Monday, September 20, 2010

Same old song, different year

In the news today there's a flood of articles about flu shots reducing heart attacks.   I've yet to find the original study but the news indicated British researchers reviewed charts and found a 21% decrease in the rate of heart attacks in people over 40 if they got the flu shot between september and mid november and a 12% decrease if they got the shot later in the year.  

This isn't new.  Last September stories like this were published 

Before that there have been studies showing a 50% to 90% decrease in mortality in elderly people who received the flu shot.  

All of these studies suffer from what's called "selection bias".   People who choose to get the vaccine,  especially elderly people, are more likely to seek medical care for their chronic conditions like high blood pressure or heart disease.   Influenza doesn't account for more than 5% of deaths in the elderly and yet the shot somehow lowers mortality 50% or more?   Additionally some of these studies found the "benefits" growing the following summer after the shot and flu season were long gone.   

The theory that reducing the stress of the viral infection,  influenza, will prevent heart attacks makes sense to me but it is equally reasonable that the myriad other viral infections that replace it (in the minority of vaccination recipients who do benefit) will stress the system just as much if not more.   Influenza unlike a few other respiratory viruses isn't known to directly infect the heart muscle for example.  

It's flu season and the stories touting benefits to vaccination are coming out of the woodwork again.  

Here's a good summary of what we know about influenza from vaccinations to antiviral drugs.

This one specifically talks about the misleading results on mortality

gotta hit play on that one

So when you hear the flu shot prevents heart attacks say "that's selection bias bubba".