Vaccinating people against the flu is something that governments recommend all over the world, including in the US, where the Centers for Disease Control and Prevention (CDC) asserts that "flu vaccination benefits public health."
Exactly how effective are immunization programs against the flu? It's actually very difficult to say for sure — though it's nothing close to 100%.
But here's something all those "get vaccinated" messages don't usually emphasize: Getting vaccinated isn't really about you — it's about protecting society as a whole.
Why it's so hard to protect against the flu
Developing a flu vaccine is like trying to hit hundreds of targets that are constantly changing, and you're never sure which one will turn out to be the most dangerous.
There are over 200 different types of the influenza virus. And they are evolving inside our bodies and in animals in a constant arms race.
Each causes the flu, but our bodies can only be protected against a specific strain if we've seen it or one similar to it before, either because we were infected or vaccinated.
Every year, the CDC decides which three or four strains are most likely to be a problem (based largely on what emerged the year before), and puts those strains in the vaccines. Since it's a bit like trying to predict the future, the flu vaccine never offers complete protection. On top of that, a small percentage of people don't respond to the vaccine.
How effective is the flu vaccine?
Last year — an especially dismal year for the vaccine's effectiveness — the CDC reported that the flu shot was 18% effective against the influenza A viruses included and 45% effective against the influenza B viruses. That made it 19% effective overall.
In other words, if you got a flu shot, your risk of going to a doctor with the flu was 19% lower.
The flu vaccine has been more effective in other years; in 2011-12, it was 47% effective overall and reduced flu-related hospitalizations by 71%, according to one study. It really varies. And these measurements are not perfect, since they are based on a limited data set and measure a reduction in flu-like illnesses, many of which are not actually caused by influenza viruses.
But as an article in HealthNewsReview points out, that effectiveness number — which shows something called relative risk — doesn't tell the whole story. If your risk of something is already rather small, lowering that risk by 19% or even 47% might not mean that much to you as an individual.
On a population level, however, it matters.
How the flu vaccine compares to other vaccines
One way to understand the true impact of a vaccine is by using something called the "number needed to vaccinate" (NNV), which is frequently used by epidemiologists to answer a simple question: How many people do we need to vaccinate to prevent one person from getting a disease?
If an imaginary disease infects everyone who is exposed, and the vaccine against that disease offers 100% protection, the NNV would be one. In reality though, that's not how diseases work.
Take flu shots, for example. A widely cited review of the research by an independent group called The Cochrane Collaboration found that 71 people need to get a flu shot in order to prevent one person from getting the flu (the actual flu — not just a flu-like illness).
For comparison, between five and 12 people need to get a chicken pox shot in order to prevent one case of chicken pox. It's generally more than 85% effective. Unlike the flu, the virus that causes chicken pox does not change quickly or frequently, so developing an effective vaccine is much more straightforward.
The Gardasil-4 vaccine, which like the flu vaccine targets a virus (in this case, HPV) with many different types, has a NNV of 100. That means 100 people need to get the shot in order to prevent just one person from getting the abnormal cervical cell changes that can lead to cancer.
For influenza, the NNV of 71 only applies to healthy adults; the number is likely different for children, for whom the flu vaccine is usually more effective, and for the elderly, who don't respond to the vaccine as well. And the study found a NNV of 40 for preventing those influenza-like illnesses, which many flus (and other similar bugs) are grouped under. (Most cases of the flu are not confirmed with a definitive diagnosis.)
So is it worth it?
All of this might lead one to conclude that the flu vaccine is pretty disappointing overall, with what seems like middling effectiveness. Is it really worth vaccinating 71 people just to save one person from a disease that's not usually serious? While the NNV helps illustrate individual risk, it still obscures the larger picture.
The flu vaccine isn't by any means a guarantee against the flu, but public vaccination programs are critical for protecting society overall. One person with the flu is usually not a big deal — but thousands of people, every year, year after year, many of whom are already weak or suffering? That's huge.
The flu "is so frequent that even if it's fleeting, it's a massive burden," Dr. Tom Jefferson, who co-authored the Cochrane review, explained.
The changing effectiveness of the flu shot from year to year doesn't mean that you shouldn't get it, according to the CDC. As infectious disease specialist Dr. Mark Crislip explained in a story on the site Science-Based Medicine, it's not just about you.
The true benefit of the influenza vaccines [is in] decreasing the morbidity and mortality of populations. The benefit for populations is derived through vaccinating individuals. That requires a bit of altruism on the part of those receiving the vaccine, as they may be getting vaccinated more for the benefit of others than for themselves.
Between 3,000 and 49,000 people die every year because of the flu, according to the CDC.
More people getting vaccinated means the herd immunity of the population will be higher: One infection is less likely to spread to someone truly vulnerable — since that person will be surrounded by people who have somewhat altruistically vaccinated themselves — and then fewer people will get the flu overall. (People can even spread the flu even without having any symptoms.)
Dr. Trish Perl, a professor of medicine at Johns Hopkins University, said she often appeals to people's conscience when she explains the societal benefits of getting a flu shot.
"You may not be getting sick, but the person with cancer who won't respond as well to the vaccine might," she said. "If we protect you, we can cocoon grandma."
