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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 evidence-based 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 what's going on here. It's true that flu vaccines don't 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 isn't always effective
The flu vaccine has been our main strategy for warding off seasonal flu for more than 60 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 can't just make one kind of vaccine that'll last a lifetime. Every year, public health agencies essentially make educated guesses on what strains and mutations will make the rounds. And that's not easy. In February, the World Health Organization offers a prediction — 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 won't 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 meta-analyses (looking at the results of lots of individual studies) on whether the shot prevents flu in healthy children and adults.
In kids, the highest-quality evidence — a randomized controlled trial — suggests the vaccine works well enough: On average, if you give six kids under the age of 6 a flu shot, you can expect to prevent one case of the flu. For children under age 2, 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, however, the vaccine's effects are more modest. "Depending on the season," explained Tom Jefferson, an author on these Cochrane reviews, "you need to vaccinate anywhere between 33 and 100 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 33 adults flu shots, on average, to prevent one case of illness. In a year when the WHO guesses badly, you need to vaccinate 100 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 it's hard to detect major effects on public health in the data.
On the other hand, the evidence suggests that there's no harm in getting a flu shot, either. The meta-analyses 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 doesn't hurt. In 2012 researchers analyzed 15 meta-analyses 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, where researchers observe and gather data on people who take the flu shot compared with those who don't. Because these aren't experiments — ideally we'd have randomized controlled trials — they're more prone to bias and therefore less reliable.
In the case of the elderly, the situation is particularly troublesome. As this Lancet study points out, because annual flu shots were recommended for the elderly since the 1960s, it has been unethical to conduct high-quality randomized controlled trials where you give only one group the shot in order to compare it with another group that didn't get the vaccine. So, as this Cochrane review notes, it's impossible to draw firm conclusions on the effectiveness of the flu vaccine for those over 65 because the data is so biased and poor.
Again, though, this doesn't 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. There's also little by way of alternatives. Beyond public health measures (making sure you have good hand hygiene and avoiding the office when you're sick), the flu shot is the best medical intervention we have for preventing infection. So there's little downside with at least some potential benefit. I personally continue to get the flu shot for this reason.
As Roger Baxter, the co-director of the Kaiser Permanente Vaccine Study Center, 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 benefit-to-risk ratio is high and the cost is relatively low."
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