Seasonal flu: Why I got vaccinated

I just got my seasonal flu shot. It was free and my medical center is encouraging everyone to get one. I wouldn't be telling the truth if I said I didn't feel it at all, but in all honesty, I hardly felt it. They must be using smaller needles these days. Anyway, given that most circulating flu virus is pandemic swine flu H1N1, for which a vaccine is not yet available (coming soon to a clinic near you, we're told), you might wonder why I -- or anyone --would bother. I'll do my best to explain my reasoning, but I'll grant at the outset I may have missed some good reasons or have reasons that are fallacious -- you decide. The pandemic has produced lots of questions that don't have easy answers. But I've been asked here a number of times what I was going to do and why, so I thought I'd give you an explanation.

As I'm sure all readers here know by now, the currently available flu vaccine is the usual seasonal trivalent vaccine (three components) designed to protect against circulating flu viruses influenza A/H1N1 (Brisbane), A/H3N2 (Brisbane) and influenza B (Victoria). Since flu virus changes every couple of years in ways that aren't always predictable yet the vaccine has to be produced before we see what strains are actually circulating, there is always the risk of a mismatch -- i.e., the prediction was wrong. That's happened with one or another component of the vaccine a few times in recent years, although it's thought that even mismatched vaccine gives some protection (the unhappy corollary is that even properly matched vaccines often fail to protect a significant fraction of vaccinees from infection; see our post about efficacy here). So how are we doing with matching this year?

Since the seasonal flu season hasn't gotten underway in earnest (if indeed there will be one with seasonal subtypes), we don't know yet, but the data up through week 35 (beginning of September), which is the most recent, shows the seasonal H1N1 is a complete match, that is, since October 1st of last year (the administrative start of CDC's flu season), all of the seasonal flu A/H1N1 matched the vaccine component for this year, including up to last week. That's a good sign that if seasonal H1N1 starts circulating again in earnest, the vaccination I got will give me maximum protection. In my age group (65+) it's not clear how good that protection is. The immune systems of older folks doesn't respond with the same alacrity as that of younger folks (as in many other things, alas), but if there's seasonal H1N1 around like what we're seeing now I've done the best I can as far as a vaccine goes. The news for the H3N2 and flu B components is not quite as good, although (so far) not terrible. The match over the last year for H3N2 is 93% and for flu B 89%. So these subtypes are changing and if both or either start circulating and change even more the vaccine won't have done as well by me. But it will still have some efficacy -- and potentially quite a bit -- so I judged it netted out positively.

Let's review for a moment what vaccine efficacy means. Really well matched vaccines can have efficacies of 70% or 80%. What that means is that if you compare the amount of influenza in a vaccinated group to an unvaccinated one, the amount of flu will be 70% (or 80%) less in the vaccinated group. That's not complete protection, but it's pretty good. Let's be sure you understand exactly what this means. For clarity, let's take 50% efficacy. This means that if you are exposed to enough flu virus that would have infected you without vaccination, your chance of actually getting the flu is now like a flip of the coin (50%). That sounds bad until you realize that the people you are being compared with are people who aren't vaccinated and who are playing with a coin that has tails on both sides. I know which coin I'd rather play with.

But of course there's more to it this year. We are in a very complicated situation, because the vaccine I got is thought to give me no protection against the virus that, at the moment, is the overwhelmingly predominant circulating influenza virus, the pandemic swine flu H1N1. The epidemiology of this virus is quite different from seasonal flu. For one thing it has been infecting people since April, straight through the summer, during a time when seasonal flu is normally at very low levels. Although we haven't been doing surveillance during summer months since we didn't think we had to (because we didn't think there was any flu around), we could have been wrong that there's hardly any flu in the summer. We just weren't looking for it. But this year we have been doing virologic surveillance in the summer and it shows hardly any seasonal flu -- 1 - 2% at most. So there are two possibilities. One is that seasonal flu is behaving as normal, and more or less disappears in the summer (that's why flu is call "seasonal," after all); or that it's been there in other years and we've never looked for it but that this year pandemic swine flu has crowded it out of the host marketplace (however that happens). Either way, almost all the flu A virus that's out there now is pandemic swine flu, the virus for which the vaccine I just got gives me no protection. So why did I get it?

Below is a graph I showed last week, from CDC's FluView surveillance webpage. It's the product of CDC's Emerging Infections Program (EIP), a population-based surveillance network that monitors trends in laboratory-confirmed influenza-associated hospitalizations. The font is kind of small, so if you want to see the it full screen you can go here. What it is showing you is how the risk of winding up in the hospital with a lab confirmed case of influenza (any type or subtype) is shaping up since April, when the pandemic started. The horizontal dotted line in each panel is the average risk incurred in that age group over the last three flu seasons. Thus if this year is like the average of the last three years, you would expect the risk in your age group to rise to the level of the dotted line by the time the flu season is over. You can see that the risk is quite different this year for different age groups. I'm in the age group in the bottom panel (>65 years old):


The risk level is expressed on the vertical axis. For example, the number 2.0, the top value in the second panel, means that the risk of winding up in the hospital with a laboratory confirmed case of flu is 2/10,000 or 0.02%. It's important to note that the scales in the top and the bottom panels are different than the four in the middle. Looking at these age groups what you see is the risks have ramped up most quickly in the 5 - 17 year old and 18 - 49 year old age groups, where the risk of hospitalization for lab confirmed flu has already reached or exceeded what it was at the end of the flu season, even though we are now at a time when we haven't even started CDC's official flu season. In the under 4 year old and the 50 to 64 year old groups, we're already half way there, as well. In my age group, we've hardly budged. It looks like a normal seasonal flu picture, with risks just beginning to edge upward as fall is coming on. This confirms data from other surveillance activities that, for reasons that aren't clear, the over 65 age group is being affected dramatically less than normal for flu. For swine flu deaths, my age groups is contributing perhaps 2% of the mortality, whereas for a normal (seasonal) flu season, the over 65 contribute about 90% of the deaths.

Remember, however, that I mentioned the difference in scales. CDC says (correctly) that the age group at highest risk is the under two year olds. If we look at the risks, we see why:

Rates for children aged 0-23 months, 2-4 years, and 5-17 years were 2.5, 1.0, and 0.8 per 10,000, respectively. Rates for adults aged 18-49 years, 50-64 years, and >= 65 years, the overall flu rates were 0.5, 0.6, and 0.5 per 10,000, respectively. (CDC FLuView, Influenza-associated hospitalizations)

What this says is that although the rates for children and adults ramped up much faster than the normal (low) rates, rates for my age group are so high that we are still about even, all somewhere around .5/10,000 people in our respective age groups, or 0.005%. Thus my risk of winding up in the hospital as a lab confirmed flu case is about the same as an adult over the age of 18. The risk for the under 2 year old age group is five times higher (2.5/10.000), which is why CDC says they are at greatest risk.

What I don't know (and I don't know if CDC knows either) is what is putting the 65+ group into the hospital at this moment. More to the point, I don't know what will put my age group into the hospital as the flu season progresses. Here are the possibilities:

i. Pandemic swine flu A/H1N1 2009 almost completely replaces both subtypes of seasonal flu (seasonal H1N1 and seasonal H3N2) and there's hardly any of it around. Then I get no or little benefit from getting vaccinated (except for the flu B part, which is significant). Since last week 2% of the isolates were seasonal flu, it's hard to say what will happen. We don't usually see much seasonal flu virus this early in the year anyway. Will the future be different?

ii. Pandemic flu and seasonal flu co-circulate. We don't know if that can or will happen. But since I'm in the age group that dies from seasonal flu, I've done the right thing.

iii. Pandemic flu and seasonal flu are out of phase. For example pandemic flu has a sharp but short history and by virtue of using up susceptibles and vaccination it's essentially gone by January, leaving an opening for the seasonal viruses to return and act as they usually do. Again, I've done the right thing.

iv. Pandemic flu changes character and starts to infect my age group, crowds out seasonal flu or something else. Depending on the nature of the changes (maybe it reassorts with something the vaccine works for) I might or might not have come out ahead and everyone else, of all age groups, might or might not come out ahead if they are vaccinated with the seasonal flu vaccine.

The truth is this. No one knows what's going to happen. We're all guessing. But in my estimation, the risk-benefit calculation for vaccine side-effects and flu is so markedly in favor of the vaccine that I made the decision to get vaccinated and that's what I'd advise others, too. How confident am I? I'm confident it is the most rational thing to do given what we know. But flu confounds us at every turn, so being confident about anything else in this case is not something I'm confident about.


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I got mine yesterday too, mostly because I just DO every year, but I also had option iii above in mind when I did. And it hurt less than my allergy shot, which I got at the same time.

Pediatric versions are still not available in my community.

Got mine a week ago. It's not optional for me. I'm a diabetic and I've been hospitalized with severe bronchitis in the past. There's no doubt that the seasonal flu is out there, swine flu or no swine flu.

By speedwell (not verified) on 17 Sep 2009 #permalink

I am getting my seasonal flu shot next week; I always get the flu shot. I appreciate your spelling out the reasons for doing so. Another good reason is that I don't want to get the flu and then spread it around to others, possibly to someone more vulnerable than my 50-something self - my pregnant next door neighbor and her 2 year old, my senior parents, friends who might have the so-called "underlying health problems". And when the swine flu vaccine is available, I'll be standing in line for that, too.

What about pneumovax (vaccine against pneumococcal pneumonia)? I'm a bit younger than the age where it's standard (age 65?), but thinking that getting it this year might be useful.

Got mine last week as well. I rarely catch the flu (as in about once in around 15 years), but I would rather be safe than sorry.

Everyone at my office (about 8 people), and my wife, caught the flu a couple of weeks ago. Except me. I don't know why, but as I said, I rarely catch it.

By Benjamin Franz (not verified) on 17 Sep 2009 #permalink

It certainly makes a lot of sense for people offered free and convenient flu shots to get them. It also makes sense for a whole lot of people based on their own particular conditions and ages. I will get any shot offered free and easily and may pay to get a shot for myself (our pediatrician has advised my daughter not get a flu shot so I think vaccinating myself would improve her odds).

I also think you expressed well the issues that will probably dominate a lot of people's thinking about the "other" flu.

I do think you oversold the case for a bit. The absolute risks above of hospitalization are pretty low so the benefits can be low. Also, you exhibited a classic "fallacy of boosterism" by expressing the effectiveness of "really well matched" vaccines at 70% or 80%. The rational expectation should not be that high, particularly when the "match rate" has recently been so low.

Even with these adjustments it makes sense to get the shot for many (most?), I just believe in low-key salesmanship.

By floormaster squeeze (not verified) on 17 Sep 2009 #permalink

rosie: Pneumovax: yes. We discussed it here.

floormaster: I thought using the best case (70%) was being pretty negative. Notice in my example I used 50%. So I plead non guilty.

The football article you linked (thanks!) is particularly troubling. I am not in the least surprised that this is happening as the ethic of the players (suck it up) and the economic interests of the institution (very big money involved) reinforce each other. I suspect this will keep happening until the first well publicized death and then the finger pointing will start.

In the "New scientist" today:…

More possible reason to want to get the seasonal flu vaccination!!

Headline is: "Mystery Immunity could boost swine flu protection."

A couple of snippets:

"But last week the Swiss firm Novartis and the Australian firm CSL reported in The New England Journal of Medicine (DOI: 10.1056/nejmoa0907413) that nearly 300 adults given their experimental pandemic vaccines "unexpectedly" developed protective antibodies after just one shot."

""Somehow people's immunity has been primed," says Michael Greenberg of CSL. His team found antibodies that reacted with the pandemic virus in 1 in 3 of the people tested. "

"These antibodies can't be the whole story, though, because people without them also responded swiftly to the vaccine. Seasonal H1N1 infection may have primed another part of the immune system, called cell-mediated immunity,"

>>>>> End Quotes. <<<

Interesting, eh? There sure is a lot we don't know.

I got my seasonal vax and pneumo-vax this Monday!

Revere, I received the swine flu vaccine in 1976. Will that provide any protection at all against the novel H1N1? You may have addressed this but I can't remember.


By Julie, RN (not verified) on 17 Sep 2009 #permalink


Helpful info as always! So could you reassure me about this somewhat troubling piece on the potential link between accumulated vaccine exposure to mercury and higher risk of Alzheimers. Our flu clinic is coming up and I need to make a choice!


Thanks for this discussion. How do you feel about mandatory flu (H1N1 and seasonal) vaccination for healthcare workers?

The NYDOH recently put this into effect with refusal possibly leading to dismisal. Another policy trend is for healthcare employers to require mandatory vaccination and for workers who can't or won't get vaccinated, mandatory surgical mask wearing during all shifts during the flu season.

Helpful info as always! So could you reassure me about this somewhat troubling piece on the potential link between accumulated vaccine exposure to mercury and higher risk of Alzheimers. Our flu clinic is coming up and I need to make a choice!

There is no evidence that repeated flu vaccines increases your risk for Alzheimer's disease. That is a claim that Bill Maher likes to repeat and originated with a real anti-vaccine crank named Dr. Hugh Fudenberg based on no evidence. I explained this all a while back:…

Also, the key phrase from the article you cite is this:

One expert at the 1997 National Vaccine Information Center (NVIC) International Vaccine Conference stated that anyone who had five consecutive flu vaccine shots increased their risk of developing Alzheimer's disease by a factor of 10 over someone who received only two or fewer shots.

The NVIC is one of the oldest anti-vaccine groups there is out there. I would not trust anything reported at an NVIC conference.

Revere: Given the immune response to vaccination in the >65 population, why not delay one's seasonal flu shot (for those >65) until closer to the months when the local flu season starts to take off and peaks? For example, if this occurs in January-February, why not wait until November or so to get seasonal flu vaccination and, thereby, boost the odds of decent protection at the time you need it?

SoCal: I would never try to game out flu. You'll lose (you might lose anyway, of course, but you want to maximize your chance of winning). It's like the stock market. You should wait until it hits bottom and buy or the top and sell. If you knew when that was going to be. But you don't. You might see that seasonal flu is taking off by looking at the news, but you might be one of the news items. The vaccine will work throughout the flu season but it doesn't work immediately. So the earlier you get it the longer you are protected. If you intend to get vaccinated (and I have already said that's my advice, FWIW), then do it now, while there's vaccine to get and you'll be protected the longest.

Orac: Thank you for supplying those details. Alas, the thimerosol fear will dog this vaccine, but I believe there will be thimerosol-free single vial vaccines for children and pregnant women in the US. I don't know how available it will be. That's more a public relations issue than a scientific one, as I tend to agree the evidence is that thimerosol is safe for all ages (although I don't like the idea of it for children or pregnant women) and certainly certainly for adults. I had my doubts about it (on principle) which I expressed to you and you directed me to some relevant literature. I checked with some of the scientists I trust who helped advise on the methods in the study in question (and one of these people is very hard nosed about neuropsych testing methods and is one of the world's authorities on the subject) and was satisfied with the answers. There is no demonstrable neurodevelopmental effect from thimerosol containing vaccines in children. The study was competently done by people who know what they are doing and don't have an axe to grind. That's from multiple informants whom I trust.

Mark: I am not in favor of mandatory vaccination, but I consider HCW a special case as they come in contact with people at high risk. So I would consider that part of the job -- getting vaccinated.

Julie: I don't know if we know about cross reactivity with the 1976 strain. It is an open question if there is residual protection. I wouldn't let that stop you from getting the new vaccine which is to a different strain.

Where are the studies to support your statement that "Really well matched vaccines can have efficacies of 70% or 80%. What that means is that if you compare the amount of influenza in a vaccinated group to an unvaccinated one, the amount of flu will be 70% (or 80%) less in the vaccinated group." I've been looking for days and could not find anything but a few very equivocal (i.e. flawed circumspect) studies. I would really appreciate some references for this, please -- you can and should post them publicly or send them to me privately to my email.

That statment seems to me to be the pivotal kernel of truth for your entire position but if it cannot be substantiated with any credible studies it seems to me to be just a matter of faith with everyone participating in some strange primitive rite to appease the God's of Science that we will not be the ones to suffer the coming year's influenza.

By Michael Tierra (not verified) on 17 Sep 2009 #permalink

I'm 60 and will be getting my first ever flu shot when the H1N1 vaccine becomes available. I'm kind of nervous because my husband had all sorts of mysterious health issues pop up out of the blue shortly after he got his first ever flu shot a couple of years ago...including liver function problems and severe Psoriasis. And those health issues still persist. Coincidence, or not? Who knows.

Michael: I provided a link in the post that will take you to a review by the world's expert on vaccine efficacy. Here's the link again with a partial summary and a link to the original article. I hope you find it useful.

Revere, it may be just my formatting on my screen but it seems the right side of the posts are being clipped a bit? Anyone else having this problem?

If so whats the fix? If any?

By M. Randolph Kruger (not verified) on 17 Sep 2009 #permalink

Randy: I'm not seeing it with Safari. What browser are you using?

Rhett: I've responded to a couple of people about this. These are unpublished observations that may or may not be significant. In particular we don't know which direction the causal arrow is pointing. Helen Branswell's article makes note of the caveats from some of the experts she talked to. This will have to be confirmed and most observations like this turn out to be nothing. We'll just have to wait and see. Immediate scrutiny should occur. Other than that, anything else is premature.


We Truly and fastly need your help in Canada and UK.

Glaxo-Smith-Kline adjuvant AS03 as been rejected by the FDA in USA (and rightly so) IMHO, but is going to get into all Canadians and some U.K. citizens.

Dr SusanC of Flu Wiki and I at Flu Trackers are doing our best to get the real infos out.

Despite the facts recognized by GSK
cf to

You will notice the morbid effects of the adjuvants on foetus and its neurological effects specifically on children.

We need your opinions on this Reveres, may there will be a enlightenment forewarning for the citizens of Canada and UK in whom the Govs are preparing themselves to inject this adjuvants in the bodies of their citizens in the weks ahead.

This is my deepest call to you Reveres.

Snowy Owl

Very nice explanation, young man. Sorry to have missed this Friday when I was singing the praises of Tara Smith's post.

I'm concerned, of course, about the young adult and middle-aged hospitalization numbers already. From what you've taught me about 1918-9 flu, this is the major concern, correct?

This all makes me wish I had studied public health. At least I know I can come here for adult education.

Abel: Thanks for the kind words, and the intentionally misleading appelation of "young man." If you're only as old as you feel, I'm 78. But that's another story

I think you are right that the big issue with H1N1 is not the virulence of the virus per se (since virulence is a combination of host, agent and environment, not just a property of the virus) but the fact that the epidemiology of this virus is different. In particular, as you note, it is striking a younger age group (and sparing an older one), just the reverse of seasonal flu. So how the new cases react to the flu will make a big difference. So far there's isn't any solid evidence this is some new hideous version of flu (like H5N1), but flu is always a disease to be treated with the utmost respect from the public health point of view. We have a very brittle health care system. In many big cities, including my own hospital emergency rooms are routinely on "diversion" (meaning they divert emergency cases to another hospital) and usually the main reason is lack of critical care beds. We are full to the brim under ordinary circumstances, and any increase in the need for people in ICUs on ventialators will be a terrible problem in many places.

But also remember flu is notoriously unpredictable and also very patchy in space and time. So while one place could be having a horrific time another place, even one near by, could be having just a normal flu season. And it will hit different regions at different times. The good news is that this is a major event in the history of science. We are watching a pandemic unfold in real time and we have unprecedentedly powerful tools to watch it with. We'll learn more about flu in these few years than we did all last century.

Hi Revere,

Great job of outlining the efficacies of this year's flu shots. I also agree with your final assessment -

But in my estimation, the risk-benefit calculation for vaccine side-effects and flu is so markedly in favor of the vaccine that I made the decision to get vaccinated and that's what I'd advise others, too.

But, what you've provided does not compare the risks versus benefits of getting a flu shot (side effects vs efficacy). It compares the risks versus benefits of a flu shot as a function of hospitalizations. The charts show the risk of hospitalization is greater without a flu shot compared to being vaccinated against the flu. There is no mention of the side effects from vaccination.

However, it is true that in certain populations, the benefits of a flu vaccination have always and will probably always outweigh the risks of not being vaccinated.

Mike: I think the confusion comes from the fact that I was referring back to a previous post where I ran the numbers of rare adverse events and likely benefits (I think it's this one).