"I'm not getting a flu shot because I never get the flu"

Basing medical practice on science helps us avoid the pitfalls of relying on our own reasoning and experience. If I want to start a patient on a new medicine, the individual characteristics of the patient are important (Is the drug meant for their condition? Will it interact with other drugs they are on? Are they allergic to it? Can they tolerate it?) but at least as important is how the drug performs when used on large numbers of people. This attenuates the large differences that can be seen among individuals, and allows us to predict how in general the drug will act.  

One of the metrics we can use is something called the "number needed to treat/harm" (NNT). This is a nice measure which is a bit more intuitive than other statistics. For example, in the recently released USPSTF recommendations on mammography, it was reported that nearly 2000 women in their 40s needed to be screened to avert one breast cancer death (the number in the 60's age group was in the 300s). Deciding what that means is a value judgement. Is it worth it to screen so many for each life saved? Is it worth the pain of chasing down abnormal finding that are ultimately found to be benign?

These are questions that many of us will be asking over the next few months; every primary care doc in the states is already fielding calls.  Whatever we decide to do with this data, it is the statistical analysis combined with societal and individual values that will help us decide what to do about breast cancer screening.  We tend to think that our individual experience gives us more data than it really does.  A woman may decide to have a screening mammogram based on the mistaken idea that she is more likely to benefit from it than most women.  A similar cognitive fallacy affects us when contemplating flu shots.

One comment I hear daily is, "I'm not getting a flu shot; I never get sick."  This is what is sometimes called the "hot hand" or "reverse gambler's" fallacy.  Setting aside the problem of confounding "flu" with other illnesses, this kind of thinking is a problem.  Whether one gets the flu in a given year is independent of whether one has gotten the flu in other years (mostly).  In years such as this one, where the current strain has found a particularly susceptible population, the fallacy is even more likely to get you in trouble.  Our own experiences are powerful, but often deceptive.  With medical decisions, it's often useful to write out the question, examine the data, and ask for an outside opinion, because our own judgments often lead us astray.


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My idiot sister tells me she has never had the flu or a flu shot. I know she's lying.

I don't even remember the last time I had a flu and I still got my flu shot yesterday cause I'm not to keen on the idea of getting the flu this year either. Yay vaccine! Sorry for the display jubilation it must be the big pharma mind control they warned me about kicking in ;)

A person's exposure to the virus is independent of whether they got flu in the past or not. A person's susceptibility to the virus may or may not be. Prior exposure to variants similiar to H1N1 may confer some resistance. Animal studies clearly show the efffect of innate resistance to inflenza virus. It is unclear what predisposes some people to mild versus severe complications. Is it possible that some folks possess enough resistance to completely avoid infection? I don't know, but I don't believe it is as common as there are people who claim not to get the flu each year.
In addition, memories are imperfect, and people may not recall years when they had a mild case of influenza. I don't think it best to write off someone's claim of never having influenza, and I just put it into a basket of factors that don't affect my recommendation to be immunized.

I was wondering if you could expand on the statement "Whether one gets the flu in a given year is independent of whether one has gotten the flu in other years (mostly)."

I think I understand the fallacy you're referring to. If you flip a coin 5 times and it comes up heads every time, it's still a 50 50 shot the next time. But I was under the impression that your chances of getting the flu are related to your immune system, diet, risk factors, exposure, whatever. So it's less like a coin toss and more like loaded dice. Past results would serve to determine the chances, not a trend, but just the statistical probability of what the results would be.

Am I putting too much emphasis on the chances of actually contracting the flu or is there some other aspect I'm overlooking?

The "mostly" refers to some of the things Dan brought up. For example, this year's H1N1: folks over 64 seem to have some resistance, which is posited to be due to exposure to a strain/strains they were exposed to decades ago.

But due to antigenic drift it is unlikely that any recently-acquired flu infection will protect you from any strains that are closely related temporally.

The current H1N1 is a product of antigenic shift, which makes acquired resistance even less likely.

I'm wondering about the inverse of this: that is, if someone has had the flu several times in the past, are they more likely than average to get the flu this year?

That is, are there factors other than the possibility of innate resistance in a small number of people, and past exposure to a similar virus? Does general strength of the immune system make a difference, for example?

For that matter, does self-reported "never get the flu"/"always get the flu" map onto (lack of) time spent around young children? All else being equal, I would expect greater exposure to lead to higher incidence of symptoms.

I "never" get sick myself. I credit the fact that I don't go nuts with antibacterial socks and antimicrobial air fresheners and such so that my immune system gets plenty of practice on relatively benign stuff. (Slightly tongue in cheek there, of course)

But seriously, couldn't some people have a more effective immune system than others? Couldn't some people's personal hygiene habits make then less susceptible than others?

As an example, I once had a case of adenoviral pink-eye. I know that because the optometrist I went to some months afterward saw evidence (scarring?) of a recent infection. I was very careful not to rub my eyes with my hands, always used a paper towel or tissue, and always washed my hands if I thought I had touched my eyes. None of my family were ever infected. The optometrist was surprised that it didn't spread.

So I teach high school kids, and I'm not a "germ-o-phobe," but I am reasonable cautious, especially when a kid seems not to be feeling well. I do sometimes get a fairly mild cold. I treat the symptoms, and get over it. The last time I had something I thought was the flu was over 30 years ago.

All that said, I did get the seasonal vaccine when it was offered this year. First flu vaccine I have every had, as far as I know.

How does one know if they are "blessed" with some sort of super-immunity, or just lucky?

Well, I'm going to guess that either my mom is totally impervious to smallpox, or utterly susceptible, because she was vaccinated for it regularly and never had the vaccine take.

(Clearly very different from flu, I know, but an example nonetheless.)

By JustaTech (not verified) on 19 Nov 2009 #permalink

A question no one seems to be answering: Should we still get the vaccine if we think we have had h1n1?

Our doctor is no longer testing and just assuming any ILI is h1n1, due to the vast amount of cases he has seen. My daughter's high school, at its peak two weeks ago, had a 23% absence rate. Records were kept and nearly all students reported influenza-like symptoms. My family of four has all been struck. Are we free and clear? Or should we be safe and vaccinate?

Why rely on "lucky" when you can get the damn vaccination? What about this is so difficult? I'm an asthmatic who tends to have VERY uncomfortable symptoms when attacked by a respiratory illness (and has had visits to the emergency room when a cold or the flu triggered a serious breathing problem that didn't respond to the emergency inhaler). I *beg* for the flu shot each year (and I'll get the H1N1 as soon as I can). I practically wash the skin off my hands during cold/flu season. I also avoid contact with sick people. Coincidentally, I typically don't get sick. Is it luck? Maybe, but I'm not gonna bet on it.

This sounds to pat to be true (but it is); a woman called our health dept last week to ask the earliest time she could get two of her children vaccinated. The third child, a thirteen year old boy with no underlying health issues, came down with H1N1 novel influenza and ended up on a ventilator.

She had not gotten her kids vaccinated because none of them ever got the flu. Her son is going to recover but I am guessing it will be a long time before she forgives herself.

(Our PIO asked the family if they would be willing to talk to the media; they are, but the local paper thinks H1N1 is "old news").

I had a friend in college who insisted that he "just never gets sick". the next week, I was letting him borrow my notes because he was too sick to make it to class. A few weeks later, he was back to bragging about how he never gets sick.

For people that really haven't had the flu in many years and think they can't get it, pert of it is because of woo-pushers advertising all these "immunity boosters". Even people who don't buy the woo often buy into the idea that you can avoid disease nearly 100% by just boosting immune system or having a naturally good immune system. There are things that can make people less likely to catch infections, such as eating breakfast and getting adequate sleep, but a lot of people just don't understand how things work. Decreasing the chance of catching something is important, but it's not the same as eliminating the risk, and people over-estimate the benefits of these things while somehow underestimating the effectiveness of vaccines. I think part of it is the belief that if you just live a good life and take care of yourself, that nothing bad can happen to you. Some people like to think that others who are sick are that way because of some moral failing, like poor diet.

In my mind, "boosting the immune system" is like turning the amp up to "11". Amusing as satire about the "more is better" fallacy, but just shifting the goalposts.

Consider someone who says "I've never gotten the flu in 30 years, and I've never gotten a flu shot." Let's estimate how many such people there would be if nothing more than pure luck were going on. Typical worst-case infection rate in a flu season is 20%, so someone of average risk has an 80% chance of making it through a flu season uninfected. 80% to the 30th power is .012%. Assuming that slightly more than half the US population is 30 or older, .012% of that population is around 200,000. Not a very big percentage of the population, but one especially likely to tell anecdotes.

momkat: I'm convinced that when people make statements like that, they're using "immune system" to mean something very similar, if not identical, to qi or prana or similar variants of "vital force." Similar to the way "toxins" in altmed means something much closer to religious concepts of "demons" than anything toxicologists deal with.

Well, I'm 34, get flu shots every year. In my line of work I am exposed to EVERYTHING that's going around (I'm a GP).

I have never had flu either. I have, however, had RSV, typhoid, chicken pox, shingles, measles, norwalk, and strep, plus the usual four colds a year.

Which is the anecdote I come back with when someone tells me they're not getting the shot.

How does one know if they are "blessed" with some sort of super-immunity, or just lucky?

Of course that's the big question, along with this one: How do you know that you didn't really get infected, but have such minor symptoms that you were walking around infecting everyone else?

I often get a cold, but no one else realizes I even sick. Maybe I have actually had the flu each and every year and just didn't think it was that big a deal.

You know, like Lil' Abner, strong as an ox and almost as smart.