How Effective Are Flu Shots?

Sometimes 60% isn’t all it’s cracked up to be

Mon, 01/28/2013 - 11:15

This flu season has been worse than normal. The Centers for Disease Control and Prevention (CDC) data show that the flu has struck early and hard. Influenza cases shot up during December rather than the more usual January or February, and 47 states report widespread influenza cases.

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I get a flu shot every year even though I know they’re not perfect. I figure they’re a relatively easy and inexpensive way to reduce the chance of having a miserable week.

I’ve heard on various news media that the shots’ effectiveness is about 60 percent. But what does 60-percent effectiveness mean, exactly? How much does this actually reduce the chances that I’ll get the flu in any given year? I’m going to explore this and go beyond the news media simplification and present you with very clear answers to these questions. Quite frankly, some of the results were not what I expected.

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Comments

Jim,

What about the 'flu shots reducing the chances of 'flu transmission in certain populations?  In other words, if I spend October to February largely with my immunized family then my chances of catching flu are much reduced.  Likewise if getting the shots was mandatory for everyone, for example, in the Army.

Also, you said:

"So let’s look at the average of these four flu seasons. If you aren’t vaccinated, you have a 7.0-percent chance of getting the flu. However, if you do get the flu shot, your risk is about 1.9 percent, which is a reduction of 5.1 percent"

I would calculate the reduced risk differently.  The 7.0 becomes 100% of the 'flu sufferers and as part of the 1.9 with my 'flu shot I would 27% as likely to catch 'flu.  A reduction of 73% not 5.1%.

Would you agree?

John

John,

If you spend most of your time with immunized people, I'd expect your risk to decrease even if you aren't immunized. For my column, the studies I reference look at the general population of healthy adults. It would not surprise me if various factors increased or decreased your specific level of risk.

Your calculations are another form of relative risk which doesn't tell you the absolute risk. Suppose that one car is traveling at 60mph and you call that 100%, and another car is traveling at 30mph, which you call 50%. You could say that is a reduction of 50% or 30mph. Both are based on the same data and true. My point is that you gain valuable contextual information by looking at it in absolute terms.

Or, put it this way. Suppose you have two pairs of cars travelling at different speeds:

1) 200 mph vs 100 mph

2) 60 mph vs 30 mph

For both pairs, the relative reduction is 50%. You get the same number for very different cases, which isn't so helpful by itself. However, if you put that in their absolute terms, suddenly the picture is more clear!

Thanks for reading!

Jim

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