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?”

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.

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0 thoughts on “How effective is the flu shot?

  1. Lets use the numbers of 9800 of the 10000 did not come down with the flu so it is 98% effective. I bet people 4, 5 and six will believe that due to their gullibility even though 2 out of those 3 still wont get a vaccination.

  2. By never getting any flu shots, am I putting myself at a disadvantage in the long term? Do these seasonal flu strains recirculate in subsequent years? I’ve never had a flu shot. I haven’t had a flu in years. Should I dread a scenario where I get hit with 47 simultaneous (including a few vintage) flus when I’m 80, or is that just crazy?

    (I don’t actually dread that scenario. I’ve grown complacent. It’s just idle curiosity.)

  3. This is a pretty well explained post; most people don’t quite understand the compounding effect of vaccinated individuals. It’s the interruption of disease transmission at above a certain level which gives us those wonderful results we see in national/global vaccination programs. There’s also a threshold limit of “herd immunity” where a few people can be without the vaccine and not be at very much risk of getting the disease because so many others are vaccinated. One of the problems with LOW vaccination levels is this: the larger the infected populations, the higher the risk of mutations which overcome said vaccine. In reality, that decrease in infected or infectious individuals does far more than just prevent the individual from getting sick.

  4. Stella, influenza is a tricky virus; it mutates quite a bit and new strains are always popping up/dying off. Chances are, the strains you aren’t vaccinated for years prior won’t show up again, and even if they do, they will not resemble what that vaccine was targeted for. Honestly, though, you should get the vaccine because it is an interruption in the transmission chain, not just because you don’t want to be sick.

  5. Jared: Maybe. I would not be surprised, though, if a person who got a flu shot every single year would end up being immune to later years’ strains. Some of the mutation that occurs consists of very basic reorganizing of the genmoe with a high probability of similarity. I have no idea how strong that effect might be, though.

  6. @Jared: Yeah, I always wondered why the people that think vaccines are dangerous are always so eager to share their “knowledge”. If they actually were dangerous, your best bet for avoiding them safely would be to not get one while shouting about how great they were to anyone that would listen. Then you could at least rely on herd immunity to protect you and yours.

  7. A co-worker asked this morning “how scared should we be of the flu?” I told him that as healthy people, not very, but that wasn’t the point. He’s an IT guy, so I said; “Imagine that flue passing through a population is like packets through a mesh network. You don’t want to be a node in that network, because somebody a couple nodes over may be immunocompromized and die. You want to break every vector you can.”

  8. We can combine these discussions. Not being immunized is like carrying around a hand gun, on a chain around your neck with no safety.

  9. Yeah, I know I should get it, Jared. I just live such an isolated lifestyle that if I ever do get the flu, I’m likely to be a dead end.

  10. I kept hearing “Dr. House” in my head with all those “Moron” lines.


    Personally, I rarely catch the flu (as in about one year in twenty). But I’ve already gotten the seasonal shot this year and will get the H1N1 shot as soon as it becomes available here.

  11. Excellent point, Greg, in fact, there is some evidence that influenza strains with high morbidity share a great deal of characteristics. as for “seasonal flu” variability, it seems these strains have much more variability. I’d have to look to make sure, but that’s what I gathered from an immunology class I took a few years ago

  12. I still have my doubts. When in the military I was vaccinated every year and then some. Still got the flu once in a while, didn’t seem any worse or any better. But one thing was for sure – I was out for a day or two due to allergic reactions. I got out in 1988, and never got another flu shot, same story, got the flu once in a while, nothing ever serious, and never out of work more than a couple of days. So I take this new job, and unfortunately, I am compelled to get the shot. The reaction was the worst yet – I blacked out. Fortunately I had gone home, so it occurred it a safe place. If it happened while working or driving – I would have been a statistic – not dead because of the flu, but because of the shot.
    The misanthrope in me says we can afford to lose a few hundred, a few thousand, a few million people. Crikey, half the population of the earth could die and we still have more people than were alive in 1900; it ain’t as if human population is endangered. Furthermore, I cannot think of any problems that were solved thanks to the addition of another beach ape. As I told the frau, we shouldn’t be working for a cure to cancer, we should be working to make the consequences of stupidity fatal.

  13. Jared: Good point, but our understanding off how the flu evolved improved by an order of magnitude in the last two or three years, so that might matter. Maybe not. Not sure.

  14. Up here in Ontario, we have a very good selling point for the flu shot:

    It costs OHIP[1] less to give out flu shots to a large portion of the population, than the amount costs for taking care of flu victims has dropped since the program began. IOW: It’s saving tax dollars, and that is before you account for things like reductions in sick days.

    [1] Ontario Health Insurance Plan

  15. I’m getting the flu shot as soon as it’s available. From the anecdotes, the Mexican swine flu is a doozy, leaving you aching and gasping–who needs it? I don’t, and I don’t want to pass it around. I live in a big city, and people who have had organ transplants have to keep their immune systems suppressed. It’s criminal not to protect them by keeping the flu away from them.

    Besides, I was intrigued by a study that said getting your flu shots could cut your chance of getting Alzheimer’s disease by 40%. And every bout of illness wears you down.

  16. How about if folks in recommended groups get their flu shots and those thirty, fourty, fifty somethings without respiratory disease (not working in health) wait until at risk people have gotten theirs?

    All this furor about ‘morons who don’t get flu shots’ is completely out of touch with the fact that I’ve been waiting two weeks for my pediatrician to get more flu shot in. Some areas are experiencing shortages.

    There is a reason they come up with the recommendations for those who should get flu shots each year. The hysteria in the comment “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. ”
    is completely counterproductive.

    Could we try to be a little sensible?

  17. micheleinmichigan: Yes, let’s be more sensible. My post was in reaction to a half dozen science teachers going with the 1% idea. Yes, I want to be more sensible than that, and a bit of gentle ribbing is appropriate.

    You do bring up a very important point. There are years when I was “told” (by the people on the TV) “don’t get a shot yet, we’re running out. Don’t get a shot yet, the high risk groups must go first” then all of the sudden “heath officials are concerned that no one got a shot, and they are not about to throw out gallons of vaccine”

    We do need to improve the system.

  18. The hysteria in the comment “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. ”
    is completely counterproductive.

    I understand your desire to “save” vaccines for those who really need them, but the problem is that a lot of people in high-risk groups just can’t get the flu shot for various reasons. So unless there is a shortage of vaccines, you’ll help those people best by getting the shot so you won’t get the flu and you won’t pass it on to them.

  19. typical pro-vaccine fear mongering fud. How about giving a reference for the 1% difference? That appears to be derived from an AMA study and, like any statistic, is amenable to being represented in different ways. In this particular case 2% infection vs 3%, with the 1% coming from the percent difference. Accurate? It doesn’t mean what some people seem to think it means, but it also indicates how infrequent the infections really are.

    As for ridiculing vitamins, you *are* aware of why people get scurvy? Dr. John Cannell for one believes that vitamin D plays a role in winter diseases. Proven? Maybe not, but ridiculing someone whose field you are not even in is far from scientific.

    Props for providing advertising for the burgeoning vaccine industry. You hit every point they want made and ignored the reality of money trails and attempts at research that could undercut the revenue stream. Don’t give the enemies any acknowledgement because it lends an air of legitimacy. All good formulas, long practiced by mankind.


  20. [25] It doesn’t mean what some people seem to think it means

    That would be the point, yes. And I could not give a reference to some number some one else mentioned and “heard somewhere” any more than you can even one more level removed.

    I do not ridicule vitamins. I ridicule people who state that if you take enough vitamins you won’t get the flu.

    Oh, and you are a dangerous nutbag.

  21. If my link to the article about flu shot shortage in my area wasn’t enough, you can do a google search for “flu shot shortage 2009” and see that many counties have run out of flu shot.

    I’m no scientist, but to me it looks like the current production and distribution system of flu shot is based on the goal of getting at-risk people AND those who care for very at-risk people vaccinated. I don’t believe the production will supply a model where the general population gets vaccines to protect those who can’t be vaccinated.

    If that’s not the case, please supply a link to source. I don’t want to be uninformed.

    Just a reminder, people who can’t get vaccinated can use protective measures, such as hand washing, etc.

  22. micheleinmichigan: You seem to be trying to disagree with me yet we are not disagreeing.

    There is now doubt that the following facts are true:

    1) A lot of people are badly misinformed.

    2) Some of those people are engaged in damaging rhetoric (see above)

    3) The system for production and distribution sucks and needs to be fixed.

    4) Not enough people get vaccinated because of 1,2 and 3 above and maybe other reasons.

  23. Greg “We do need to improve the system.”

    Yes, I’m in total agreement on that. And the don’t get the vaccine, get the vaccine games get old.

    But I worry about the priorities. When ever I visit the science blogs there’s seems to be at least one or two rabid posts(and yes, the minute you use the word “moron or stupid, or perhaps ignorant, you qualify as rabid in my book) regarding the flu vaccine. They are not talking about how to improve the system of flu vaccine production and distribution. They are all grabbing that low hanging fruit of “ohhhh, people who don’t get the flu shot are uniformed and stupid”

    As far as I can see the flu is not particularly dangerous to the general population.

    Another reality, a person who can not get the flu shot AND is at-risk for dying from the flu is not going to be able to go out into public safely without fear of catching the flu from someone directly or indirectly. The flu is not polio, it is not small pox and people who represent it like it is, only undermine their own believability.

    Of course if you meant the ‘moron/ and holding someone who doesn’t get the flu shot responsible for a flu’ comment as gentle ribbing…? To me it sounded like more than that. But I will admit I don’t know you or your style. Sure, gentle ribbing is in order for misinformation.

  24. Think about it like this, Michele:

    You ride the bus to get pretty much everywhere you go because you don’t have a car.

    You had a kidney transplant four years ago.

    You take medication that suppresses your immune system so that you don’t reject the kidney.

    You cannot get a flu shot.

    The other 30 people on the bus that you take to work six days a week can get a flu shot.

    19 of them don’t get one.

    14 of them get the flu.

    11 of them are symptomatic.

    5 of them go to work anyway.

    3 or 4 of them don’t cover thier mouths.

    You get sick and die because 19 people you are in close contact with nearly every day, did not get thier flu shot.

    That’s really all this post is trying to say, get the damned shot for the people you come into contact with that cannot, for whatever reason, have one themselves. There is no point of disagreement, you’re just getting hung up on the language somewhere.

  25. Kate: #30 The language matters, if 1, 2 or 6 of those 19 people didn’t get the shot because they were sick of being called a moron by the pro-vaccine folks .

    If it’s important enough to state an argument. It’s important enough to state it in an way that people can listen to and still maintain their self respect. You do not convince anyone by calling them a moron.

    Greg #28

    Yes,we basically agree (I think) Perhaps, I just hate seeing my beliefs sold in a less than productive way. Every time I see a pro-vaccine person calling someone who hasn’t gotten whatever shot an idiot or some such I feel an complete body cringe.

    2) Some of those people are engaged in damaging rhetoric (see above)
    Damaging rhetoric – “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. ”


    3) The system for production and distribution sucks and needs to be fixed.
    Sure, that was my point. I have not talked to one person in my area that didn’t get a flu shot because they were against it or thought it was unsafe. I know a lot of school kids who have gotten flu shots. Most of the people who haven’t gotten it say they should…What’s stopping them?

    4) Not enough people get vaccinated because of 1,2 and 3 above and maybe other reasons. – I just don’t know. I am unconvinced that the number falling under 1 and 2 is that significant.

    Just as an irrelevant rant. – I don’t know I just think really, people get sick. Sadly, sometimes people die. I am all for the CDC and I support their initiatives, but I also want to keep our society from getting too germaphobic/OCD. An immune-compromised person can get sick and die from any number of common illnesses, flu, cold virus, mycobacterium. Is that the target to give people enough shots and to keep every surface immaculate enough to keep them safe? My son was significantly sick 5 times last winter, he missed well over two weeks of school (which is fine, for him to miss school) and when he wasn’t getting sick, he was getting over getting sick and when he wasn’t sick, my daughter or I was.

    Yes, they had the flu shot last year and I don’t take the kids out if they have a fever (unless it’s to the doctor or drugstore when there’s no sitter). But, every time, I take a kid out to the library, store, play area with a sneeze or a cough I get glares and stares. Is that really realistic?

    So, please don’t make me responsible for some poor person’s death, but I do need to leave the house sometimes.

  26. OR you could be one of the people who get sick and die from the flu shot.. a stat not often talked about. I’ve known many healthy people who’ve gotten more sick from the flu shot than the seasonal flus of past years and I know one who died as a complication to the flu shot. As well it may very well be 70-90% effective on the seasonal flu strains going around any given year if they where right as to which strain would hit a given area on a given year without any mutation.

    The thing is the seasonal flu shot contains egg products which can cause an allergic reaction to some people. As well it contains trace amounts of mercury which is a cumulative poison (meaning with each shot you increase your risk of complications due to mercury exposure. Not to mention any injection has risks of other infection. These risks are often not outlined to the general public who are then left untrusting when thing go wrong with people they know and unable to make a proper informed decision as to their own health.

    I don’t get vaccinated because I have built my immune system up to withstand it by fighting it naturally each year. Once every 7 years or so it hits bad as my body learns to deal with new mutations and different strains, but it’s short lived. This process is what vaccines are based on and they are meant for people who’s immune systems are incapable of doing this on it’s own (or for viruses one can not do this without serious complications). They were meant for epidemics and helping immunocomprimised people, not for everyone. Proper sanitation practices, and staying home when sick is a far better way to avoid spreading viruses then vaccines. Vaccines are more convienient to peoples lifestyles however, as they do not interrup work or play. But you are still responsible for spreading the virus if you get vaccinated, get the virus and pass it on because your not aware you have it. Your logic makes no sense, unless you think the vaccine stops the virus from spreading at all… I would like scientific evidence of that please. (it may reduce the time one is infectious but if one is symptomatic and infectious they have the option to take extra precautions to avoid infecting others as those unaware would not.)

  27. Joanne: I really find it unlikely that you know “many” people who have gotten sicker from the vaccine than from seasonal flu. The number of people who get sicker from the virus are far greater than those who have significant effects from the vaccine.

    As for mercury, could you cite where you have determined that it is a problem?

    Yes, some people are allergic to egg products. That’s probably why the people who give me my shots every years will first ask if I am allergic to eggs, or have ever had a bad reaction to any vaccine, then ask me to hang around for twenty minutes before leaving.

    The normal way to build up immunity to a virus is to get sick with it and survive. Some people will die from the flu, every year. Your immunity would help protect them. Personally, I don’t like being sick and miserable for one or two weeks, and not getting any work done for that time. Then, because I am asthmatic, I cough for the next 6 to 8 weeks, and have a hard time sleeping and exercising. I haven’t had to put up with that for the last ten years, since I started getting the shots.

    Vaccines were developed so that people wouldn’t get sick. There are worse viruses out there than the flu, and the flu will eventually mutate into a more dangerous form, as it has in the past. The fewer people that get sick, the less likely it is to happen in any given year. It is people who get sick who spread the virus, and they are rarely the ones who get the vaccine.

    As for taking extra precautions when sick, I wonder how that works for the middle school student who doesn’t know she’s sick yet? And I have worked at jobs where the workers simply couldn’t take a day off, either because they needed the money or because of work policies. A vaccine given to enough workers at such a place would prevent a local epidemic. If you prefer getting sick and staying home, more power to you. Not everyone has the option.

  28. H1N1 = Conspiracy?

    H1N1 notepad

    H1N1 notepad

    October 11 to October 20 2009
    On October 12 2009, Prisonplanet publishes that ‘The vaccination propaganda is now kicking into overdrive’ but that ‘corporate mediaâ??s own polls reveal that around two-thirds of parents have serious reservations’ and writes about nurses suing the state to block H1N1 vaccinations. While Khaleej Times publishes that the ‘WHO Denies Reports of H1N1 Vaccineâ??s â??Dangerous Effects’ and that ‘The side effect of administering H1N1 vaccine is limited to a slight fever and pain at the site of injection’. October 13, the LoudounTimes writes about Desiree Jennings who can move sideways and backwards, but not forward. On October 14 The New American publishes that a growing chorus of doctors and researchers is claiming that being injected with the swine flu vaccine may be more hazardous than catching the flu and Florida Today writes ‘Swine flu vaccine here’. FootConsumer publishes that ‘Novartis, GlaxoSmithKline, MedImmune, Australian drug maker CSL, and Sanofi-Pasteur will likely make a great deal of money’.

    Read the full article with all media references @

    notepad publishing (a non-profit news agency based in Switzerland)

  29. A day after I receive the flu vaccine for the first time, I became severely ill. I ended up in the ER, needing IV. I can’t remember the last time I ever even had the flu, but I had to get the vaccine for work. Would I get the vaccine again? I think I’ll pass. For you who call us morons, just hope you never felt the way I did.

    How does a moron like me stop spreading the illness around? Basic precautions. If I ever get the flu in the future, it’s my responsibility to stay home and not spread the illness around, wear a mask to keep others from getting sick. The reason the flu is so easily spread is because people infected with the flu decide to go out into public, coughing without covering their mouths, wiping their noses and not washing their hands, spreading illnesses to their family members because they choose not to wear a mask at home. This should be the most important teaching in preventing the flu. Not instructing people to get the vaccine. People need to learn how to keep germs to themselves.

  30. Mookie, at least you understand how the flu is passed around and what needs to be done to not pass it on. Many vaccine denialists believe that if you take enough vitamins or eat certain foods, or if you pray enough, or whatever you won’t get the flu.

  31. In regards to the thimerosal concern, are we not exposed to mercury in food all the time? Also, isn’t there thimerosal free flu vaccines? My daughter had the seasonal flu vaccine and according to what I read it was supposed to be thimerosal free.

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