Each winter, people vulnerable to flu are offered jabs which reduce the chance of infection.

The make-up of these changes every year, in response to the constantly mutating and evolving flu viruses, and last year's vaccination probably will not protect you against this year's outbreak.

There are so many different types of flu virus that it is impossible to map them all, and mass-produce a vaccine to protect against them all.

So scientists try to identify some of the most virulent strains likely to pose a threat over the winter.

This year the World Health Organisation has recommended that flu jabs should contain a strains of the influenza "A" virus from Sydney, Australia, and Beijing, China, and a strain of the "B" virus from Beijing or Shangdong in China.

Minor symptoms

The vaccine cannot cause flu as it does not contain any "live" virus, and merely alerts the body to its threat without actually reproducing and causing symptoms.

However, the immune response may still cause some soreness near the injection site and, less often, a slight temperature and aching muscles for a couple of days.

However, as it is produced using hen's eggs, those with a serious allergy to them should not be vaccinated.

The best time to be vaccinated is between late September and early November - not once the flu outbreak has started.

However, there may still be some benefit to being vaccinated now if you have not yet had the illness.

Not everyone in the UK automatically gets offered the influenza vaccine.

Those given the option fall within the following groups.

• The young - they have not developed any natural immunity to flu yet
• The elderly - they have higher risks of developing severe complications such as pneumonia, have a worse immune response, and often have underlying diseases which weaken them
• High risk groups - other people whose medical condition makes them more vulnerable to flu. Conditions include:
  • Chronic respiratory disease including asthma
  • Chronic heart disease
  • Chronic kidney disease
  • Diabetes
  • Lowered immunity due to prolonged steroid treatment or cancer treatment

Some countries automatically offer jabs to the 0ver-65s, but in the UK the cut-off point is 75, although younger people living in residential homes - who are at greater risk for the spread of the disease.

People who think they have a medical condition which makes them vulnerable to complications of flu should consult their GP.

Those who fall outside these groups are not normally given flu vaccinations, as they are far better equipped to fight off flu, and it is thought that the occasional bout of flu actually improves their long-term resistance.

Patients who live with vulnerable relatives can ask for jabs, as can those who work in a healthcare setting.

Jabs can be obtained privately - Boots the Chemist launched a service this year charging £20 for the shot.

Vaccine uptake

Vaccine uptake is still fairly low, despite concerted campaigning by doctors and government.

Approximately a quarter of those aged 65 or above had the jab in 1996/7, and only 12% of those younger people in the high risk groups.

There is no firm UK data about the effect of the uptake rate on the size of the subsequent winter flu outbreak.

Certainly in 1995/6, when uptake was the highest ever, the size of the outbreak was significantly higher than in some years earlier in the decade when uptake was low.

So how effective is it? One Dutch study, conducted in the winter of 1991/2, looked at 2,000 elderly people given flu jabs.

It found that the flu rates were approximately halved in the group given jabs.

Two cases of flu were expected in every 23 elderly people, and the vaccination prevented one of these.

In addition, a recent study suggested that vaccinating health care workers significantly cut the number of flu deaths among patients in their care.