by revere, cross-posted from Effect Measure

I’ve been asked a number of times why I am bothering to get both flu vaccines this year (the seasonal flu trivalent vaccine and the swine flu vaccine when it is my turn). I am in the older age group (last in line for swine flu vaccine) and it is my group that is hit the least hard from the swine flu virus. But there are a lot of us and we still being hit. I don’t know if I will be one of the unlucky few in my age group who draws the short straw or not, and I’d rather get vaccinated with an acceptably safe vaccine than take a chance in winding up having a machine breathe for me or not breathing at all. Moreover, we aren’t sure what’s going on with the relative lack of impact in my age group. It is possible (although truthfully I don’t think it is the most likely thing) that we are still being infected and shedding virus in reasonable numbers but some kind of cross-reactivity obtained in a past infection of a long history of yearly vaccinations is sparing us more serious clinical outcomes. After all, about a third of flu cases are asymptomatic under any conditions. So why care? Because it might mean I could still pass it on to family, friends and co-workers.

As for the seasonal flu vaccine, whether the seasonal flu A viruses will be a return in December or January or co-circulate with the swine flu viruses and whack the over 65 group I don’t know and neither does anyone else. Flu is unpredictable, but if the seasonal flu A strains come back, 90% of the deaths are typically in my age group. I’m not completely confident that the vaccine is highly effective for the elderly, but whatever its efficacy, it sure beats zero effect, which is what you get from not being vaccinated.

Mostly, though, I get asked the question by younger people who are (wisely) planning to get the swine flu vaccine but wonder why they should bother with the seasonal vaccine. Beside the answer that we don’t know with confidence what the seasonal influenza viruses A/H1N1 and AH3N2 will do, there is another, very good reason that applies to people of all ages. The seasonal flu vaccine is a trivalent vaccine, meaning it has three components, one of which is influenza B. Influenza B can make you plenty miserable and it co-circulates with the other viruses every winter. Last flu season about a third of flu cases were flu B (for more information on the three types of influenza, A, B and C, see a nice concise post at Vincent Rancaniello’s Virology Blog).

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The green bars are flu B. And last year was not unusual. Here are the four years before that, again with green being influenza B:

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There is no reason to think the swine flu will squeeze influenza B out of circulation, although with all-things-flu anything seems possible. But I think it’s unlikely and that means the seasonal flu vaccine will be providing significant protection against influenza B for whoever gets it (NB to grammar pedants: whoever is the subject of a subordinate clause; it the clause that is the object of the preposition “for” so it isn’t “whomever”. This is a pre-emptive strike. It’s the internet, after all.) That’s a reason for a person of any age to get the seasonal flu vaccine.

That’s my reasoning, anyway. I won’t say different people can’t weigh the odds differently. I’m a Bayesian so I allow subjective probabilities, but I think the evidence suggests you should move your posteriors and get vaccinated (I know; lame statistical joke).

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