How effective is the flu shot?

There are several answers to this question. One was overheard the other day among a bunch of well educated people oriented towards science who were taking a break from their job.

Person 1: "So, how effective is the seasonal flu shot?"

Person 2: "I heard about 1%. If you get the flu shot, you'll have a 1% difference in if you get the flu."

Person 3: "That's crazy. I don't know where you are getting your data from. It can't be 1%, but I admit I don't know what the actual answer is, but it can't be that."

Persons 4 through 6: "Well, if YOU don't know, and HE says 1%, I'm going with the 1%. Too much trouble to get a flu shot anyway."

Person 3: "Wait, wait! That's crazy! That makes no sense!" as persons 1, 2, 4, 5 and 6 are filing out of the break room to go back to work. "You can't leave thinking that 1% is correct! It can't be correct!!!"

Person 5: [Over her shoulder on the way out the door.] "Hey, why don't you go ask your husband. He's a Scienceblogger, right?"

[This is a repost in honor of Get Your Flu Vaccination Week. Which is now. Did you get your vaccination yet?]

Another answer is found by checking with the CDC web site. They say that in persons under 65 who are otherwise not at risk, the flu vaccine reduces the chance of getting influenza by about 70 to 90%

But, if you go to one of the authoritative science blog sites like Effect Measure, you get a somewhat different story:

CDC wants us to get vaccinated for flu every year. ... They want us to get vaccinated because they think the vaccine works ... citing figures that the vaccine is 58% effective or 91% or effective or some other number, depending on what group is being talked about ... This post is not about contradicting CDC, since I mostly agree that flu vaccination programs are sound public health. It is about clarifying some things that are glossed over when CDC talks ... bla bla bla bla bla

and so on and so forth and I'm pretty sure I know less than when I started after reading that post. (... in a good way ... it is an excellent post.)

It turns out that the issue is quite complicated, but I think I can explain it. But you have to be ready to change how you think about a couple of things.

First, you have to understand that at some level, a vaccine only works on people who get the flu. The vaccine does not shield you like a force field on Star Trek so the virus does not get at you. No. The virus gets at you, gets in you, and is inside your body .... so in a sense, you've "got" the flu, and then your immune system fights it off hopefully before you become symptomatic. Or maybe it fights it off enough that you get sick but not too sick. Or maybe it fights it off enough that you end up not spreading it to someone else. The vaccine sets things up so that your immune system is prepared to do this much more effectively than if you had not had the vaccine. (Oh, and by the way, does that person who did not get the flu from you, because you had the vaccine and thus did not pass it on, count when counting percentage of effectiveness of the vaccine?)

So you can see that it is fairly complicated. How to count the flu as an infection or the vaccine as an anti-flu agent is a matter of infection, symptom, virulence, reinfection, and so on.

There is another level of complication: There is not one flu. The so called "seasonal flu" is, in a given year, several different strains of influenza virus. The so called "flu vaccine" is a mixture of vaccines for each of several strains that are believed to be the most likely to circulate. Some of these vaccines may actually give cross immunity to other strains, or they may not. Generally not much, it would seem. Usually, the strains that are addressed by the vaccine are the very ones that are circulating, but some years, a strain or two circulates above minimal levels that was not included in the mix. A very high number for infection rates for a non-vaccine strain might be 20 or 30%, but that is rare.

Here's the question one really wants to know the answer to: Imagine two alternative universes. In one there is seasonal flu but no vaccine, in the other there is a vaccine. Within both of those universes we pick out a large number of people matched up in both ... the dopplegangers if you will ... and vaccinate all of them in the second universe. Then we ask, Of these paired-up people, how many got the flu in the first, no-vaccine universe, vs. how many got the flu in the second, vaccine-capable universe?

If we picked 10,000 people to vaccinate, and in the non-vaccine universe, 1,000 of them got the flu badly enough to be as sick as a dog or worse (like, they died) and in the vaccine-capable universe, of the 1,000 dopplegangers only 200 got the flu to this level, we could say that the flu vaccine is 80% effective. I'm pretty comfortable saying that this 80% is a useful, meaningful number.

But the reality is more complex. In truth, the vaccine might be 100% effective for one strain, 50% for another, and there might be a rare strain circulating that is not addressed in the vaccine, but the prevalence of each strain vs. differential effectiveness resulted in the observed apparent overall effectiveness of the strategy of vaccination of 80%. In truth, 78 of those people who did not get the flu didn't get it not because the vaccine worked but because they were never exposed to the flu in the vaccine capable universe because the person who was going to give them the flu did not get it themselves. In truth of those 800 people who "didn't get" the flu, 123 of them did get it, and did get sick, but not too sick and they didn't think it was the flu even though it was. In truth, 18 people in the first universe were counted as having the flu but it was a false positive. And so on.

In conclusion, no, it is not 1%. It is closer to 70 to 90% just like the CDC says, on average, for people not otherwise at risk who are under 65.

If you don't get vaccinated because you think vaccines are dangerous or you think you can avoid the flu by taking vitamins then you are a moron. If you do't get vaccinated because you think the flu vaccine is only 1% effective then you are ... an entirely different kind of moron. Either way, get a shot and don't be a moron. The person you give the flu to because you didn't get the shot may be one of those that dies. Wouldn't that be smart. Not.

UPDATED (in relation to a comment posted below): Effectiveness of the shot varies across various demographic and health related variables such as age. It will have less of an effect in very young children simply because vaccines rely on a person's immune system, and young children have less effective immune systems. One study showed that kids between 6 months 5 years had half the incidents of flu if they were vaccinated. The risks of the flu to children under 2 are much higher than for others, so even if the vaccine is less effective for them, it is more essential that they get it. The only group for which the vaccine is not recommended is age 0 to 6 months. Kids that age don't have enough of an adaptive immune system for a vaccine to work. If you have an infant under 6 months old, you need to avoid exposure to the flu.

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I got my shot. Fourth year in a row.

By NewEnglandBob (not verified) on 24 Sep 2011 #permalink

I'm a regular blood donor (176 units as of Sep '11).

They blood donation place gave me a free flu shot.

When I was a civilian employee of USAF I got free flu shots almost every year.

I had no idea it was Get Your Flu Vaccination Week, but I did get mine a few days ago! Now I feel like I'm owed a sticker or something, but I'll settle for not getting or spreading the flu. :-)

It is really hard to pull stats showing that flue vaccine works. It does show a lot less hospitalization for those vaccinated, but that might be because people who take shots are likely to take care of themselves.
I get the shot, it makes sense.

The CDC would disagree.

But yes, as I said, it is complicated, and in fact, last year or the year before the method of estimating the baseline (how many people get the flu) changed.

Awesome post Greg! It should be required reading. I make all of my family members get the vaccine, every year. The flu doesn't always kill you right away. It can cause an auto-immune reaction and cause your body to attack your heart causing failure of that organ, or of your insulin producing cells, causing diabetes.

I have gotten the vaccine for years, since they offer it to health care workers at my job, free of charge. I became a believer when the 2000 H1N1 outbreak hit our ICU. Our beds were full of patients down with this flu. It also wiped out our staff. The unvaccinated staff who came down with the flu were out for up to EIGHT weeks on average. The vaccinated staff (like me)who did come down with this flu were only out a week. I received no sympathy at all when I went back to work, because everyone else was so much sicker, and some were hospitalized. The same thing happened last year with THAT H1N1, one of our (unvaccinated)staff was hospitalized, and one of the hospital staff died.

@Gwen: Can you please provide some references about flu potentially causing an autoimmune reaction? I've never heard of that! Thanks.

I really - really - wish Revere would start blogging again. Ive never found anyone so good on epidemiology and public health... if anyone has discovered someone else approaching Effect Measure I'd appreciate it if they could provide a link.

This is an interesting question because a lot of things affect the response. For instance if the test persons were already vaccinated and what dose applied.
Vaccination of healthy children aged 2 to 16 years against influenza vaccine prevents influenza illness. In contrast, in children under 2 years inactivated vaccine's efficacy is similar to that of placebo.

[This is misleading, incorrect, and non-attributed at this time. See update above in the original post.]

In contrast, in children under 2 years inactivated vaccine's efficacy is similar to that of placebo.

That is interesting, or very bad misinformation and I'd like to know witch. Please provide a source for this as soon as you can. Thanks.

It is not what I thought to be true.

So, you're now running two _different_ blogs? I'll have to update my blogroll.

I can add anecdotes about flu and diabetes. Two people that I know, out of the limited number whose illnesses I hear about, suffered a "flu-like illness" and then developed diabetic symptoms about 18 months later. I suspected a separate disease flying under the medical radar (as Legionnaire's did) that provoked an auto-immune response. But maybe it was just the flu.

I think "Person 2" has a different definition of
"effectiveness" than you do. I think he is saying something like "I hear that the chance of getting the flu is 1.25 percent, and that getting a shot reduces that by 4/5, or to 0.25 percent. That one percent reduction is not worth getting a flue shot." Now I suspect that your 10,000 - 1000 - 200 hypothetical [which works out to "eight percent effectiveness" under Person 2's definition] is probably closer to the actual numbers that are Person 2's, but it is only a hypothetical. What are the real numbers? That is, what is the "effectiveness" under Person 2's definition?

Have you ever seen someone die of influenza? I have, numerous times, during the winter I was an intern. We had our own little flu epidemic of seeming 1918 proportions. The hospital was filled to overflowing. The ICU was packed. Patients were on ventilators all over the hospital. Positive swabs for influenza were so common we basically stopped doing them. Patients died directly from influenza related respiratory faliure, or heart failure and/or bacterial pneumonia which are common complications. Especially in the elderly and chronically ill. I and my fellow residents were exposed daily to numerous highly infectious cases. It was terrifying. Very few of us took ill, however. None of us were sick enough to miss work. Of course, flu vaccines were mandatory for all healthcare workers at our hospital, and I and all the other house staff had been vaccinated earky in the season. I now work with an immunocompromised patient population. I get the flu vaccine as early as I can every year. I will not be a vector for that terrible disease. I will not expose my patients, or my family, to influenza. Get your flu vaccines. Don't be a vector. Don't put yourself or your loved ones at risk.

By Nonsense On Stilts (not verified) on 25 Sep 2011 #permalink


Yeah, I'm splitting it out: Non-science (but including some creationism/evolution stuff) rants and such (politics, atheism, etc) at the xblog and everything else here.

Interestingly, despite my requests, no one has identified the symbolism behind "The X Blog" (the name of the blog) even tough the clues are everywhere (especially in the first post).

I do not disagree with the usefulness of influenza vaccination - especially in vulnerable populations and for healthcare providers. I do disagree with getting 'flu shots'. Strongly. Flu shots proivide very limited protection when viruses have changed slightly - and that's basically what the influenza viruses do for a living. They also give short term protection that blocks the formation of long term immunity to strains that might become much more dangerous with a very slight mutation that your immune system would accomodate if it had been exposed and reacted to the entire virus. So, I suggest getting FluMist - the nasal vaccine - instead of the shot. Anyway, that's my two cents worth.

Incorrect, undemonstratred tripe that is thinly disguised vaccine denialism. The clock is running. This post gets deleted shortly without something to back it up.

By ThayCrazyPharmacist (not verified) on 25 Sep 2011 #permalink

Your two cents worth is bordinging on vaccine denialism: What you say is inaccurate, misleading, even dangerous. I'm giving you 24 hours to produce peer reviewed, current information to support your claims then your comments gets detleted because this blog will not be a launching place for vaccine denialist commentary and propaganda.

Meanwhile I'll be investigating who "ThayCrazyPharmacist" is and if I find out you are one of the usual trolls then you don't get yoyur 24 hours.

Got stuck on Friday. My arm still hurts. In the several many years I've been getting the vaccine, the only time I've had the flu was in a year that the CDC guessed wrong.

There is a high-dose formula available for those over 65 and it is covered by Medicare Part B for those living in the U.S.

By Susan Silberstein (not verified) on 25 Sep 2011 #permalink

Flu vaccine is a classic case of medical over-servicing: a costly procedure including attendant risks is performed which has a fairly average chance of actually performing in relation to something which is very, very low-risk in the first place.

By Vince Whirlwind (not verified) on 25 Sep 2011 #permalink


I wasn't aware that the flu shot was costly...

It is low risk...I suppose. But people die from the flu. People are miserable from the flu. People miss work that they cannot afford to miss because of the flu. Children miss school because of the flu. Again, people die from the flu.

Tell me again, how flu vaccines are "over-servicing?"


Interestingly, despite my requests, no one has identified the symbolism behind "The X Blog" (the name of the blog) even tough the clues are everywhere (especially in the first post).

The nature of my plan here, a mere blastula of an idea, is partly hidden in the name of this blog, which is a reference to something. are getting a sex change, an experimental sex change that turns all of your Ys to Xs? And then being inseminated with your own semen???!!? YOU MONSTER!!!1@1111!!!

Or perhaps...

That will be a topic addressed at The X Blog. You may not realize this, but for reasons that need to remain unspoken (there are rules) I am more than a little responsible, personally, for Michele Bachmannâs rise to power. Ooops. That certainly isnât what I meant to do, but something I did was instrumental in the advancement of her political career. I am very, very sorry and I feel the responsibility acutely.

So, weâll try to fix that over the next few months, but Iâll need your help.'ll be doing something like this...

Some sort of tell-all blogoir (blog+memoir)?

Or maybe something X-Files-y? But....more in blog form?

By Drivebyposter (not verified) on 25 Sep 2011 #permalink

I do not disagree with the usefulness of influenza vaccination - especially in vulnerable populations and for healthcare providers. I do disagree with getting 'flu shots'.

So you agree with "influenza vaccination," but disagree with "flu shots?" What, in your pointlessly hairsplitting little mind, is the diference, exactly?

You sound like a creationist who insists he's totally okay with evolution, but not with "neo-Darwinism" or "the neo-Darwinian paradigm."

Hadn't noticed, thanks.

I'm fending off a number of anti-vax commenters.

I've been getting the flu shot for years. After it had been available for several years, the Ontario government made it free for everyone on the grounds that it saved money by eliminating a lot of work time lost to illness and a lot of healthcare costs treating the ill. I haven't had the flu for years and I don't miss being "sick as a dog" one bit.

Meanwhile, Mr. I'm-too-busy-to-get-the-flu-shot has had the flu a couple of times, and each time he pisses and moans about how he hates to be sick.

i work at a very large MN company. they give FREE flu shots to any employee. there is always a line to get in. i am scheduled to get mine in a couple weeks.

to all you anti-vaxx asshats: you're welcome. i am doing my part to keep you selfish ignorant bastards from getting sick.

Here's my totally subjective anecdote.

I work in healthcare (graphics/advertising, actually, for healthcare), and we get our seasonal jabs like everyone else. I've had mine 6 years running now.

What's happened to me - or what I believe I have noticed - is about twice a year I feel just a very little run down, just a very little punky for a couple of days, after which it clears up.

What I believe is happening is I've been exposed to one of the flu viri I've been vaccinated for, and it's been flattened by my amped-up immune system.

What I do not recall is having had these kinds of effects before I worked in healthcare. What I do recall from my pre-vaccination days is getting nailed, once a year, by the current damn virus.

In the last six years, I've only been hit once.

So do flu shots work? As I see it, in my experience, yes.

Hey Greg, YOU'RE a MORON. Pure conjecture with no evidence to back up your bullshit 70-90%. Most people do NOT get the flu anyway. What is the NNT Greg? Or do you even know what that is?

By Mike Gould (not verified) on 17 Jan 2014 #permalink