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In parts of the world where winter is approaching, this also heralds the start of the flu season. Over the last few weeks many people will have made a decision about whether or not to receive the latest vaccination against flu. Every year a new vaccine is produced, aimed at combating the latest strains of the virus. Recommendations vary from country to country, though many places target pregnant women and the elderly, as they are particularly at risk if they contract flu.

But every year doctors and practitioners encounter similar resistance from many people. How often have you heard people say that they’re convinced the flu jab gave them flu, or that the one year they were vaccinated they had the worst flu they’d ever had? They swear they’ll not be making the same mistake again. If you fall down with the flu after being vaccinated, you can see why you would be reluctant to receive it again, but is there any evidence that this is down to the vaccine itself?

Part of some people’s unease comes down to the nature of the vaccine. There are two types: an injection that contains an inactivated form of the virus; and a nasal spray that is becoming more common in places like the United States, which contains a live form of the virus, but in a much weakened form. Side-effects from the nasal spray can include a runny nose or sore throat. But the injection is not a live vaccine like, say, the rubella jab, where you are given a tiny amount of the infection to induce your immune system to create antibodies against it. The viruses involved in the flu vaccine are alive, but they are inactivated during production and batches of vaccine are tested to ensure that it is no longer virulent.

To study whether there are any side-effects in the days following a vaccination, US researchers vaccinated two thousand people, but only half were given the real vaccine, while the other half were injected with harmless salt water. There was just one side-effect reported more frequently by those who had received the genuine vaccine, and this was soreness in the arm in which the injection had been given. Health authorities warn that occasionally people feel slightly feverish or achey afterward. This is a rare side-effect, but it still doesn’t mean that people have flu.

Prediction test

So what’s the explanation for the people who find they get flu within days of vaccination? It takes two weeks for immunity against the virus to develop, so these people could have contracted flu just before or after they had the vaccination. Research such as the US study shows that no more people develop flu symptoms than those who had salt-water injections. It is simply that a proportion of people vaccinated were already about to get flu.

But what about the people who get flu during the winter, despite having received a vaccination? Are they somehow more susceptible to the illness, as many people think? The answer is that this is less about susceptibility and more about how the vaccines are created.

Each year the World Health Organisation selects the three viruses they believe will be most likely to be circulating the upcoming winter season in the northern or southern hemisphere. The selection for the northern hemisphere is announced in February, and vaccine production begins in March, ready for people to be immunised in the autumn. (Here’s the WHO’s latest recommendations for vaccines in the northern and southern hemisphere.) They are effectively making an informed prediction on the strains, but there is always the possibility that the flu virus you catch won’t be one of these three. And this is why it is never claimed that the vaccines are 100 % effective.

All of which begs the obvious question – how effective are they? Different studies are hard to compare because they measure different outcomes. Some look at rates of hospitalisation, others at death rates or at the number of laboratory-confirmed flu infections. But to take an example, a large study from 2007 published in the highly regarded journal, The New England Journal of Medicine found the vaccine protected seven out of ten people. Protection also varies from year to year, depending on the accuracy of the WHO’s predictions. Preliminary data from the US Centers for Disease Control estimate that the 2010/2011 flu vaccine was approximately 60% effective.

Persistent fear

That said, some people respond to the vaccine better than others, and this can depend on age. Protection is lower for those over 65, but the consequences of contracting flu can be more severe, which is why older people are targeted for vaccination. If people get flu despite having the vaccine, it’s not that vaccine gave them flu that winter, but that it didn’t protect them against it. And if you do get flu despite having had the vaccine, the chances are it would have been more severe without it.

Then there is another possibility, which is that you have a cold. And as horrible as it might feel, this is not the same as flu. There is a tendency to describe a bad cold as flu in order to convey to people quite how ill you are. I’ve done it myself. But the first time I had real flu I realised the difference. When I was recovering I made the mistake of going to rent a DVD from a shop five minutes walk away. The whole trip proved so exhausting that I had to sit on a bench for an hour to muster the energy to get back home. Colds and flu can both can involve sneezing, coughing and aching muscles, but flu usually comes on much more quickly, often starting with a sudden fever.

Yet despite the evidence available, many people remain convinced that vaccines can cause flu. In a study conducted by health psychologist Lynne Myers last year – not on the seasonal flu vaccine, but on the swine flu vaccine – only 53% of people surveyed correctly answered that the vaccine can’t cause swine flu. The evidence may be in favour of the vaccines, but the problem is that when we experience symptoms ourselves we make assumptions based on our own ideas about causality. It’s very hard not to connect events when the timing seems to fit. But that doesn’t always mean we’re right.

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You can hear more Medical Myths on Health Check on the BBC World Service.

All content within this column is provided for general information only, and should not be treated as a substitute for the medical advice of your own doctor or any other health care professional. The BBC is not responsible or liable for any diagnosis made by a user based on the content of this site. The BBC is not liable for the contents of any external internet sites listed, nor does it endorse any commercial product or service mentioned or advised on any of the sites. Always consult your own GP if you're in any way concerned about your health.

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