of studies.
The gold standard study is called a randomised controlled trial (RCT). The second type is the observational study.
Randomised controlled trial
In an RCT, about half of the subjects get the vaccine and the other half receive a placebo (a shot that contains only saline or some other safe, inert material).
Those participating in the study don't know which one they've received until the study has concluded.
Researchers design the study so that people recruited to participate are very similar in terms of age, health status, and gender. I
t's important that both the groups are as alike as possible to ensure their bodies' response to the shots are directly comparable.
The design of randomised controlled trials also allows for more control over variables that can affect how well a vaccine works under ideal conditions.
Participants are checked each week to see if they have any symptoms of an influenza-like illness. If they do, they're tested.
This allows researchers to know with more certainty that they have tested everyone with symptoms in their study.
After a time, researchers determine the frequency of influenza in both groups and determine how well the vaccine works. This is called vaccine efficacy.
The most commonly used observational study is known as a case-control study.
This type of study helps estimate how well influenza vaccines work in a real-world setting, which is different to lab conditions for a number of reasons.
In a real-world setting, people unwell enough to seek out care and those with risk factors for complications become your primary study population.
This tends to reduce the impact of the vaccine when compared to RCTs (although improvements in study design have minimised this reduction significantly), but reflects how influenza vaccines are actually used.
With this study type, people seeking care for an acute respiratory illness (known as an influenza-like illness) at a doctor's office or medical clinic are voluntarily enrolled in the study.
They're identified as a case if they test positive for the influenza virus - meaning they were actually infected - and are designated a control if they test negative.
In the analysis, researchers compare the frequency of recent influenza vaccination among the cases and the controls, taking into account the same factors considered with the RCT.
This estimate of how well the vaccine protects is known as vaccine effectiveness.
So, how effective is it?
During the 2008 and 2009 influenza seasons, an RCT was conducted in Australia and New Zealand in healthy non-elderly adults.
Vaccine efficacy against all three influenza strains included in the vaccine was 60%.
Case-control studies conducted in Australia found similar results.
While the estimates of how well the vaccine works each year varies (because the formula of the vaccine changes), the average effectiveness from 2007 to 2011 was 62% in healthy adults under the age of 65.
How well the vaccine works in a given year also depends on the dominant strain of the virus in circulation and the population group that's most impacted by it.
All this means that, on average, healthy non-elderly adults in Australia can expect the vaccine to prevent an influenza infection serious enough to require medical care about 60% of the time.
Based on data from clinical trials, up to 10% of adults and up to 20% of young children will be diagnosed with seasonal influenza in a given year.
In children aged between six to 59 months, results from an observational study showed the vaccine works about 70% of the time in preventing influenza that requires medical attention.
Influenza vaccine efficacy and effectiveness studies including both observational and RCTs take place around the world.
The results of these Australian studies are similar to a recently conducted review of influenza vaccine efficacy and effectiveness from multiple countries.
But data are limited on how well influenza vaccines work in adults over age 65 and in people with chronic health conditions and other risk factors that increase the likelihood of complications from influenza.
Yet these are the populations for which the influenza vaccine is strongly recommended and actively promoted in Australia and around the world.
What the data shows is that the influenza vaccine doesn't work as well in these populations as it does in healthy adults of working age.
Indeed, during the past influenza season, vaccine effectiveness in people over 65 in the United States was 27%, while overall effectiveness for all ages was 56%.
While influenza vaccines don't provide the level of protection desired by public health, they are still the best way to prevent influenza for most people.
They should continue to be used while better vaccines are developed.