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.
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.