Would you want to be vaccinated against a virus that killed 12m people worldwide in 1919 - more than died in the first world war? A virus that puts you flat on your back for at least a week, with a raging fever, sore throat, hacking cough, aching muscles and such weakness that using the TV remote control is too much effort? A virus that, if you already have a medical problem or have reached retirement age, can land you in hospital? Now is the time for influenza vaccinations, and the start of the season of flu surveillance by doctors in the northern hemisphere, who are keeping their fingers crossed against another epidemic.

But however debilitating the illness - and once you've had it, you never again confuse it with a bad cold - there is only enough vaccine in Britain for 11m people. So while some employers offer it to their workforce to avoid the risk of everyone taking flu leave, most healthy people may find it tough to get vaccinated. The government intends the vaccination to be given free only to people aged 65 and over and those at high risk: a category including anyone with a serious medical condition, such as diabetes, chronic respiratory disease (including asthma) or kidney or heart disease, and those with impaired immunity.

The vaccine will also be given to people in residential or nursing homes because it is in these vulnerable and enclosed communities that flu - which is spread by droplets, for example in coughing - wreaks havoc most rapidly. This is the first year that the government has lowered the minimum age for free vaccination from 75 to 65.

"The main reason for preventing flu is to prevent the complications of flu," says a spokeswoman at the Public Health Laboratory Service in London. "The people being targeted by the Department of Health are those at high risk of bronchitis and pneumonia, which are the serious complications."

A new vaccine against flu must be given each year because the flu viruses cunningly change at least annually. Each vaccine is made from killed strains of the three main flu viruses, decided upon by the World Health Organisation each February after examining several thousand influenza viruses from its laboratories in London, Atlanta and Melbourne. But there is still a slight risk of epidemics because the viruses can mutate in mid-season.

Production of the vaccine starts in March, and GPs are encouraged to place their orders in the summer. But in an ideal world, wouldn't it be worth everyone getting vaccinated? "It is worth having a flu jab if you are a fit, healthy adult," says Dr Joe Neary, a GP and chair of the clinical and special projects network for the Royal College of General Practitioners.

Neary cites a paper in the New England Journal of Medicine in 1995 which compares the amount of sick leave taken by two groups of fit working adults (there were more than 400 in each). One group had been vaccinated, the other had not. Those given the vaccine took almost half the amount of sick leave and had a quarter of the the rate of sore throats, fevers and coughs. The economic savings of vaccination were modest (about £30 per person), but the health benefits were arguably more significant.

"You can look at whether you'd benefit from a flu jab in the same way as having a gamble," says Neary. "The stake is the cost of the vaccine" - around £7.50 from some chemists - "and its side effects are a sore arm and sometimes flu-like symptoms for a few days. Only people who have a serious allergic response to eggs shouldn't have the vaccine. The odds on whether you would get flu depend on your job - there's a higher risk if you touch lots of people who might be infected.

"The vaccine also offers only between 80-90% protection - but if you do get flu after being vaccinated, the chances are you'd be able to struggle on with it. The payout is that you don't get what is a very incapacitating illness that can last two weeks."

But Neary accepts that unless more vaccine is produced, then government priorities are right. "You can't afford to do everything for everyone. For some people flu is an inconvenience, but for others with impaired health it is life-threatening."

Neary pays for his own flu jab every year, despite being able to justify having one on the NHS because the health service virtually collapses when its staff get flu. "It's just that when my patients who aren't eligible for a free jab ask for one, I wouldn't be able to look them in the face if I didn't pay for my own vaccination," he says.

• Further information is avaiable from www.phls.co.uk/seasonal/influenza, and www.nhsdirect.nhs.uk.