http://scienceblogs.com/effectmeasure/2010/04/did_getting_vaccinated_with_se.php"...
Observational studies (by definition) don't allow that random allocation. The two groups are now given to us by circumstance and may well be very different. If we know all the differences that are important that's not a problem. We can adjust for them in a variety of ways (stratifying, using statistical models, matching of various kinds). It's the differences we don't know about or have no information about that are the problem. It could well be true that there are characteristics that people have, characteristics we don't have any information about, that affect both the chance they will get vaccinated and the chance they will get pandemic flu and that the two groups differ in these characteristics. For example, suppose that care-seeking behavior were different in the two groups. Then people who were more likely to go to a doctor or clinic to get vaccinated might be more likely to go when they have an influenza like illness (ILI) and hence more likely to have a "medically attended" case of pandemic influenza. This is something the authors were attuned to and took pains to check indirectly to see if this was a plausible explanation, but there are many other possibilities, including ones no one has yet thought of. In an RCT you don't have to worry about them. In an observational study you do. It is called residual confounding or hidden bias (hidden because you don't have any information that would allow you to control for the effects of the factor).
There are a number of ways to control for confounders about which you have information and that was done in these substudies. There are 3 very similar substudies that are essentially replications. They use different populations and somewhat different protocols but all have the same problem that they don't -- they can't -- control for residual confounding. They are observational. The authors suggest that having three such substudies makes the problem of residual confounding less likely, but replication doesn't do that, since the same problem is seen in each. In my opinion these studies don't meaningfully eliminate uncontrolled confounders found in one and not in another. I qualified this with the word "meaningfully" because there is room to object that the studies aren't identical but I don't think they are different enough to be significant (not in the statistical but in the semantic sense). The 4th substudy is the weakest because it has small numbers but it does solidly eliminate the care seeking and some other hidden biases and for young adults also shows that seasonal flu vaccination makes later diagnosis of pandemic flu more likely.
Residual confounding -- a hidden difference in who was and wasn't vaccinated that affects pandemic flu risk -- is a type of bias, but it isn't the only type. Epidemiologists are also adept at uncovering, inventing and ferreting out all kinds of systematic error (meaning, not random error) that are not related to confounding. For example, supposing the test for pandemic flu consistently gave false positive readings. That would be a type of systematic error that isn't confounding and could as easily plague a RCT. RCTs allow efficient handling of one kind of error, random error, but no others. Any RCT can have many different and disqualifying kinds of systematic error ("bias"). So the difference between the observational studies and the RCTs pertains only to a specific kind of bias, residual confounding. Both kinds of studies are subject to other kinds of error. Indeed there are good RCTs and very bad RCTs. RCTs, like observational studies, are often discordant for this reason. An RCT is no guarantee of validity any more than an observational study is a bar to it.
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What's my bottom line on whether seasonal flu vaccine ups the risk of pandemic flu? It well might. But it might not. Or maybe it does some times and not other times. We don't know yet. This carefully done work opens up an important set of questions we now have to pursue. It'\s immediate public health import is probably small, because the pandemic strain will be a component of the next seasonal vaccine strain. This study confirms that the flu vaccine is quite effective in preventing the specific flu virus infections it is directed against or those quite similar. The surprise was the possibility it might go the other direction in some strains that are not as close but perhaps close enough to cause mischief. But those are words in the puzzle that still need to be filled in.
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Please stop spreading bad science journalism... THANK YOU!
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