"Dear Julia How well do flu shots actually work Just about every influential public health figure urges people to get their flu shots each fall as a necessary precaution during influenza season. So over dinner I asked a couple of respected evidencebased medicine researchers what they thought of the vaccine. I was taken aback by their response. They basically laughed at the idea that the flu shot was particularly effective. This posed a conundrum. It seemed like there were two groups saying very different things. But after reviewing the research I think I understand whats going on here. Its true that flu vaccines dont work spectacularly well at least according to the best evidence we have and most of the evidence on flu vaccines is of poor methodological quality. But the downsides to getting a vaccine are extremely minimal and flu shots may still be helpful in reducing some illness and saving lives which is why public health experts say you should get vaccinated anyway. Why the flu vaccine isnt always effective The flu vaccine has been our main strategy for warding off seasonal flu for more than  years. Seasonal influenza which surfaces as a respiratory illness is caused by influenza A and B viruses. The flu shot is designed to protect people against three or four strains of the A and B viruses that researchers believe will be most common that year leading to the nasty fevers headaches coughs and runny noses that make many people miserable in the fall and winter. The challenge however is that the flu virus is highly unstable mutating all the time. So this means drug companies cant just make one kind of vaccine thatll last a lifetime. Every year public health agencies essentially make educated guesses http on what strains and mutations will make the rounds. And thats not easy. In February the World Health Organization offers a prediction http  based on surveillance and laboratory and clinical studies  and then each country uses that prediction to approve vaccines in its jurisdiction. In the US the Food and Drug Administration makes that final call. The potential for error here can dramatically alter the effectiveness of the flu shot in a given season. If vaccine makers prepare for one combination of viruses and different ones end up circulating widely the vaccine wont work that well. This may help explain why the evidence on the effectiveness of flu shots is relatively mixed. The Cochrane Collaboration an independent nonprofit that reviews the evidence on medical questions has conducted two major metaanalyses looking at the results of lots of individual studies on whether the shot prevents flu in healthy children http and adults http In kids httponlinelibrary.wiley.comdoi..CD the highestquality evidence  a randomized controlled trial  suggests the vaccine works well enough On average if you give six kids under the age of  a flu shot you can expect to prevent one case of the flu. For children under age  the benefits are less clear the evidence the researchers found was scant and of the research that was available it seemed the efficacy of the shot was similar to placebo. In adults http however the vaccines effects are more modest. Depending on the season explained Tom Jefferson an author on these Cochrane reviews you need to vaccinate anywhere between  and  people to avoid one set of symptoms. In a good year when the WHO guesses correctly and the flu shot matches the strains in circulation you need to give  adults flu shots on average to prevent one case of illness. In a year when the WHO guesses badly you need to vaccinate  people to prevent one flu case. Jefferson pointed out that his Cochrane review on adults also found that flu vaccination rates have barely any effect on the number of sick days taken and no major impact on rates of hospitalization. So yes the flu vaccine can prevent illness but its hard to detect major effects on public health in the data. On the other hand the evidence suggests that theres no harm in getting a flu shot either. The metaanalyses found no evidence of serious side effects beyond the odd case of soreness at the injection site. That seems to be the same conclusion that other studies have drawn  the flu vaccine is modestly helpful and doesnt hurt. In  researchers analyzed http  metaanalyses on the flu shot and concluded Most influenza vaccines have been shown to confer some protection against naturally acquired infection and no evidence for major harms has emerged. One key caveat here is that the evidence overall is pretty poor quality. Most of these analyses rely overwhelmingly on observational studies http where researchers observe and gather data on people who take the flu shot compared with those who dont. Because these arent experiments  ideally wed have randomized controlled trials http  theyre more prone to bias and therefore less reliable. In the case of the elderly the situation is particularly troublesome. As this Lancet study http points out because annual flu shots were recommended for the elderly since the s it has been unethical to conduct highquality randomized controlled trials where you give only one group the shot in order to compare it with another group that didnt get the vaccine. So as this Cochrane review http_vaccinesforpreventingseasonalinfluenzaanditscomplicationsinpeopleagedorolder notes its impossible to draw firm conclusions on the effectiveness of the flu vaccine for those over  because the data is so biased and poor. Again though this doesnt mean we should throw away the vaccine. Stanford researcher John Ioannidis told me how he makes sense of this issue after reviewing the evidence Overall the flu vaccine is effective and it does save lives. I cannot put an exact figure to how many but it may well be millions of lives. Generally it is safe and rather cheap so even if the effectiveness is less than optimal sometimes overall it is a good deal. Why people still bother to get the shot Despite the limited evidence of effectiveness there are still good reasons why public health experts recommend the shot. For one in those years when the WHO guesses correctly which strains will circulate the shot tends to be a lot more helpful than some of these averages suggest. And the vaccine is overwhelmingly safe. Theres also little by way of alternatives. Beyond public health measures making sure you have good hand hygiene and avoiding the office when youre sick the flu shot is the best medical intervention we have for preventing infection. So theres little downside with at least some potential benefit. I personally continue to get the flu shot for this reason. As Roger Baxter the codirector of the Kaiser Permanente Vaccine Study Center http told me Flu vaccines are not a panacea and do not prevent all cases of flu even in the best years when there are no manufacturing problems and the match is perfect. He continued However the vaccines provide moderate protection and can prevent huge numbers of cases of this serious illness. Many studies have shown that the vaccines are very safe so the benefittorisk ratio is high and the cost is relatively low. Welcome to Dear Julia http a column where readers can submit everyday health questions. Which overthecounter painkillers http work best Is it better to run or walk http for exercise How much harm does frequent flying  http to your body Julia Belluz will sift through the research and consult with experts in the field to figure out how science can help us live happier and healthier lives."