Influenza is caused by a viral infection which mainly affects the nose, throat, bronchi and, in some cases the lungs.
Every year the winter months see a surge in cases of seasonal flu which has a significant impact on public health and health services.
Most people recover within a week or two but vulnerable people – such as the very young, the elderly and those with a serious underlying medical condition – can suffer severe complications. In some cases, infection can lead to pneumonia and death.
Is flu preventable?
Yes. Flu vaccines can protect against influenza.
Annual flu epidemics affect between 10% and 20% of the population every year, leading most developed countries to embark on immunisation ahead of the flu season.
How is seasonal flu vaccine composition decided?
The flu shot is usually made up of vaccines against the three most common viruses in circulation. Experts at the World Health Organisation (WHO) monitor flu disease for any changes in the viruses and then make a recommendation to health authorities across the world on the composition of the vaccine for the forthcoming season.
According to Prof Bruno Lina, Professor of Virology at the University of Lyon, national reference centres play a key role in determining the make-up of the vaccine.
He told Vaccines Today that changes to the composition of the vaccine are necessary because influenza can quickly transform itself in order to evade the body’s defences.
“The body builds up immunity to viruses but influenza can adapt itself and escape the immune response. The viruses can change in a way that makes it difficult for the body to recognise them,” he said.
“So, if you want to protect people you need to have an optimised vaccine that contains the latest viruses that are circulating.”
In 2009 a major change in the virus was observed as a new strain of flu – known as H1N1 – started spreading across continents and lead the WHO to declare the first full-scale influenza pandemic in decades. Specific single vaccines against this form of influenza were produced at that time and the H1N1 strain was later also incorporated into the annual flu shot for the following season.
Two winters, two seasonal flu vaccines
The WHO consults its network of influenza surveillance centres twice a year – once to help decide on the composition of the vaccine for the northern hemisphere, and once to decide on which viruses people need protection against in the southern hemisphere.
The seasonal vaccines given north and south of the equator are usually similar but can differ slightly given the pace with which flu viruses can change.
“Every six months we have an opportunity to change the composition of the seasonal influenza vaccine. Sometimes there are no changes – this year the northern hemisphere and southern hemisphere vaccines are identical and with no changes from last year – but other times strains undergo changes or new strains emerge which must be included,” Prof Lina says.
In 2009, the EU already recommended countries to ensure that 75% of their elderly population and 75% of people with some specific underlying health problems are vaccinated every year against seasonal flu. However, the majority of EU countries are falling short of these European targets for uptake of the seasonal flu vaccine. This, say experts, is partly due to misconceptions around the safety and efficacy of the influenza vaccines.
WHO, health authorities and academics say flu vaccination is effective, the vaccine has an excellent safety record and warn against complacency.